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209.163.221.154
Posted on October 4, 2010 at 08:26:03
acres verde
Audiophile

Posts: 723
Location: Big Easy
Joined: November 13, 2004
 So after 10 years and after consulting with Ralph, I decided to pull the trigger and remove the feedback loop from my M60s which feed AG Duos. Man....what a difference 2db made in the sound all those years. 2 db doesn't sound like much on paper but it sure made its imprint on the music. The spatial enhancements I'm getting now along with endless decay trails and increases in intelligibility and micro-detail more than offset the small losses in low bass articulation. If your speakers have a well behaved impedance plot, I highly recommend this easy to implement and to reverse running change!

 

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RE: feedback, posted on October 4, 2010 at 13:52:27
pathologymd
Audiophile

Posts: 22
Joined: August 9, 2003
 Removed feedback from my M-60's years ago and never looked back. Running them with Merlin VSMs.

 

RE: feedback, posted on October 4, 2010 at 15:08:24
powolny
Audiophile

Posts: 14
Location: Helmond
Joined: January 26, 2007
 I removed the feedback from my M60's about a year ago. My speakers are well performing QUAD ESL57's, with all panels refurbished.
There was a change for the better in detailing of the music, but nothing dramatic. Well below the tweaking that can achieved with tube rolling, but noticeable nevertheless. The performance obviously was already great to begin with, also with the feedback in place!

 

Feedback, no feedback and selective feedback, posted on October 5, 2010 at 00:35:34
6AS7_6SN7
 Driven by the strong criticism of many expert inmates about negative feedback I have personally verified (by experimenting with my DIY PP amplifiers) that, depending on the number of parallel output tubes and the amount of maximum feedback, frequency selective feedback can get the best relative performance in subjective listening tests.

By frequency selective feedback I mean that feedback is fully applied only at mid-low frequency, with no feedback at all above 20-40 KHz.

The simplest way to implement this kind of feedback is to place a small capacitor in parallel to the standard (feedback) cathode resistor.

From the frequency response point of view such selective feedback is the same as a gentle high boost, but it sounds different from a standard high tone control, because the high emphasis is the result of reduced feedback rather than due to a LC resonant circuit.

The likely rationale is that low frequency feedback is certainly beneficial to obtain optimal speaker damping at low frequency and that above about 5 KHz the perceived benefits of negative feedback (if any) become vanishingly small.

I think that, due to common hearing impairments due to age (most tube lovers, like myself, are quite 'seasoned') the key factors in assessing subjective audio performance are:

- perceived quality of the critical audio band 200 Hz - 5 KHz;
- perceived tonal balance of this critical band w.r.t. low (50-200 Hz) and high band (5 KHz - 10 KHz).

Perceived tonal balance explains why low level listening is quite unimpressive and we need to adjust volume to a sufficient level for a pleasant listening experience.

When removing feedback from a commercial amplifier you likely alter tonal balance and sensitivity (but I suppose that you are not influencing so much the quality of the critical audio band, expecially if the 'basic feedback' is low).

In fact, in the lower spectrum you reduce damping and increase low cut frequency (and reduced damping can be even perceived as stronger bass) whereas in the upper spectrum you introduce some gentle high frequency roll-off. Overall, the increased sensitivity allows a lower volume setting for the same listening volume.

This change is often perceived (and reported) as a 'spatial improvement', 'micro-detail' or 'enhanced soundstage'.

The obvious conclusion is: as the ear is the final judge......

Best Regards
Luca






 

RE: Feedback, no feedback and selective feedback, posted on October 5, 2010 at 07:34:39
acres verde
Audiophile

Posts: 723
Location: Big Easy
Joined: November 13, 2004
 I think you make some valid observations. Strictly from a listening point of view, the only range where I seem to feel that I "miss" my feedback loop is in the lowest registers. For lack of a better word, it sounds as if it "tailors" the decaying edge of low bass notes as they disappear into the ether, thus imparting a sense of greater definition or articulation. Bass weight, however, does not seem to be compromised.

 

RE: feedback, posted on October 5, 2010 at 11:09:15
baileyler
Audiophile

Posts: 359
Location: Chadds Ford, PA
Joined: January 17, 2007
 Wondering if I could get details of the mod to remove the feedback loop, as well as to leave it in place but just add a small parallel cap (what value?). I would actually prefer to start with the latter.

I have an orignial M-50 upgraded to an M-60 (Mk1 status).

I also have an S-30 (Mk2.3). Is the mod applicable to that as well?

Thanks in advance.

 

RE: feedback, posted on October 5, 2010 at 12:27:31
acres verde
Audiophile

Posts: 723
Location: Big Easy
Joined: November 13, 2004
 I hauled my M60s in to a qualified electronics tech. He called Ralph and five minutes later we were replacing the covers on the bottoms of the amps. Since I wasn't directly privy to the actual procedure, I defer to Atma-sphere. But it's a piece of cake.

 

RE: Have you seen this?, posted on October 5, 2010 at 13:03:52
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 IMO, this is the issue with feedback.

 

The very best money can buy - 'power paradigm' gear, posted on October 6, 2010 at 00:59:35
6AS7_6SN7
 If I understand correctly, Ralph points out that the most desirable speaker/amplifier combination is the one that gets closer to the so called 'power paradigm' operation, that is, when the amplifier/speaker combination delivers the same ACOUSTICAL power irrespective of the speaker ELECTRICAL impedance.

As the acoustical power irradiated in the ambient is always a very tiny fraction of the electrical power delivered by the amplifier to the speaker, this makes sense to me and I could not agree more with this vision.

Then Ralph states that, when approaching 'power paradigm' operation, negative feedback usually creates objectionable distortions and that 'zero feedback' is better.

Also this makes sense to me, as any feedback, by its inherent automatic control of the amplifier output impedance, certainly interferes with 'power paradigm' operation, that, on the contrary, would require precise matching of amplifier output impedance and speaker input impedance (in order to get a flat acoustical response).

Unfortunately it must be taken into due account that, to my best knowledge, 99.999999 % of the available commercial speakers are designed in such a way to guarantee a more or less flat ACOUSTICAL frequency response (in anechoic chamber) when driven by a constant voltage source.

As one of the practical ways to force an electrical amplifier to behave like a more or less constant voltage source is by using some negative feedback, this means that 99.999999 % of audiophiles, because they buy (or they own) standard commercial speakers, they are forced by the 'audio status quo' to be quite reluctant to completely abandon feedback.

Simply because if you own standard commercial speakers, you are likely to trading in 'zero feedback' in exchange for 'tonal imbalance', as the ambient acoustical frequency response is going to be less flat than with some carefully selected feedback.

In conclusion, I admire that 0.0000001 % of audiophiles that can afford the luxury of purchasing the right amplifier/speaker combination (with zero feedback, of course) able to deliver a flat acoustical ambient response (according to 'power paradigm' operation).

Those fortunate people will certainly experience the 'less objectionable distortions' on earth but please, don't recommend to the vast majority of other audiophiles to 'blindly' resort to the supposed 'zero feedback' miracles.

This in my opinion can not be considered correct information.

Best Regards
Luca

 

RE: This has nothing to do with acoustic power, posted on October 6, 2010 at 09:33:18
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 The idea is that if an amplifier has no feedback and a high output impedance, it becomes a power source rather than a voltage source.

SETs are an example of this, as are Nelson Pass's First Watt amplifiers.

Speakers that are designed around the Power Paradigm include all ESLs, nearly all horns (except the Avantgarde Trio), 'full range' single-driver speakers, and a good number of bass reflex and acoustic suspension speakers (Coincident Technology is a good example).

Note that the impedance curves are not flat- that is not a requirement of the Power Paradigm. What is happening here is that the Power Paradigm is saying that distortion products are a greater source of coloration than frequency response is. However it is possible to take advantage of the amplifier characteristic in speaker design, some of Duke LeJeurne's (Audiokinesis) designs are examples of that.

IOW, this is a fairly large portion of the high end market, where that not the case, there would not be the equipment matching conversation that is everywhere in high end audio.

 

RE: Trio, posted on October 6, 2010 at 12:45:09
acres verde
Audiophile

Posts: 723
Location: Big Easy
Joined: November 13, 2004
 Ralph...what is it about the construction of the Trio that excludes it from the concept of the "power paradigm"?

 

RE: Trio, posted on October 6, 2010 at 14:45:42
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 The speaker, depending on the version, is built without chokes in the crossover, only caps, and none of them are shunts.

So what happens is in the low frequencies the speaker is about 19 ohms. Then the mid horn rolls in and since the bass horn is not rolled out, the two are in parallel, so the impedance drops to about 8 ohms. When the tweeter rolls in, neither the bass or mid is rolled out, so they are in parallel with the tweeter.

This is why so many owners of this speaker have really had to dig to find a tube amp that will play the speaker properly. The reason the speaker is built this way is that the designer used a transistor amplifier.

 

As power paradigm has nothing to do with acoustic power...., posted on October 6, 2010 at 23:46:50
6AS7_6SN7
 I see a serious efficiency problem and an 'out of control' ambient frequency response.

I a recent post of mine
http://www.audioasylum.com/forums/otl/messages/3/33624.html
I recalled the classical graph of (electrical) power transfer/efficiency vs. the relative amplitude of source and load impedance.

Assuming that source impedance is high (w.r.t. speaker load impedance) this means that the amplifier/speaker combination, when operating according to power paradigm, lies at the very left of the peak of the curve of electrical power transfer, close to the XY origin, where efficiency is extremely low.

If this is correct, power paradigm is basically negating the very purpose of audio power amplification.

What is even worst (in my opinion) is that the power paradigm portion of the power transfer characteristic is so steep to actually preventing any tonal balance with the vast majority of commercial speakers (that are designed for optimal performance when driven by a voltage source and whose electrical impedance varies quite a lot over the operating band).

Pushing to the limit your argument, if the source impedance were infinite we would even obtain a flat frequency response (irrespective of any variation of speaker load impedance), but with NO POWER TRANSFER at all to the speaker.

If this is the price of power paradigm I would rather stay with moderate feedback (and voltage source paradigm)!

I still do not know how to faithfully reproduce the orchestra dynamic range (more than 90 dB) without enough 'raw' power.

Best Regards
Luca

 

I respectfully disagree, posted on October 7, 2010 at 01:31:07
6AS7_6SN7



"distortion products are a greater source of coloration than frequency response is"

The influence of frequency response depends on listening level.

Because of ear sensitivity, for low level listening a 'narrower/less flat' frequency response is acceptable, but this is not true for faithful orchestra reproduction.

For the interested readers I recommend to take a look at Chapter 14 "Fidelity and Distortion" of the Radiotron Designer's Handbook (IV edition, downloadable at the Peter Millet site).

In my opinion your point of view would be barely acceptable only in case of low level listening, because distortion products are usually vanishingly small and hence frequency responce can be somewhat degraded.

To summarize our discussion:

According to the proposed 'power paradigm/zero feedback' approach,

- on one hand we should accept a less than optimal (and variable with different speaker types) frequency response because distortion products are a supposed 'lower source of coloration',

- on the other hand, due to the extremely low power transfer to the speaker, we will certainly get the highest distortion products (because we are forced to push harder the output tubes for getting and adequate acoustic dynamic range).

Power paradigm now looks to me what it should look like to all un-biased audiophiles: a minor niche of high-end market of small or no interest for the average people.

Best Regards
Luca




 

RE: feedback, posted on October 7, 2010 at 10:56:01
Banat
 Personally Always like OTL Circlotron Power Amps sound when they play without any GNFB.(Global Negative Feedback-from output stage to input stage)
But must pointed next:Local Negative Feedback isue

OTL Circlotron Output Power Stage allready work with 100%negative feedback,basicly beacose this stage are built from Two(SE OTL) Cathode Follower power Amps joined together in PP Circlotron Output Power Stage.(or four or more SE OTL stage joined together in case of Multiphase Output Circlotron Power stage system)

Same thing( 100%local Feedback) is valid for DC coupled CF Diferencial Driver stage too.

Input Diferencial Dual Cascode Gain stage(in case of ATMA-SPHERE Amps) is very good Engineered too, so produce very low distortion.

All This separate stage relative low distorsion, stage by stage together, results in High Definition OTL Power Amplifier which dont need any GNFB to sound Good.

Relative Lot of GNFB is actually general `cure` fore Bad designed(stage by stage) OTL power Amps,but that (GNFB) general `cure` made only good results for Osciloscope or Distortion Analyzer and NOT for listener(Human) Ear`s.
Actually OTL and other Amps with relative Lot of GFNB have some better punch in Bass area, but lose significant in overall sound definition( 3 D space,highs and natural definition)

With all that above in our mind`s only which can we do is too choose asociated and appropriate (LOAD)SPEAKER for our High Definition OTL Circlotron Amplifier, this (LOAD)SPEAKER is usually with relative higher impedance but with Sonically Superior final Results.

Good Luck to All

 

RE: from your post it looks like you took the idea, posted on October 7, 2010 at 11:38:53
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 to some sort of weird extreme. "high output impedance" refers to an amplifier that is perhaps more than 0.5ohms output impedance- some appreciable fraction of the load impedance...

IOW power production is the issue; look at all zero feedback SETs made, they are all part of this.

 

RE: you seem to be missing the point, posted on October 7, 2010 at 11:44:51
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 I know about Fletcher-Monsun, this is not what I was talking about. I **was** talking about how the ear perceives distortion as a coloration, for example distortions of only 1/100th of a percent of the 5th, 7th and 9 th harmonics are perceived as brightness, despite the frequency response of an amp with such distortion as being otherwise flat.

The same is true of low-ordered harmonics will make the amp sound 'warm' or 'full', perhaps even bloated in the bass, despite flat frequency response.

Some CD players can exhibit so much inharmonic distortions that no amount of turning down the treble control can tame their brightness.

Do you understand now my point??

 

High impedance means 'high', posted on October 7, 2010 at 23:07:18
6AS7_6SN7
 0.5 Ohm difference is nothing.

English/American is not my mother tongue but I understand very well the meaning of the words.

High source impedance in technical world means many, many times the load impedance.

If the difference of input and output impedance is so small (0.5 Ohm or the like) I see that 'power paradigm' is nothing different from common impedance matching (for almost maximum power transfer).

Nothing new, nothing particularly interesting, in a single word, 'moonlight'.

Good for advertisement, not for engineers.

Best Regards
Luca

 

Maybe I am missing the point....., posted on October 7, 2010 at 23:27:55
6AS7_6SN7
 But I prefer to stick to audio science.

My impression is that you seem to escape any meaningful technical discussion by never making clear your points.

You look too often so vague, so far away from common technical vocabulary, always trying to referring to something else (that is not the topic of the matter under scrutiny).

I won't follow you on this terrain anymore.

If you want to positively contribute to this forum discussion please stick to the points at hand, don't ever assert: "low-ordered harmonics will make the amp sound 'warm' or 'full', perhaps even bloated in the bass, despite flat frequency response".

This is basically nonsense as long as you do not put numbers there.

And again, what the hell your statement "some CD players can exhibit so much inharmonic distortions" has to do with 'power paradigm high impedance"?

Which CD models?
Who is the manufacturer to be blamed?
But who was ever asking your opinion on CD players?
Who cares?

In my experience CD players are excellent audio sources.

My point is clear: masked advertisement maybe good for you but it is extremely disturbing for me.

Best Regards
Luca

 

RE: Maybe I am missing the point....., posted on October 8, 2010 at 09:02:16
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 I am assuming that it has something to do with language.

Had you been following along, then I doubt you would have said this:

And again, what the hell your statement "some CD players can exhibit so much inharmonic distortions" has to do with 'power paradigm high impedance"?

So I am assuming that either you did not read something, or read it and misinterpreted it based on a misunderstanding of the language.

To flesh things out a little, General Electric did a study in the 1960s, wherein they found that humans find odd-ordered harmonics to be objectionable, and very small amounts, less than 0.1% . By contrast, the same study showed that humans may not even notice lower ordered harmonics (2nd,3rd, 4th) until they get over 30%! The former is easy to demonstrate with a sine/square generator, an amp, a speaker and a VU meter.

If you want to positively contribute to this forum discussion please stick to the points at hand, don't ever assert: "low-ordered harmonics will make the amp sound 'warm' or 'full', perhaps even bloated in the bass, despite flat frequency response".

I *thought* this was common knowledge, and germane to the conversation- I refer you to Norman Crowhurst.

 

RE: High impedance means 'high', posted on October 8, 2010 at 09:08:45
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 When you are referring to a conversation about the output impedance of amplifiers, in the US anyway, the engineering language is such that 4 ohms would be considered a 'high output impedance' and 0.5 ohms would be somewhere between low and high, as most amps that have a 'low output impedance' are often 0.1 ohms or less.

There is no such thing as a power amplifier that has an output impedance that is say, 100 ohms or more. That's not an amp that can make any appreciable power, so by definition its not in the conversation.

So it does appear that the mother language is playing a role here.

 

Again, you start talking about something else....., posted on October 9, 2010 at 01:27:27
6AS7_6SN7
 What a pity. It's not a language problem, it's you that are likely running out of meaningful argumets for defending the supposed advantages of 'power paradigm/zero feedback'.

As in your recent 'clash' with Tre' you escape again and again the technical points and start referring to something else.

As far as the argument "General Electric did a study in the 1960s, wherein they found that humans find odd-ordered harmonics to be objectionable, and very small amounts, less than 0.1%" is concerned, please re-publish some relevant excerpt of this 'unknown/unpublished' study on this (or your site), so that we can all check what the study assumptions were at the time (istead of relying on your statements only).

And as odd ordered harmonics seems so dangerous according to this GE study I will be glad to see the measured distortions of your audio products, let's say at 12 volt peak across ESL loading, 330 Hz, just to see if the odd-ordered harmonics fall below 0.1% as you consider so much essential.

Best Regards
Luca

 

RE: feedback, posted on October 9, 2010 at 13:34:11
Satelliteman
Audiophile

Posts: 122
Location: Napier - The Art Deco City of the world
Joined: May 21, 2006
 Yep - I wired one M60 with it , one without .....mono testing , swap out with myself and a trusted listner for detail in all he does ...he picked the same amp out of the two as me... result - No Feedback .
It also pleases me that the feedback wiring is not even installed .

 

RE: Feedback, no feedback and selective feedback, posted on October 10, 2010 at 15:08:35
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Luca, Can you tell us more? For example, exactly where do you put the feedback loop in one of your circlotron designs? Are there decent equations for calculating the values of R and C, such that a non-EE could do it? What output impedance do you shoot for and how do you shoot for it (vs some other lower or higher value)? I am receptive to trying a touch of NFB to bring the output Z down into the range of 1 ohm or below, and I have no problem about your idea of making it frequency-dependent. Thanks.

 

RE: Feedback, no feedback and selective feedback, posted on October 10, 2010 at 23:18:33
6AS7_6SN7
 I will likely come back on this (and other NFB topics) in the next future.

I am currently carrying out a personal study in parallel with the construction of the 829B PP amplifier (depicted some time ago in the DIY section) and I need some more time.

Thanks for your interest.

Luca

 

The ear is the final judge, posted on October 10, 2010 at 23:49:27
6AS7_6SN7
 Mother Nature gave us such a wonderful gift, our ears.

Nevertheless, trying to figure out the reason why we prefer something may be rewarding too.

Best Regards
Luca

 

RE: Again, you start talking about something else....., posted on October 11, 2010 at 13:47:28
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Luca, Tre and I figured it out even if you didn't.

Now, since you don't believe me, you now have to prove me wrong. This is an easy test. Get a sine/square generator, an amp (any functioning amp will do) a speaker and a VU meter.

Set the generator for sine, at 0 VU and listen to the result through the speaker. Now cover up the meter. Turn the volume down, set for square wave and bring up the volume until it sounds as loud as before. Now observe the meter. You will see that it is a good ways down from the sine wave- probably about 20-25db.

This is because we use odd ordered harmonics to see how loud a sound is. A square wave is all odd orders. So naturally it sounds a lot louder than a sine wave.

The fact of the matter is I have not been changing the subject- instead I get the sense that you are actively trying to not understand. But you need to perform the test above!

 

A possible selective feedback for circlotron, posted on October 12, 2010 at 01:49:57
6AS7_6SN7









'Mutatis mutandis' I think this selective feedback network can be applied to the majority of commercial circlotrons.

The selective feedback network is made of R13, R22, R21, R20 and C5.

The feedback signal is picked across the normal speaker output (paying much attention to the polarity to prevent wild oscillation).

In this example there is room for about 11 dB feedback at low/mid frequency, 7.5 dB at 10 KHz, 3 dB at 20 KHz and zero feedback above 30 KHz.

Changing the value of C5 will allow to increase or decrease the 10-20KHz feedback.

Changing the value of R13 and R20 allows to trade in more low frequency feedback (i.e. lower output impedance) in exchange for reduced mid-band gain.

Best Regards
Luca

 

RE: Feedback, no feedback and selective feedback, posted on October 12, 2010 at 08:37:37
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Thanks for the response. Am I correct, if C5 is increased, then the frequency at which feedback is reduced is lowered? It's a simple enough circuit to try it. I am much more interested in reducing output Z than in lowering distortion per se. This would allow me to run my ESLs without the Zero autoformers (or not, if I don't like the effects of NFB on sonics). As you probably know, there is no problem with ESL impedance at low frequencies, where ESLs have very high input Z. With my Sound Labs, the impedance problem only arises in the mid-band, from about 500Hz to about 2kHz. I wonder if one could design a NFB network to work in the midband and not in the bass OR extreme treble, without destroying the magic midrange sound.

 

RE: A possible selective feedback for circlotron, posted on October 12, 2010 at 09:03:49
Banat
 Hi Lucca

Basicly agree with Your posted schematic,results and conclusion,but I think next:
Is better to have Only for Anode Follower driver stage separate supply direct from oposite output(Circlotron)power stage two B+ supply.
This way of`bootstraping` between driver stage, AC coupled to grids of output power stage, achive some low value of positive feedback for anode of driver stage and results in bigger voltage sfing and overall lower distortion.
Of course ewerything else including GNFB remain the same like You posted in Your schematic

This solution for driver stage is actually esential idea of Mr Tapio Koykka,inventor of Counter Parallel Push Pull(CSPP) Amplifier(Circlotron),and this solution is succesfully used in many Circlotron Amplifier,VOIMA,Philips,Electro Voice...

Best Regard

 

I know there is always room for improvement..., posted on October 12, 2010 at 09:38:21
6AS7_6SN7
 Thank you for your suggestion.

Best Regards
Luca

 

For sake of clarity, posted on October 12, 2010 at 10:03:27
6AS7_6SN7









This is the family of frequency response curves wher C5 is (from top):
22nF
220nF
470nF

For me, however, the preferred value in this case is 68nF.

Luca

 

RE: A tricky issue, posted on October 12, 2010 at 10:25:11
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 By installing a resistance between the cathodes of the input tubes, the coupling between them is reduced. There is a price paid for this: reduced differential effect, which results in lower gain and higher distortion. There is also the need for some means to adjust the resulting plate voltages- a big mismatch in them means distortion.

A variable resistance installed in the cathode circuit, with the wiper tied to the CCS can be used to solve this problem, but it will be immediately seen that other problems are introduced, leaving the distortion issue nearly untouched.

Also, to be truly effective, a 2-stage CCS is highly recommended any time a differential amplifier is executed, along with a B- voltage of sufficient value such that a true high impedance effect can be realized in the CCS.

It is possible to mix the feedback signal with the incoming signal with a resistive divider network, thus eliminating the cathode coupling problem in the first stage. The increased gain can then be used to assist with additional feedback, if desired.

However, it should be noted that while feedback **seems** to decrease output impedance, that if that were really the case, the output power at lower impedances would increase, and it doesn't. What it *does* do is push the amplifier design towards behaving more like a voltage source, but unlike a transistor amp which might double power as load impedance is halved, in this case the total output power into higher impedances will be limited by the power that the amplifier is capable of into lower impedances.

IOW, a set of ZEROs will still be a good idea on lower impedance speakers.

 

RE: A tricky issue, posted on October 12, 2010 at 22:48:33
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Thought I saw an article once in Glass Audio in which it was shown that a pair of small value resistors between the cathodes, such that the current source feeds in at the junction of the resistors rather than directly to the cathode(s), actually resulted in a reduction of distortion, at the expense of a small amount of gain. Otherwise, I am not qualified to argue. In that article, the dual-differential cascode was referred to as a "Hedge Amplifier".

 

It is not a tricky issue at all, posted on October 13, 2010 at 01:14:20
6AS7_6SN7






These are the (simulated) output spectra @ 1.2A peak current (in 8 Ohm speaker) of my dummy schematic with (top) and without (buttom) the 100 Ohm resistors.

As at 300 Hz negative feedback (with the 100 Ohm resistors) is working 'full steam ahead' the overall distortion is MUCH BETTER than without the 100 Ohm resistor.

And this is exactly as it should be according to known electrical theory, because negative feedback not only reduces output impedance but distortion too.

You can also see that in the case at hand odd distortion with zero feedback exceeds 0.1%, the supposed upper limit recommended by the still unknown/unpublished General Electric study of 1960 mentioned so many times by Ralph.

Best Regards
Luca


 

RE: It is not a tricky issue at all, posted on October 13, 2010 at 07:03:14
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 Luca, The 3rd order harmonic distortion is clearly reduced by the presence of the 100-ohm resistors (what I use in my amps), but is it not true also that higher order distortion products are increased, WITH the resistors, based on what I am seeing in your data? Thanks.

 

You are absolutely right!, posted on October 14, 2010 at 00:44:26
6AS7_6SN7
 That's feedback.

Moderate feedback (like in this case) reduces low order harmonics (about 12 dB), but creates higher order ones.

In real world you can not have, at the same time, a completely filled in cask and a drunk wife (this is my best translation of an Italian proverb).

I suppose this is the reason why excessive feedback has such a bad reputation and why it is generally accepted that a good basic design (with inherent low distortion) is needed before ever thinking of applying NFB.

I realized this fact by extensively playing with simulations and this is the main driver for adopting frequency selective feedback instead.

If feeedback is vanishing away (starting from a few KHz), the creation of potential high order harmonics, that would be otherwise created by flat frequency feedback, is automatically prevented.

In conclusion, I think that, based on a careful case by case assessment, frequency selective feedback can be effective in improving audio performance.

Best Regards
Luca

 

RE: You are absolutely right!, posted on October 14, 2010 at 10:10:48
Banat
 Hi Luca

Here is some old(1966) Philips AG9007 60W OTL Circlotron HI- FI Tube amplifier design.

Found interesting in this design that Philips designers used 2x2 symetrical Negative Feedback.
First NFB loop is symetrical from output stage to driver stage only,and second GNFB loop is symetrical from output stage to input stage.
Best regards
Check this link:

 

RE: You are absolutely right!, posted on October 14, 2010 at 19:51:52
Lew
Audiophile

Posts: 10911
Location: Bethesda, Maryland
Joined: December 11, 2000
 So, should I get those resistors out of there, in my amps? Wish I could find that Glass Audio article which gave me the idea to use them in the first place. Without the FB resistors, my amps would have a very high input sensitivity (meaning a very low signal voltage would drive them to full output). I am using type 6900 triodes in the dual-differential cascode. These are super-5687s with tremendous Gm and low Rp.

 

RE: I respectfully disagree, posted on October 17, 2010 at 15:08:26
morricab
Distributor or Rep

Posts: 9178
Location: switzerland
Joined: April 1, 2005
 " on one hand we should accept a less than optimal (and variable with different speaker types) frequency response because distortion products are a supposed 'lower source of coloration',"

In todays audio landscape it is a relatively straightforward operation to fix frequency response. A decent digital equalizer will take care of this nicely (The Behringer DEQ2496 is a good choice that can be inserted between transport and DAC)

It has also been long apparent to me that listeners are far more tolerant of linear distortions (i.e. frequency response) than they are of non-linear distortions (cone flex and breakup, cabinet resonances etc.) and the same holds true for electronics. People mistakenly make a big deal out of 1 db FR variations (potentially depending on the speaker impedance curve) from a SET amp not but the bigger damage to real natural sound, NON-linear distortions and in particular high order non-linear distortions. This is what negative feedback brings to the party and its effect on the sound can be readily heard (even 2db worth according to the initial poster).

I have been experiencing the best sound of my life once I removed all negative feedback from my system. My amps don't have it, my DAC doesn't have it and my phono stage doesn't have it.

In fact, the total number of active stages has been minimized as well. There is only one active stage in the DAC (the tube output is taken directly from the DAC chip output) and three active stages the integrated amp (one JFET, one MOSFET and one big output tube). The whole system runs in Class A. The phono stage adds 1 more active gain stage but still no feedback. As a result it is about as harmonically complete and dynamically lively as I have heard from any system. I use digital equalization in limited amounts and this gives a FR accurate room response.

 

non linear distortions, posted on October 18, 2010 at 10:09:13
acres verde
Audiophile

Posts: 723
Location: Big Easy
Joined: November 13, 2004
 A big "amen" to just about everything you said up there.

 

RE: You are absolutely right!, posted on October 20, 2010 at 11:05:49
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 In conclusion, I think that, based on a careful case by case assessment, frequency selective feedback can be effective in improving audio performance.

Luca, I hope you see that in this quote we are in complete agreement, and that in fact that has been the point of my initial link, as well as the succeeding arguments.

I read about the GE study in a book, but so far I have not seen that anyone has published a report of it online, FWIW.

To be clear though, GE was saying that the 3rd, as well as the 2nd and 4th, are not regarded by the ear as irritating; it is the 5th, 7th and 9th where the problem lies. If you spend any time on the SET Asylum, you will find that that is a commonly-held viewpoint that is not unique to me. As your simulations show, these troublesome distortions seem to arise as result of the use of feedback.

It is for this reason that the cathode resistors as mentioned in the differential amplifier are not desirable- sure, they reduce lower orders (although the even orders will be canceled, so we are really only talking about the 3rd harmonic), but at the price of increasing the higher orders.

 

We made all the points quite clear......., posted on October 20, 2010 at 23:56:08
6AS7_6SN7
 And all inmates are now in the position to fully developing their own (informed) opinion.

As a final remark, I think that we all also agree that the ear is the final judge.

If somebody prefers zero feedback this is good for him.

If somebody else prefers some moderate feedback this is also good for him.

And those who still like the old Leak (30dB NFB) or McIntosh (about 20 dB NFB) tube amplifiers, again, it is good for them too.

Best Regards
Luca

 

RE: You are absolutely right!, posted on October 21, 2010 at 05:28:04
Banat
 Agree with You Ralph about GE harmonic distortion study and their conclusion.

Same is for SET example You give,everybody know why in SET Amps GNFB is more Problematic than PP Amps:Basicly beacose there is no automatic distortion canceletation like PP amp doo,SET Amps generate allmost all harmonic distortion products,so GNFB for SET Amps is `must` and value of inserted GNFB is allways compromise beetwen real `Ear` Audio performance and acceptable measurment(Osciloscope) performance,and this is allways valid for SET higher Audio spectrum reproduction.

Any way beside that from above,the Bigger problem for most SET Amps is actually OPT(price/sound limitation/ degradation isue),think the acceptable and real good `Ear` sounding SET Amp is relative low output power(~10W)A/A2 class SE OTL Amp with moderate GNFB.

Best Regards

 

In this particular case I have found that...., posted on November 28, 2010 at 11:35:59
6AS7_6SN7
 Best performance corresponds to 6-13 dB feedback.

http://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=190082

This is my last project and I have recently added a top panel switch to select 9dB or 14 dB NFB.

Even though the amplifier is very natural with zero feedback too when using 14 dB setting the absolute performance is (believe me or not) undistinguishable from my OTL!

In my opinion high plate voltage, almost zero PS impedance and high bias current (i.e. true class A operation) allow tetrode amplifiers to match OTL triode performance.

Best Regards
Luca

 

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