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I’m forever trying to find correlations between measurements and sound and recent thread on NFB got me thinking about distortion YET again. To post a response I needed to search and review my notes and measurements on some amps I’ve played with over the past few years.In doing so it reminded me about my first observation of distortion cancellation between the driver and OP stage of an SE amp. I’d like to share my findings and invite discussion, especially from those that have had similar experiences and particularly from the techos amongst us who might wish to comment.
I’ll pre-empt the following by stating that I’ve read with interest the theories of the benefits of 2nd harmonic distortion and the evils of odd order harmonics, particularly high order. At first all seemed logical, then I doubted them and now … well, I partly believe them but I remain unsure about the threshold of our perception of high order harmonic distortion.
Equipment used for my measurements were an HP8903B Signal Generator, which was later replaced with a Stanford Research DS360 Ultra low Distortion Function Generator with a typical THD of less than 109dB at 1V from .001Hz to 5kHz. Pico ADC-216 Spectrum Analyser software was used to monitor the results. This 16bit version has a specified dynamic range of 96dB with a typical figure closer to 110dB. Also used was a Tektronix 468 100mHz Digital Storage CRO.
I’ll begin with some measurements I made a few years ago on a pair of SE1 300B monoblocs using RCA 6SN7 drivers and JJ 300Bs biased at a high 80mA. Load was 4 ohm. OP power was 3W under test. Freq was 1kHz sine wave.
% Distortion at grid of OP tube -
2ndH, 3rd, 4th, 5th
2.78, 0.1, 0.006, 0.001
% Distortion at speaker terminals -
0.27, 0.54, 0.103, 0.026Same as above with 6N8P in driver position in lieu of 6SN7
Grid
2.09, 0.17, 0.035, 0.004,
Spkr Term
1.00, 0.60, 0.072, 0.030Observations
1 – Distortion cancellation between driver and OP stages is evident,
presumably due to the effects of opposing transfer curves.
THD is mainly comprised of 2ndH and (more than expected) 3rdH.
2 - Different tubes have slightly different distortion characteristics.
3 - OP distortion can be reduced by increasing out of phase
distortion at the driver stage. However, this could also change the
relative harmonic spectra of the distortion.Similar results were noted for an early version of my previous single tube SE 845s, which were the forerunners of my current amps. Driver tube was CCS 6N8P, Primary load was 8k, HT was 960V, Idle current was 80mA, Power was 10W.
% Distortion at grid of OP tube -
2ndH , 3rdH, 4thH 5thH
0.11, 0.004, 0.003, 0.001
% Distortion at speaker terminals -
0.40, 0.015, 0.002, 0.001Same as above with CV1988 in driver position in lieu of 6N8P Grid
0.035, 0.002, 0.001, 0.001
Spkr Term
0.55, 0.027, 0.001, 0.001,Observation – Similar results to the SE 1 amps, albeit with lower overall distortion. Note the significantly lower distortion of the CCS loaded driver in this example.
Question.
Can you engineer lower total amp OP distortion by adjusting bias on the driver stage? Well, apparently you can. I accidentally found this to be the case by substituting the 6SN7 driver with an ECC33. It sounded much better than the 6SN7 but intuitively, it shouldn’t have because as expected, due to its lower gm the direct swap had the ECC33 biased at such low idle current (<2mA) that it was running in its non-linear region. However when I “fixed” the situation by increasing the current it sounded worse than the 6SN7.In order to properly compare different tubes and bias currents I made the cathode resistance adjustable. I discovered that contrary to the norm, biasing the driver tubes to lower currents than normal sounded better … sweeter. Low and behold this was also entirely consistent with my distortion measurements.
Further, I found that the more current I ran through the OP tube for lower local stage distortion, the more current I needed to run through the driver tube for optimal linearity. In other words, make the OP stage more linear and less compensation (read distortion)is required in the driver stage. BTW, there are limits and there is an optimal driver current (which depends on the current through the OP tube), below which distortion rapidly increases.
Conclusions?
1) I can now understand the disagreements that continue to rage between inmates regarding tube bias. In fact I recall one of my own with Jeff (drlowmu). I have always advocated higher currents and whilst my belief remains intact for an individual stage, I can see that this will not always be the case depending on the preceding or following stage.
2) The same principle outlined in 1) applies to tubes. Low distortion tubes are generally better but depending on the preceding or following stage this will not always hold true.
3) It’s surprising how easily our ears can pick minor differences in distortion when we have a reference.
4) I’m convinced that SE amps do not sound good because of their high 2ndH distortion. It is possible to engineer an SE amp with very little overall distortion including 2ndH (although it should remain dominant), without the use of Global NFB for correction … and still have that SET magic.
5) Lower distortion is generally perceived as a sweeter sound with less grunge that, I might add, you sometimes didn’t even know existed until it’s removed.Obviously we cannot perfectly match (in opposition) the distortion characteristics of the OP stage in the driver so this method of cancellation seems suspect mainly due to the possibility of increasing high order distortion. From the results I can only surmise that this is not a problem provided that neither stage contains high order distortion beyond very low levels ... very easy to accomplish with triodes, particularly when CCS loaded.
I apologise for the extremely lengthy post. However I felt the need to balance it with sufficient detail to make sense.
I’ve been meaning to put this forward for discussion for some time so comment is most welcome. Please remember that this is only my perception, based on my experiences. BTW, I’m also not claiming anything new … just trying to put a few things together because I think we so often isolate them to the point that we cannot draw meaningful conclusions.
In this case what I measured is what I heard and not necessarily in that order. In other words, sometimes I heard things that just happened to be supported by later measurements.
Naz
Follow Ups:
Naz, I have done similar measurements and got similar results (on a 300B with 6SL7 or 6SN7 mu stage) Initially it seemed that better tubes got less distortion, but the biggest influence was the bias current in the driver tube.I recall that when reducing bias on the driver tube overall the amp became more linear. However, I also noticed that the bandwitdth of the amp and gain reduced by doing so.
Have you seen a similar phenomena?
Thanks, Pieter
Pieter, but the results for the two amps were predictably different. The gain and BW of the 300B amp reduced although not significantly because the driver was a conventional grounded cathode stage. By reducing the idle current the tube works in the less linear and lower gain region of the curve. Rp also increases and, depending on the circuit, that could affect BW.The 845 was my own design using a CCS 6H8P driving a 6H30P White CF driving the grid of the 845. Because the WCF has a little under unity gain and does not invert I varied the idle current through the 6H8P since this is is the stage that could cause the cancelling effects. The gain varied even less significantly but the BW remained the same since the CCS is buffered by the WCF (which BTW also makes the CCS work better).
Rgds,
Naz
I think I agree, in that SET amps sound best....and not because the listener is inclined to accept high levels of even order distortion, but because of the inherent superiority of the devices and simplicity of circuitry involved. My experience indicates that either distortion is not absolutely inherent in triodes or SET circuits, or harmonic distortion is an invalid construct...a misdirection, if not outright bastardization of science.This is also apparent when I listen to PP amps. Whether using triodes or pentodes, they always have a deliterious effect on the musical presentation. Although presenting what initially sounds clean and 'fast' they also sound constricted and dry by comparison to SET amps. The latter aspect seems directly proportional to degree of NFB in the circuit. The human result is a lack of involvement [loss of interest] or outright listener fatigue. But they do measure better....at least I have been lead to believe.
The other obvious variable [the elephant in the room] in any 'test' of an amp is the speaker. If you fail to utilize a sufficiently high quality, high sensitivity speaker or worse, you miss the point and your experiment fails. IMMHO.
The other, other obvious variable [flaw] in your test is your acceptance of CCS as a standard PS. If you really want to conduct the test of absolutes, you must rid your circuit of ALL compromises. I could pursue this aspect a long time.
Of course I could be wrong....but I don't think so since the ultimate test is on real subjects, with real ears. Or did I miss something? :)
Later
D
....that speakers are actually relatively unimportant! Now, before I get a truckload of bricks thrown at me, let me explain: yes, there's a difference in quality between speakers, but my impression is nowadays even cheapo's are pretty darn good. It's like phono cartridges - even a cheap MM cartridge today blows away anything available in the 60's except the highest of the high-priced boutique stuff - but perhaps that's a different argument - what I want to point out is that the jaw-dropping differences I heard with two amp mods - one a PP, the other a SET - when I went to triode strapping were obvious and stunning, and in both cases I was using cheapo speakers. In the first case a cheapo vertical pair of fullranges with a tweeter in between - there's a special name of that configuration - and in the second case a plain old 12" high efficiency guitar speaker in an unenclosed small cabinet. So, I think speakers make a difference in terms of the absolute quality, yes, but in terms of relative differences between mods in an amp, the differences are so essentially staggering that the speaker becomes relatively unimportant. Note that I am leaving volume consideratiions out; both of my present choices are high enough efficiency that in neither case could Max or I turn up the volume to more than 2/3 without it becoming deafeningly loud.By the way, when I triode-strapped the guitar amp, Max turned to me with his mouth open and said, "How did you do that? It was already great, I didn't think you could make it any better!!" heh heh heh
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
When I read your first statement I had already started to write "quite the contrary" but you went on to explain an observation that I totally agree with ... speakers have indeed improved dramatically over the years and they are now not the weak link (by a light year) that they used to be. This, in my book makes subtle changes in downstream components, particularly amps all the more discernable and perhaps more can be gained my giving it more attention. Kinda like good wine eh, you don't know what you've been missing until you try it and then your paradigm changes completely!I concur with triode strapped anything, and find it hard to believe that some have trouble hearing a big difference. But, again it may have to do with other components masking the improvement.
Best,
Naz
They are good when you need power: orchestra, big choir, organ.They allow using broad variety of speakers.
Pentodes sound natural and undistorted when used properly. I have been there, I've heard sound of triode and pentode connection, and I like them both.
Saying pentodes are not good is same as saying that classical tube amplifiers are no good. Macintosh, Marantz, Scott, Harman Kardon, all no good. Sorry guys, I disagree.
probably anything can be made to sound good if optimised. I personally don't believe in absolutes, especially if I don't understand the forces at play that makes one thing different from another. And I can't say I understand or agree with (yet) the theories I've read for what makes Triode strapping sound different.I can only comment on my personal experience thus far and in every case Triode strapping subtly and sometimes dramatically made the sound more relaxed and three dimensional. I'm also reasonably sure from memory that the distortion characteristics are improved in Triode mode. How much of this is due to the fact that internal rp is reduced (and its consequential effect on the load line) I don't know.
If that has not been your experience it may be interesting to explore how you are using Pentodes in a different way to try and make some sense of it. Can I ask what sort of load criteria do you apply?
Naz
in a heated triode-vs-pentode exchange here a couple of years ago. My points were:1. One of the ways to reduce distortion is to use the amp at a small fraction of its power. Can be easily done with pentode amps, and not so easily with triodes.
2. Pentodes are quite linear with Ug2
3. Triode sound is euphonic, but pentode sounds closer to reality. 3. Due to high plate resistance, some NFB is needed to reduce amplifier's Zout. I know that for many NFB sounds like obsenity, but it is my belief that 10-15 dB of NFB don't do much harm. Pentode amplifier can can work very well without NFB at all if it is used for midrange and highs only, preferably with a fast decay driver, like BG Neo-8.
Its a pity that the overwhelming majority here are so convinced in the superiority of triodes that they never bother to experiment with pentodes.
With Ug2 I meant that it should be considerably higher than Ua and have good regulation.
If you have a class A amp with no feedback, distortion is a function of signal level and since most listening is done at small (average) signal levels the sound will be nice and clean. But with a class AB amp the feedback loop introduces a distortion that is not signal dependent and might hurt low level detail.If my thinking is correct it would imply that the class A amp would measure best at low level but the class AB amp would measure better at higher output (perhaps best right before class B). Maybe someone has the ability to measure and report on this.
Throw in the facts that pentodes are not as tolerant of load variation (and we have a big one when we transistion to class B)and the fact that I for one can't think of a class A pentode amp with global feedback and it makes it even harder to say just what impact global feedback alone has and how much it is a mixture of design compromises that seem to be included with global feedback amps in general.
Russ
We are mixing terms a bit here. In theory, NFB is applied as a ratio and therefore should affect all levels by the same percentage. Whether it does in reality is another thing. Obviously NFB can be applied to any type of amp using any tube but you are correct that zero feedback Class A will indeed produce lower distortion at low levels.Regardless of tube type, Class A is inherent in SE but could be used in PP whereas Class AB refers only to PP. AB suffers most on the transitions from A to B and that translates into higher % distortion at low levels due to the fact that all tubes are very non linear near the cutoff point. This could be improved by reducing the applied load but it would have to be much less than would normally be used in the OP stage of a power amp. The problem with this type of distortion, as you rightly put it, is that it occurs at levels where we typically do a lot of our listening.
If there is a consistency in my book it's that those who like PP tend to prefer them for their high power or their sound at higher OP levels. Few seem to choose them for their sound at low levels whereas for SE it's the opposite. This does seem to coincide somewhat to the distortion characteristics of the two topologies ... at least as far as ratios go.
The high output impedance of pentodes compromises the OP stage too much and considerable NFB would have to be used to improve the situation. In this regard my comment was borne from the observation that every time I've triode strapped any tube it sounded better and since no other changes were made I wondered how much of this was due to the fact that triode mode offers lower OP impedance and therefore lower distortion by default. Personally, I think it's not the primary reason and would like to hear other theories.
I also think that NFB is the least damaging to amplifiers that are inherently wide-band before NFB is applied. In other words, I think it can somewhat successfully be used to correct for non-linearities but probably causes too many phase related issues where its primary use is for extending bandwidth.
Naz
When you triode strapped a pentode it was a Pentode amp with a typical ( read CRUMMY ) G2 supply. sser2 is talking about a Pentode amp with an optimized G2 supply.I built such optimized G2 supplies in my Paoli 60M projects ( posted on this forum ) and I must say, these amps were nothing like the amps we typically experience. A whole new ballgame, with detail and lotsa good old fashioned and TRUE authority.
Jeff Medwin
You are correct in that the mod was the standard typical mod. I only comment on what I've experienced so I can't argue that what you say is wrong. Where a new experience supersedes a previously held belief then I have no problems reporting that. A good example is the difference of opinion we once had regarding bias. I have not changed my mind per sae but have since experienced that under SOME conditions lower idle current can be beneficial eg, in the driver stage of an SE amp.This is something that I observed to sound better and because it was at odds with almost all of my other experiences I investigated in pursuit of a plausible reason ... hence the primary reason for this whole thread. BTW, I still believe that higher idle current sounds and measures better in most situations.
sser2 did make the comment that triodes are euphonic, and that, I most definitely don't agree with. Done right they are very natural and provide a real sense of space that I have not heard from other tubes ... not that I'm saying it can't happen. I'll try optimising the G2 supply one day and report back with my impressions and with measurements. I wonder if it might happen to work well as a driver, ala lower bias on a triode?
Naz
Hi,Well, someone like Dennis suggests on this forum that tubes, transformers, lawnmower and car engines, etc., all work best at about 5/8ths to 2/3rds of their maximum ratings.
The rating on a tube is derived of two components which we can adjust........current and voltage.
Maybe we need to routinely use lower voltages and "decent" current, ( below the published A1, etc. " by the book" operating specs ), to gain a more relaxed sound. Also gained, MUCH higher component reliability and tube life.
I've heard this effect, a "sweet spot" on Finals listening to music, and Wm. Lee has made similar reports up here. Listening to the "by the book" ratings sounded strained in comparison. This listening was done by me on amps whose supplies were quite optimized, and dare I mention, low in DCR.
Jeff Medwin
I see the goal of the thread as an attempt to explore two main concepts. One, can we make a correlation between measurements and sound quality. I think many here feel we can do much better in this area. Two, are we copping out and short changing ourself's by rejecting global feedback. Again, I think many here feel we could do better in this area. Not all may be interested in exploring such things.Now don't get me wrong. I am quite certain the large majority of amps the fine fellows here build do not have global feedback. Likewise I am sure the ears win over the scope. But I do think most are moving towards more linear tubes operated in more linear ways and hearing an overall improvment in all amp types. So I do think distortion is important.
Russ
Dave,We are on the same page on all points contained in your first two paragraphs. I think you have misunderstood where I'm coming from thereafter.
Firstly, my speakers (of moderate sensitivity) were used in two ways. The first was to listen and observe ... what I liked and disliked, before any measurements were made, which were done in an effort to correlate what I was hearing. The second was to observe the measurements, perhaps after making a change and then see how this resulted in what I was hearing. What I've presented here is a mere summary of some interesting points ... nothing more and nothing less.
On the subject of CCS I was referring to a Current Sourced Grounded Cathode gain stage. This to me is about the purest way of running a triode; others may differ.
Ultimately, the tests were not made in pursuit of absolutes. They purely comparative and are presented only to back up some observations ... both ways. I'm certainly not claiming to have found the Holy Grail.
Thanks for your comments,
Naz
Would you explain this to me?
I seek clarity....as always. :)How does it differ from fixed bias?
Later
D
Well, it really has nothing to do with it as a CCS gain stage can use either fixed or cathode bias. The bottom tube is pretty much a conventional grounded cathode amplifier (and can be biased as such); the only difference being that it has tube on top acting as a high impedance load in lieu of a simple resistor. This provides a much better load line (almost horizontal), leading to far less distortion.Naz
Some primitive manipulation of the figures given, in the same order :2HD 3HD 4HD 5HD THD (wt)
drive: 2.78; 0.1; 0.006; 0.001; 3.002
spkr: 0.27; 0.54; 0.103; 0.026 ; 1.763
dB: -20.3; 14.6; 24.7; 28.3;
drive: 2.09; 0.17; 0.035; 0.004; 2.551
spkr: 1; 0.6; 0.072; 0.03; 2.536;
dB: -6.4; 11.0; 6.3; 17.5;
drive: 0.11 0.004 0.003 0.001 0.131
spkr: 0.4 0.015 0.002 0.001 0.44
dB: 11.2 11.5 -3.5 0.0
drive: 0.035; 0.002; 0.001; 0.001; 0.046
spkr: 0.55; 0.027; 0.001; 0.001; 0.611
dB: 23.9; 22.6; 0.0; 0.0This is interesting because it strongly confirms the results suggested by modelling, namely that the increase in higher order even harmonics roughly correlates with the decrease in second but that the increases in odd order (especially third) are complicated by the importance of the delta mu terms (variation in amplification factor with grid bias and with current) which in turn are somewhat reduced at higher levels of 2HD (because of the presence of cancelling intermodulation terms).
I have also applied a rough "weighted" THD = 2HD = 2 x 3HD + 3 x 4HD + 4 x 5HD.
Mark Kelly
Hi Mark, your input is much appreciated; it's great when something makes sense.Pico actually provides a THD figure but more often than not it appeared to be too low. I assumed some cancellation of the harmonics must be at play but perhaps not.
Interestingly it was much closer on the 845 amp, which happened to have lower OP distortion. Figures for the 845 amp were Drive o.11THD and Spkr 0.41THD for example one and 0.035THD and 0.55THD for the second.
However, for the 300B they were Drive 2.78THD, Spkr 0.61THD and Drive 2.1THD and Spkr 1.18THD respectively.
Can you offer a possible explanation?
Thanks,
Naz
The program is probably calculating the THD figure that would result if a notch and measure type distortion meter were used. This gives the vector sum of the distortion components and like all vector sums the result depends on their relative phase.
Mark Kelly
that if the relative phase angle of the harmonics is low then the THD figure would closely match the theoretical calculation? In other words, applying this to the measured results could this be interpreted to mean that the 845 amp had lower phase distortion than the 300B, if so it appears to be consistent?Thanks,
Naz
Now I got to take the time to figger this out, since it is obviously meaningful.Why didn't you just forget to turn your machine on today; you could have saved me so much work.
Yer pal,
Poinz
Fascinating report. What about measuring at the 300B plate instead of the speaker terminals, so as not to mix the OPT in? The measurements reveal a wild world of cancellations, as far as I can see. Thanks for posting this... --keto
Hi Keto,What happens in at the plate is largely a reflection of what occurs at the speaker terminals but is more difficult to measure due to high voltages.
BTW, I should have stated that the 300B amps that I measured should not be taken as representative of all 300B amps. They are also capable of producing very low distortion if done right. But I think that too many designs make power a priority to the detriment of linearity.
Naz
Naz
I can bet your not using the 300b on the Logans..Anyway that is strange as I look at the input distortion numbers to the 300b is lower with the Russian 6N8P as compared to the 6sn7 and this is on the 2nd,3rd,and 4th harmonics and yet the output distortion numbers are higher inspite of having lower input distortion. I dont see how you can put in lower distortion figures with one tube and yet the output numbers are higher than the tube with the higher distortion in the 1st place..I believe your measurements but are you using any global feedback going back to the driver stage? That would account for some of that..The numbers are very low in all cases anyway and if you used different equpiment that could account for some of it to..Anyway thats excellent data you collected and it can help a lot.
Hi Mike,Nope ... 845s for me.
Russ is right, the point is that generally in a properly designed SE amp the OP stage produces most of the distortion as the tube is loaded much more than the preceding stages in order to transfer power to the load efficiently. This is predicted by the load line on the plate curves. Imagine for a moment that we are driving the OP tube with the same type of tube with exactly the same load but in opposite phase ... cancellation! However the mechanism is completely different to PP amps.
Zero NFB was used but the low numbers are real. Admittedly the OP voltage level under test conditions was only 10V P-P on both amps, however you can sacrifice power for linearity by using a higher than normal impedance load. Many commercial amp manufacturers do not do this and either have to put up with high OP distortion levels or use NFB to correct things.
Rgds,
Naz
"Imagine for a moment that we are driving the OP tube with the same type of tube with exactly the same load but in opposite phase ... cancellation!"
Naz, is the distortion product out of phase with the fundamental at each stage?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre,Yes, each stage that inverts the signal eg a typical grounded cathode circuit will be (at least for in band signals) 180 deg out of phase with the previous and therefore some of the distortion components will cancel. The degree of cancellation will depend primarily on the matching of the load lines and transfer curves.
Naz
The phase relationship between the fundamental and the distortion at each stage is what's important. No?For cancellation to take place the distortion needs to be out of phase with the fundamental at the output of each stage. If the distortion is in phase with the fundamental at the output of each stage the distortion will add.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Mostly it's not a problem with SETs, it happens naturally as they tend to be simple in nature using a lot of grounded cathode based designs at each stage. In a multi-tube non-inverting stage you could look at the collective output but it's harder to predict what is happening.In a three stage SET the first stage adds to the third. So for instance if you made the first stage extremely linear, the cancellation that you could engineer between the driver and OP stages would not be greatly affected since the first stage distortion is comparatively small.
Naz
Naz,Isn't the interstage cancellation inversely proportional to the gain of the second stage? Your measurements and these other posts make me think even more that direct-coupling between stages might be a good thing, because of (at least) one less element in what is already a complex mix.
--keto
Yes, one of the reasons I use direct coupling everywhere I can. It takes away any possible arguments about the quality and or relative merits of coupling caps and IS trannies. I figure you can't get better than a straight piece of wire ... no phase issues!Naz
But I'll let Naz answer. In the meantime think about phase of the signals and how a "distortion" in one could counteract a "distortion" produced by the second.Russ
Russ, what is the phase relationship between the fundamental and the second harmonic distortion tone in the output of a single triode stage?Thanks...Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
We measure, discuss and debate harmonic distortion, but somehow IMD gets left behind. Show me a SET (or any amp) with rich harmonic distortion, and I'll show you an amp with lots of intermodulation going on with music. It's not just a matter of how much total %IMD we observe, but the character of it: the order, the ratios or orders, the phases, etc. THIS deserves more attention, IMO.
yea, along with back-emf effects of the output/ to a reactive load like a passive crossover and motor load (especially if feedback is used back to the - input), driver headroom before the onset of clipping, open loop bandwidth uncompensated of each individual stage. The list could get kinda large! It's just difficult to build with imperfect devices. Ray
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
I thought that harmonic distortion and intermodulation distortion are different ways to measure the same thing: the nonlinearity of the amplifier?
Kinda. Sorta. The kind of IMD that is usually measured derives from the same transfer functions that give rise to harmonics. But an amp will have a different transfer function at one frequency than at another due to reactances varying as a function of frequency. IMD testing gives a chance for the amp to be tested with more than frequency at a time, a bit more like music than a single frequency sine wave. Furthermore, there can be (and is) something called phase intermodulation (think FM versus AM) that is very bad on the ears and cannot be deduced from harmonic distortion results alone.
Yes, I agree. IM measurement can be used anywhere in the pass band. I think that the real problem is exactly that: distortion levels changing with output level and frequency.With modern computers and a good quality soundcard it should be possible to do automated IM measurements for a good range of power levels and frequencies. Would that be helpfull?
Yes, I think so too. Sound cards have gotten pretty good, at least from a specification perspective, if not for listening. I'm looking at an E-MU 1616M right now for measurements. With that and cheap or free software, a laptop can measure individual spectral components of IM and HD to very low levels.
that IMD is extremely important, especially with complex music where all sorts of things are going on simultaneously. I didn't want to overcomplicate things in one post but my take is that if every stage is engineered to be as linear as possible to begin with, then within limits, the driver stage can be "tweaked" to produce a better result at the OP that is both audible and measurable.My equipment has the ability to measure the IMD products produced by two simultaneous tones. In this case I was surprised to see how low this was and therefore did not document the measurements at the time. This is something that I have always intended to revisit but just haven't got around to. In my mind I was comfortable with the fact that each stage was as linear as I could get it and the amp sounded so good that I kinda bought the evidence that IMD must have been insignificant. To be honest, deep down even I have minor doubts about what I measured so I will try to revisit soon.
As for phase distortion, I must confess that I don't know how to measure this. I will make the point that if each stage is VERY linear AND we stick to VERY wide bandwidth in each stage AND VERY low if not (as I prefer) zero global NFB, then we have done most of what we can to minimise phase distortion. BTW, in each case I'm assuming a very good power supply.
Naz
Although I do not have proper lab equipment or the free time to back up my comments, I said in the past IMD products are a serious issue regarding great audio.
Too bad work won't leave me alone long enough to digest Naz's post. I can see right now I am going to have to get some serious measuring gear and a quick education if I hope to contribute much.I feel the remarks about some amps falling apart on complex music to be in agreement with what I hear. I'll give the nod to push pull as well, but remain open to the concept that a high enough wattage SET could do the job. Personally I have always felt that a SET has an inherent problem with the plate's Rp changing as it moves up and down the load line.
I look forward to what can be accomplished. In a perfect scene we would all have the same "benchmark" speakers/front end/etc.
Russ
I am going to have to take so much time out from my workload to follow this damned thing, I am just utterly irritated.As a fellow adventurer, how can I say 'no!'?
Shit!; er, Poinz
Thanks Russ,I think your observation about the tube's rp changing is generally correct but this is always the case with tubes, even with gain stages ... a reason why I like CCS. To minimise the effect in OP stages I always sacrifice some power by keeping the primary impedance high. My point about optimising the driver also seems to help offset this limitation.
In the case of my parallel SE 845s I run them into A2 on peaks to make up for the small power sacrifice. These are by far the best amps I have been able to come up with so far ... a very good balance between realistic, three dimensional sound and sufficient power to provide enough headroom to do justice to large scale music.
Incidentally, we know that SETs have limitation at high levels where 3rd harmonic (followed by other high order distortion) increases dramatically even prior to clipping and I think that insufficient headroom is probably a factor affecting the reproduction of complex music (that often coincides with high levels) in many systems.
Naz
I usually listen to records with big orchestras and choirs like Wagner. I listened to live performances in the Bayreuth Festival theater from the early days of my life. There are sitting about 110 musicans in a partially closed orchestra pit, because Wagner didn't want to kill the voices of his singers. There are reached sound levels above 130dB in the pit. Belive me, I know the distortions of my ears very well. I never listened to a SE amp below 100W which can reproduce a sound close to a live performance, also with normal levels at home. A well done tube PP-amp with feedback comes closest acording to my ears, despite the fact that I am using AER speakers with a high sensivity. Also single instruments with a high spectral density like the cotrabass trombone (harmonical content up to 15kHz at a deep bass note of 50Hz) couldn't be reproduced by a sE amp. It sounds horrible with such an amp. The same with more than 30 violins playing the same note.
My experience is that an amp, that could play complex music, also could play more simple music. It is wrong vice versa.
Hi, sorry I missed answering your post. In the power stakes I'll admit that it's hard to beat PP and perhaps for complex music PP generally does have the edge. But I find I have no problems driving my ESLs to quite load levels with no discernable problems on complex music.My parallel 845 SETs are up around 75W. What they do much better than any PP I've tried including my own modified Ref600s is provide a gorgeous 3D soundstage that just doesn't cause fatigue.
Mind you I have also heard some SETs that have been very disappointing.
Naz
wow, now that's what I call a truly helpful post.
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
Naz, interesting post.Let me just add another dimension to the discussion. I have found that the complexity of the music being auditioned makes a big difference in listening preferences, at least for me. When listening to SETs, I am most impressed with music that is simple; for example a female vocal with acoustic guitar, or a frugal arrangement of jazz with sax, bass and drums with a lot of “black space”. With just the right music, SETs can put you “there” in a way that most other amp types can’t. But with many SET amps, when the going gets rough, the sound gets confused. Sometimes we rightly blame this on overload, given the small output power from most SETs. But I’m trying to separate this confusion phenomenon from gross overload. Listen to a very broadband complex piece with a large number of instruments through several kinds of amps, while keeping volume levels reasonably low to keep overt clipping out of the picture. Now do the same thing with a simple piece of music. With the more complex selections, I tend to prefer good PP amps, even some with NFB. I’m very sensitive to distortion in complex music. To my ears, the cacophonous confusion with complex music is very irritating and can be a show-stopper. Many amps with NFB will add a different kind of grit, harshness and an edge to complex music, but then again some don’t. A well-designed PP triode amp with low or no NFB can usually do complex (as well as simple) best, IMO. Speakers and cartridges also show their relative worth with complex music (I think ESLs win hands-down with complex music). Have you noticed how some audiophiles (and manufacturers at trade shows) will select demo discs, perhaps unwittingly, based on how well their equipment performs on complex versus simple? I’ll bet some folks’ musical collections are even influenced or limited by their choice of equipment.
I suspect, in amps at least, that the confusion with complex music is due more to the inevitable production of intermodulation products rather than just the added lower-order harmonics. Anything with a bent transfer function, especially seen in SETs, will make lots of IMD too. IMD is potentially less noticeable when the instantaneous spectrum contains relatively few frequencies; hence the better performance with simple music. I’m NOT bashing SETs, just pointing out possible strengths and weaknesses, and asking if others have observed the same.
Would you elaborate on the amps, tube compliments and speakers you did your testing/ comparisons on?Would you also elaborate on the relative nature of the music on each? By that I mean an explanation [however crude] of how the music sounded on the different setups you tried would be helpful.
I ask this because my experience, and especially my conclusions directly contradict yours.
SET creates a music that is the utmost clarity, tonality and living, breathing quality...not to mention scale that PP has never even approached. IME.
I don't mean to say it is impossible, but I haven't come across anything that even approaches the next block, so I remain pretty skeptical.
Of course, a 45 or 2A3 SET on Maggies won't float my boat either, but a 304TL on ALE 5-way horns sure does...and an 845 on my PHYs works pretty well too. :)
Later
D
Equipment used? Where to start... I've been playing with tubes for almost 35 years, commercial amps, modifications of commercial amps and my own (and friend’s) new designs. From flea power 12B4 SETs and 7119 PP triode amps through a monster 833 PP ESL direct-driver. In between, 845 and 211 SETs, 845 PP, 300B SET, 300B PP, a smattering of 2A3 and 45 SET auditions. My KT88/6550 designs in both UL and Class A PP. As a former high-end sales person, I got to audition all kinds of wild and crazy commercial offerings. My first vivid SET memory comes from the grand Uesugi 211 SET of the seventies, which I still recall as an amazing experience. My overall observations have been built on years of listening, not just on yesterday’s amp in residence. Some people might say “if you haven’t tried a hand-blown, cryogenically treated 45 with the Z shaped getter and coupling caps made of ancient papyrus and organic flax oil, then you haven’t heard what SETs can REALLY do”. This line of BS can be taken by any side of any audio debate. After a lot of critical listening to a variety of amps, speakers and music, you start to develop certain consistent observations (biases?). Speakers? Klipschorns, high sensitivity Fultons with ESL tweeters, Quads of both kinds, LS3/5As, my own DIY ESLs, a touch of just about everything that’s come down the pike; but I do not have much experience with full range Lowther/Fostex/whatever types, although I have heard those more casually in stores and trade shows.You said “SET creates a music that is the utmost clarity, tonality and living, breathing quality”.
I’d have to agree with that, especially with simple musical arrangements. But I think that when you put on Wagner, or the Mahavishnu Orchestra in full tilt, or the rich layering of a latter-day Bryan Ferry studio tune, or a dense percussion ensemble, there can be a congestion/confusion/intermixing that occurs more in SETs than in decent class A triode PP amps (yes, even at lower listening levels). Individual instruments can get befuddled and less distinct. A sense of harshness, in one sense of that word, albeit different from the glaring ice of some lesser amps, but still there. I find that effect pretty annoying. To me it’s a bit reminiscent of inner grove distortion in analog. So, again, I do respect, even love, SETs, especially on simpler music. My point was not to say that PP is absolutely better than SET, or vice versa; I was just recounting my experiences with complex music versus simple music and how these musical choices sound to me with various tube configurations - for the purpose of further discussions of distortion mechanisms, the point of this post :)
Thanks for the rundown. I have found, however, that the speaker DOES make a huge difference. I normally use a PHY KM30 full range [approx 99db eff 13ohm load] and it has many outstanding qualities, but it will congest a little on loud, complex musical passages. This happens with low power SET 2A3 or 45 more often and quickly than higher power 845, 75TL, 304TL amps. It occurrs to me that with low power amps, the congestion could be accompanied by some clipping as well....a problem exascerbated by the wrong speaker load.When listening to a system of higher efficiency, say 106db 8ohm, where the impedence and efficiency are optimized for SET, even using a flea power globe 50 SET will reproduce music with a minimum of [at least noticable] compression by presenting the music with a much higher level of dynamic attack and resolution than is possible from low imp, low eff speakers.
I say 'minimize' because the introduction of a higher power SET amp seems to improve the sound and make that experience even more dramatic. Then the introduction of a SET amp that uses ultra low distortion triodes like 304TL, makes even more improvement to the experience.
Then the upgrade to the drivers makes yet another improvement, etc. And remember these improvements are NOT the subjective 'improvements' that high tech power cords or esoteric IT and speaker wires make either....nor the 'dramatic improvement' of cryogenically treated triodes and wall plugs make. [How's that for a troll? :)]
My other observation is that the human ear tends to get confused/congested when presented by a lot of instruments playing simutaneously, and loudly. This seems to be independent of room/venue. I am talking of orchestral music/settings rather than amplified stuff. There are times when I find that I [often] can't hear specific instrument placement, they all become like a huge aural tapestry where the instruments all weave themselves into a sort of sonic textural blaket.
Quite different from typical multi-mic recordings, but I experience much the same effect that I would recognize as sonic compression.
So I wonder about the necessity of 'trading off' what I love about SET amps for the purported benefits of PP [forget NFB], especially when we all know of the deleterious drawbacks. When we can just get better speakers.
You guys are right about one thing, the quality of SOME speaker drivers have improved dramatically in recent years, but not all work well with SET amps. Trying is only fitting the square peg...
Later
D
Have you tried ESLs? After living with, say, a pair of Quads, everything else sounds so "dirty". By the way, all modern ESLs are inherently push-pull. Maybe someone needs to reintroduce those ancient high-distortion single-ended ESLs; we'll call them "SEEs". A new craze.I'm sure the ear congests somewhat with complex live music, but unless the volume is crazy high, I can decode complexity a lot better in a decent live venue (say a concert hall) than I can with most hi-fi systems. But I've been in too many dance clubs and nightclubs where the "live" sound was horrendous - talk about congestion! But then what we're really hearing is being amplified by mediocre equipment, thrust at us at unholy volume levels.
I understand what you like about ESL speakers, but you know that they hardly present the kind of load that works with SET amps.And Klipsch, as good as they are, don't quite match up with many other vintage offerings like Altec, etc, and are a far cry from modern compression driver technologies.
Regardless, I still have some difficulty accepting that congestion originates in a suitibly matched SET amp/speaker combination, which makes me wonder if pursuit of your line of reasoning will ever bear fruit, or not....but you will like what you will. :)
Heard the 'Rippingtons' last PM in an outdoor venue. I think that current pro-sound set up philosphy may be the 'ultimate evil' when it comes to quality audio. Of course, I don't like soft [fluffy] rock/smooth jazz either.
Later
D
So then you have to choose either ESLs or SETs. I've made my choice ;)
Actually guys, I've pretty much proven (at least to myself) that you don't have to choose. After playing with PP for quite some time I tried a commercial 300B SET and was unimpressed. As a result it took me some years to rediscover how good SET can be and that occurred only as an evolutionary process.First I started modifying my huge PP ARC Ref600's with some great success ... choke, Triode Mode etc. Then I wondered whether smaller was better and bought a little Cary Signature Superamp. Anyway, after doing the rounds and auditioning all sorts of gear in my system since I have a great relationship with a top high end retailer here, I was sufficiently encouraged to give SET a fair go and set about building some serious 845s. BIG improvement over anything I had previously tried in my system including (and especially) some serious (and seriously expensive) class A and AB SS monos. This was the sound I wanted, the 845s stopped the "beaming", music became so 3 dimensional I couldn't believe it. Detail, dynamics, delicacy ... these amps had it all except for sufficient power. So I set about building some parallel SE 845s driven into A2 on peaks for around 70W or so.
A few years earlier no-one could have convinced me that parallel was good but this was another quantum leap. Perhaps it was mainly attributable to the simultaneous introduction of all tube rectification and better PS caps I surmised so I set about proving it. Easy to do ... I removed one 845 from each amp and changed the OP TX tapping accordingly and the sound went back to almost where I had it with my previous single 845s.
Big SETs can drive ESLs very well indeed. MLs love them but I must point out that my ESLs only handle frequencies above 180Hz. An array of 7" drivers handle mid bass and huge subs driven my high powered SS amps handle frequencies below 50Hz. I honestly can't tell you if this would be as successful with full range ESL's ... perhaps not. The questions I was asking about headroom and power trade-off were aimed at trying to better understand the reason I don't seem to have a problem with complex music. BTW, my speakers have an efficiency rating of about 90db m; fair but hardly high efficiency.
Dave, re your comment about the load that ESL's present to an amp; if you use a higher primary impedance, effectively a lower impedance tapping on your OP tranny, then speaker load is really don't an issue. You will obviously lose a little power but you gain better linearity to boot.
Naz
Thanks Naz.I have tried low eff speakers with SET amps, and agree that it still works [to a degree], but high eff is another world that I simply wish to encourage all tubies to taste/enter, because it really completes the paradigm IMHO. Of course I like the fact that HE allows me to use more simple, relatively lower power amps, and I have not heard an alternative to [Josh's] SET amps yet that convinces me to switch.
Even with a 106db speaker, I agree that higher power is the ticket. I just prefer to keep it simple and pure with [DIY] DHSET amplification throughout.
Later
D
Having played with a lot of that stuff over many years too I kinda know where you are coming from. Overall everything seems to be a compromise rather than, as some tend to believe, absolute. I'm always looking for answers to what I hear because you stand half a chance improving things more efficiently than meandering around the place (which I've also done a lot of) from time to time. When you find something that works it's also harder to keep an open mind to other possibilities ... I guess we are all human.Like to put a spin on why you think SETs are more prone to be congested on complex or large scale music? Do you think that trading off some power for lower distortion or providing a lot of headroom helps? At least that's been my experience. And at the risk of opening up a can of worms ... what about the effect of SOME NFB?
Naz
Well, without claiming to know for sure, I suspect that the same bent, curved transfer function that makes those rich lower order, mostly even harmonics with a touch of third in SETs, also makes a lot of IMD that may be the big factor with complex music. This thought is nothing new, but sometimes needs to be repeated. When music is very simple, the bent transfer function makes harmonics that are mostly consonant with the music, especially if they are in the lower orders, as we all know. Harmonics of one overtone might then intermodulate with others, but that just makes more harmonics that are still mostly consonant. When you add more instruments, you add more spectral components (frequencies) to the mix, and these are not necessarily harmonically attuned to one another. Now further add in noise-like signals like percussion, and you've got a lot going on. In live music situations, your ear and brain can sort out all these signals because the signals are merely adding to each other, not multiplying against each other, which is what modulation is. So in an amp, or speakers, or whatever, these things intermodulate just as the word exactly describes, to create new frequencies that fill in the quiet spots in the spectrum with non-consonant garbage. Your ear has to struggle harder against the unnatural layer of junk to sort out what’s going on in more complex passages. I’m not saying this describes everything that is going on in a SET, but I believe this plays a big role. I would think that a giant SET such as that 833 Wavac beast would have a “blown-up” transfer function. Music would ride up and down a smaller section of a bigger curve. When a smaller section of a smooth curve is viewed up close it looks straighter, ie: more linear. It might even sound less like what we’re used to hearing with SETs because the distortion, both harmonic and IMD will be less for the same volume level. But that effect should be good thing.
Seems like a plausible explanation. If so, then it would seem that using a higher impedance load for the OP tube(s) would help lower IMD and the amp's ability to cope with complex signals. Perhaps maximising the cancellation I've observed also helps. I can tell you that my parallel SE 845s are designed this way, (not that I knew the extent of the benefits at the time) and they handle complex music very well.This also raises some other questions. For instance how much does NFB affect IMD? You made the comment that PP does a better job of handling complex music. Part of it would indeed be attributed to even order cancellation but it's also common for PP amps to utilise some degree of NFB. PP amps also generally have greater power than SETs. Could it be that SETs tend to get overdriven a lot more? Complex music tends to be loud more often than not so this could be a contributory factor.
Your point about high powered SETs is also very salient. Obviously, if one amp can produce say 10W at 3% THD and another at 100W at 3% before clipping, then the latter should have a much easier time at say 5W. This in part the headroom I was referring to.
Incidentally, so far all of the commercial amps I have heard using inherently non-linear tubes such as the 833 or the 6C33 have been very disappointing to my ears. Again, these amps must use NFB to correct for their non-linearity ... how much is this hurting realism?
Naz
Having the power bills to prove it, I have some experience with 833s as ESL direct-drivers. Why do you say that they are inherently non-linear? I do think that they need a very high primary impedance to work properly, and it's hard, really hard, to make a decent monster high-voltage OPT with a very high input Z. Maybe that's what you're thinking. The published plate curves show very little in the negative grid bias region, devoting more space to the positive grid regions, since this was an RF tube after all. I charted the 833's lower current, higher voltage behavior, and it seems quite linear. It measures well too.
You know, it's been that long since I've looked at the 833 curves I was working from memory. I considered trying to do something with that tube a long time ago and was sure that I dismissed it for its non-linearity but you have jogged my memory and it was because of the OP tranny. Looking at the curves again they don't look so bad ... not quite as good as the traditional lower powered favourites but OK. My error, I should have used the term relatively rather than inherently.Actually that's a good thing because it looks like a perfect tube for direct coupling to ESL panels and I've been considering trying it for some time. Have you have any luck with it? If so, care to share any opinions?
BTW, what are your thoughts on ESLs? I like them but I've just copped some flack from Thorsten, who is not shy about his negative opinion of them!
Naz
answer some questions? If all else was kept a constant and a plate load choke of high enough inductance was used along with a coupling cap of high enough quality, it would seem to me that they would help with regards to the ideas you bring up (which I think sound very reasonable and likely). Of course the conclusions would be colored by the removal of DC current through the OPT.
Russ
Some people think that the DC current through the OPT actually is a desirable feature of a SET, despite the air gap and lower primary inductance. The DC magnetization moves the operating point of the transformer up the BH curve away from the flat spot around zero current into a more linear region where there may also be less hysteresis to confound the sound. The slope is no longer symmetrical, so any distortion will have a heavy 2nd harmonic component which is consistent with the overall SET theme. I'm no magnetics expert, so I hope others will elaborate.
Russ, you gotta remember that Parafeed still uses an OP TX, the turns ratio of which determines the plate load so there's no difference in this department.I've compared Parafeed on 845s ... almost exactly the same results when using the same load. In fact distortion was just a tad higher on Parafeed, and that was with a very good and very expensive plate choke and monster, hand-made 2kV PIOs!
Naz
I wanted that post back as soon as I hit the submit button. I hadn't logged in so I couldn't delete it. I was thinking like it was a normal RC stage and the choke would allow a more linear load line and the ability to swing more volts cleanly.
Russ
It's a bugger when that happens. Done it a few times myself but managed to delete the post and re-send. I probably grabbed it quickly ... been up at all hours here!Naz
...on PP vs. SET, except I haven't heard a zero NFB PPT amp yet.I expect that a high-power SET, or other SET with extremely efficient speakers, could also "do it all" since one could stay within an optimal, very linear operating range.
I have as I built two such designs in monoblocks. Both use quality MQ iron and simular PS of LCLC to another LC for the small signal tubes. One is PPP 45s & other is triode connected PPP 6V6s.Both are excellent amps with surprisingly simular sound. I and others give the PPP 6V6s a slight edge in dynamics. Both amps play complex music well.
Comparing the PPP designs to my DIY low voltage SET 845 with Plitron audio transformers, I will give an adge to the PP designs on more complex music. But, that 845 SET has more of an open sound in the mids that is stunning.
I also found out my ProAc speakers were less than great with new DIY 2.5 way speakers. Guess I am not finished here, perhaps never will be finished until my ears give out.
Hey Jimmy,I once removed the NFB from my huge ARC Ref 600s but the result was less than satifactory. What I did find though was a significant improvement in triode strapping and another by adding a choke to the PS.
Of all of the amps I've played with my parallel SE 845s are still the overall kings. Because I drive them into A2 on peaks these things produce a lot of power and a big sound but have the advantage that they can compete with pretty much anything at low levels ... delicate may not be the word but it's close. Mind you I'm running electrostatics.
Naz
I tried 3dB & 6dB global negative feedback on the PPP 45 monoblock amps & prefered zero feedback.
to demonstrate that amplifiers do to dynamic music? And don't tell me they already have for their modus operandi of measuring distortion is obviously highly flawed. There must be a synthetic signal that could be applied that could be more revealing than the steady state full duty cycle signals that is now employed. The DIN100 and the SMPTE IM standards are too old. Ray Hughes
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
There are indeed engineers who have proposed some very sophisticated measurement techniques to better correlate with listening. Sometimes these get published in journals such as from the AES or IEEE. How many DIYers regularly read these? A couple of examples off the top of my head: One technique is to use test signals comprising a huge number (hundreds or thousands) of spectral lines (frequencies) and then see what garbage the amp makes in response. A fellow named Cheever has proposed a new harmonic weighting system called TAD. I’ve been working on a phase intermodulation measurement system.This is one area where the solid state guys may be more open-minded than the tube guys. Try some searches outside this tube forum. I'm NOT saying the matter is closed, FAR from it. I’m just saying that there are some people trying. Are we listening to them or do we prefer to hide our heads in the sand and call them deaf?
BBeck,Good references.
I have been a Member of the AES for years and agree that there have been many methods of measuring different types of distortion in amplifiers. Measuring THD by itself is just a joke. It's easy to do but totally insufficient to see what an amplifier is really doing to the input signal.
A method that (nearly?) anyone can do is that used by Walker of QUAD fame. He used a passive network that had the same passive characteristics (frequency and phase response) as the DUT. The difference in the signals emerging from the network and from the DUT is the distortion of the DUT of every kind. By listening to the difference you will hear only the distortion products plus any residual of the input signal due to the inaccuracy of the network emulation of the DUT. The input signal can and should be music or whatever you want.Maybe I could find a reference but it would take much rummaging. Maybe Googling will come up with something about the Walker method. I think it never made it to the JAES. Worth a try to find. I think the method might have been the work of Walker and Albinson(?) both of QUAD.
Bill
Yes, I recall seeing Walker's subtraction test somewhere. I figured that it would be hard to get the passive phase/amplitude response to match the DUT, but maybe not. Didn't Hafler do something like that, too?
nt
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
...in the (lowly) heathkit aa-151 (PP, though, NOT SE):when I had it triode strapped, NFB cut, ultralinear disconnected, way high B+, the sound was stunningly gorgeous - just jaw dropping - on instrumentals (The Elected; guitar, pedal-steel with big chords, piano, drums, etc) - but when the vocals were added, the sound became rough, the vocals hard to understand.
when I went back to stock ultralinear with NFB, nice clear sound, vocals really clean, but the incredible "yumminess" GONE. (tears)
Went back to triode strapped, NFB cut, ultralinear disconnected, BUT dropped B+ AND reduced cathode current, and got a balance - fairly yummy, fairly clear, not the best of both, but still better than stock.
I need to get set up to do distortion measurements, so I can start correlating all of this.
I really appreciate Naz's specificity and both of your insight.
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
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