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OTL vs SET

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Posted on January 11, 2015 at 13:48:31
saki70
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 I would like to know if anyone can compare the average OTL sound to that of an average 300B DHT SET sound . I figure it might be a bit of a stretch for the 'average' thing but please try .

Thank You
saki70

 

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RE: OTL vs SET, posted on January 11, 2015 at 18:12:58
Cousin Billy
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 I hope I don't offend anyone.

Think of OTL's as crystal clear. If a recording has emotion, than so does the music. If the recording is somewhat dry, so will the music.

SET are beautiful. Lush recordings are even more so. Hard recordings sound sweeter.

I am in the OTL camp.

 

RE: OTL vs SET, posted on January 12, 2015 at 03:27:44
sk
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 OTL's = clean, clean, clean with lots of emotion.
SET's = romantic and not as accurate.
Yes I'm in the OTL camp and don't think will ever go back. But if you ever want to go the OTL route remember that speaker matching is even more important. High and flat impedance is very important.

 

RE: OTL vs SET, posted on January 12, 2015 at 08:17:37
collinslaw@fuse.net
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 i agree on the speaker matching. i am having some success with a pair of einstein mk 70s (about 50 - 60 watts) with golden ear triton 7s. the tritons dip in the bass and that proves to be the limiting factor. still very good, but ultimate volume cannot be achieved.
i have heard some SETs that are not overly romantic but very clear and clean while admittedly rolling off some in high treble and low bass. i think the preamp is critical in those applications. some of the most tactile, reach out and touch it sound i have ever heard however.
Tom Collins

 

The speaker's the thing..., posted on January 12, 2015 at 08:58:37
Lew
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 that you have to consider first. If you are running very high efficiency horns with very high impedance (e.g., well above 8 ohms nominal), then the two types of amplifier can compete on a level playing field. In which case, I agree with the others that an OTL will be, shall we say, less colored. If you are using some other less efficient type of speaker, but still with a high-ish impedance, I would think the OTL would prevail more obviously.

By the way, the SET lovers that I know seem to prefer 2A3 and 45 tubes vs the 300B, but those two types give even less power and require even more specialized types of speakers, as you probably know.

 

RE: The speaker's the thing..., posted on January 12, 2015 at 10:55:04
saki70
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 Thanks to all of you .
I had forgotten about the high impedance requirement for OTL's .
I have 92db. speakers that dip down to 6 ohms .

Happy Tunes
saki70

 

Actually that should work just fine, posted on January 12, 2015 at 11:19:25
Ralph
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 A lot also depends on the particular OTL and the design of the speaker, as well as its impedance. More at the url below.

The Atma-Sphere amps don't run feedback, so the speaker might require more care in the choice, but Merlins are similar to what you describe and they work great with our M-60 and other OTLs too. So impedance is not the last word.

However, FWIW:

There really isn't a good argument for a 4 ohm speaker if your goal is sound quality, regardless of the amplifier you use. Look at the specs of any amplifier made and you will see that the 4 ohm distortion value is consistently higher regardless of the amp.

The kinds of distortions in this case happen to be of the type that are easily audible (higher ordered harmonics). The result is that the amp will sound harsher and less detailed. So a simple method of getting any speaker to seem smoother and more detailed is to simply build to to be a higher impedance.

If sound pressure is your goal then you have a 3 db argument for 4 ohms.

 

I've played with a few SETs, posted on January 12, 2015 at 11:26:19
Ralph
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 -300b, 2A3 and type 45.

Of these the 45 sounded the best but also made the least power.

In all cases the S-30 played not only with more power, but more detail and if anything was smoother if not just as smooth.

A lot depends on speaker choice of course but when you are talking about amplifier powers as low as we see in these SETs, that usually is not an issue. To really hear what a 7-watt SET does in most rooms, you need a speaker that is at least 100db 1 watt/1 meter or more. That way you won't be pushing the amp past about 20% of full power, which is important. The distortion of most SETs starts to include the higher orders (5th harmonic and beyond) when power gets over about 20% of full power. These harmonics are used by the human ear to determine how loud a sound is. So the result is that SETs tend to sound louder than they really are as far as a sound pressure level meter is concerned.

When you have cleaner power (like an OTL usually has), the result is the system may not sound loud, even when the sound pressure level meter is reading a lot higher. This is the way real music behaves- so the result is you are closer to the music.

 

RE: OTL vs SET, posted on January 12, 2015 at 12:20:02
groovy.guru
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 I have both SET and OTL amps. Speaker matching is very important to get the best out of either type of amp. I just bought a pair of Coincident Frankenstein DHT 300b monoblock amps, and I have owned a Joule Electra VZN-80 Musicwood OTL stereo amplifier for the past 7 years. I’m not sure if they meet your definition of average, but here’s my two cents.

Right now, my SET monoblock amps are more analytical, and my OTL amp is more musical, which is the opposite of the stereotypes of these two types of amps. My Franks only have 30 hours on them right now. As they break in, I have heard the bass and treble ripen up; they are definitely moving more towards the musical camp, and I suspect they will end up being my favorite of the two topologies.

Here is my summary of how they sound (bearing in mind that with the Franks, this is my first impression before full break-in):


SUMMARY OF SET (8 watts/ch 300b)


Definitely clearer than my OTL, this is very easy to discern

More transparent

Better dynamics, sounds can jump out and startle you

Better low level detail, better “blackness” and “air”

Better harmonic overtones; i.e. when a guitar note is played, I can better hear the nuances after the initial sound of the note

Very good coherency from the bass to mids to treble; nothing unduly stands out or is missing.

Surprising amount of tight, weighty bass with good tone for 8 watts a channel

Mechanically very quiet

Runs relatively cool, does not heat up my room

In a nutshell (based on 30 hours of break-in): Sounds more like I am hearing everything that is on the recording




SUMMARY of OTL (80 watts/ch 6c33c)


Fuller, more ripe tone across the entire spectrum (more meat on the bones)

Fatter, weightier, and more pleasant bass

Excellent cymbals, very natural and extended without sounding harsh

Voices are more full and less edgy, and voices project more out of the middle of the soundstage

Sounds other than voices are more blended, with less air and blackness between sounds

I have to try harder with my OTL to hear the same sounds that I hear with my 300b amp

Hotter than Hades!

6c33c tubes are getting hard to find

The variac because of its large toroidal transformer has a low level hum (a common issue with Joules)

My OTL requires careful placement in relationship to my other components; it does not like being near my preamp or some of my other components (it will start buzzing through the speakers!)

Other than the variac, once it is placed away from my other components (I use a configuration with the amp between the speakers, the rest of my system on the side wall), the Joule OTL is surprisingly quiet for an amp with 8 large output tubes

In a nutshell: The best thing about my OTL is that I often imagine I am at the performance.


Hope this helps!

 

RE: OTL vs SET, posted on January 12, 2015 at 13:13:11
Catastrofe
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 I have a pair of Ralph's MA-1s. When paired with Daedalus Ulysses speakers, I preferred the sound when using Speltz Autoformers. When paired with Reference 3A MM De Capo BE, I don't need the Autoformers. As others have commented, impedance matching is important.

 

RE: OTL vs SET, posted on January 12, 2015 at 16:37:38
c1ferrari
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 What loudspeakers are your driving with either amp? Thanks.

Vbr,
Sam

 

RE: OTL vs SET, posted on January 12, 2015 at 18:22:18
groovy.guru
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 Hi Sam,

I am using Coincident Victory II's to compare the amplifiers.

I also own a pair of Merlin VSM-MXe's, but I have yet to try them with the Frankensteins. The Merlins are a nice match with the Joule VZN-80.

Tom

 

RE: OTL vs SET, posted on January 12, 2015 at 18:31:16
c1ferrari
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 Hi Tom,

Thanks for the reply. Joule and Merlins do complement one another :-)
I'm using a full Atma-Sphere rig with Dali MegaLine III speakers and am enthralled!

Vbr,
Sam

 

RE: OTL vs SET, posted on January 12, 2015 at 20:50:53
groovy.guru
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 I think that instead of using the analytical/musical dichotomy, I should have said that one is more clear (SET) and the other more romantic (OTL), which goes against the stereotypes of these amps.

 

RE: The speaker's the thing..., posted on January 13, 2015 at 06:37:12
throwback
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 If the impedance of your speakers is too low for an OTL, it may be possible to change the impedance as I did.

My speakers (GR Research LS-9's in kit form)are made of a string of 12 6.5" base/mid cones and a string of 9 BG Neo 8's. Each string was configured in a series/parallel arrangement (6x6 for the woofs; 3x3x3 for the tweets) and put together in parallel at the crossover. The result was an overall impedance of about 3.5 Ohms--pretty flat across the spectrum, but also pretty low. With my Atma M-1's there was noticeable droop at both ends.

Ralph suggested that I rewire the speakers to bring up the impedance by putting the two strings more in series. I changed the arrangement to be 6+6 (series)for the woofs and 4x4-1 (four in parallel with four, and disconnecting one tweet)for the tweets. By doing so (and making a couple of simple crossover changes)I brought the impedance up to 12 Ohms across the spectrum. Now the suckers really sing.

If your speakers are in array form, you might be able to do the same thing.

 

RE: The speaker's the thing..., posted on January 13, 2015 at 08:33:04
Lew
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 It's hard to believe that the designer of that speaker would let it go out the door with such a low nominal impedance, when it was such a simple matter to revise the circuit so as to obtain a much more amplifier-friendly impedance. But changing the crossover in such a design, once you've altered the connections of the drivers, is often not at all a "simple" matter. Were you able to make impedance measurements of the separate revised driver stages so as to maintain the same crossover points and slopes that the designer originally intended? I guess this could be done if the crossover is first-order, more easily than if it is a complex 2nd or 3rd order design. Anyway, regardless, the only thing that counts is that you are happy with it now.

 

Agree with Ralph..., posted on January 13, 2015 at 08:34:05
Lew
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 MA1 should drive that speaker easily and well.

 

A bit of a generalization..., posted on January 13, 2015 at 09:32:09
Ralph
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 Not all OTLs are the same. There's a pretty wide range of experience with them- for example Futtermans tend to sound more like conventional push-pull amps (owing to a large amount of feedback), but a lot more transparent. Our amps have no feedback and thus do sound different.

So a hard and fast comparison against the 'average' SET is a bit tricky. But then, I have to assume that not all SETs sound the same either... but I do find that the smaller ones sound better, at the sacrifice of power. With our OTLs at least it seems the bigger we make them the better they sound, which flies in the face of the conventional wisdom.

 

RE: The speaker's the thing..., posted on January 13, 2015 at 12:34:07
throwback
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 Yes to all of the above. I do not know why the manufacturer designed the speakers as he did. He uses them primarily with conventional tube amps (Dodd's)with transformers. But also, they would work just fine with solid-state amps as well.

You're right about x-over changes. We did measure the strings individually and together. Despite the x-over being 3rd order, all it took was the change of one capacitor and we achieved a very good acoustic response. Better to be lucky than smart, says Confucious.

We are going to experiment with a whole new x-over with first-order slopes. According to BG, this will not hurt the tweets any, even at 300 Hz, versus the 850Hz we have now. Anyway, we can put it back if we don't like it.

Cheers,
Chuck

BTW, Lew, I believe I passed on the news that Lou Johnson had passed away. Imagine my shock when I bumped right into him coming off the elevator at the Venetian at CES last week. Rumors to the contrary, etc.

 

RE: The speaker's the thing..., posted on January 13, 2015 at 13:28:28
saki70
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 Catastrofe ;
Very interesting , I have Di Capo i's . Do you like the way they sound with the big OTL's ? Do you find that the Auto-formers effect the sound or have a flavor ? Have you ever tried the Di Capo's with an SET ?
I know some have used SET"s and say it works . But as Ralph iterated , how much of the true SET sound is heard if one must crank them up passed the 20% point .

 

RE: OTL vs SET, posted on January 13, 2015 at 18:36:02
Caucasian Blackplate
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 They are generally very different topologies. SET amps tend to be zero feedback designs, with relatively high damping factors. They can produce incredibly low distortion considering no feedback is used, and a very simple circuit will suffice. Getting good sound out of an SET amplifier requires both a good circuit and good iron, however, and a lack of primary inductance in many single ended output transformers can lead to crappy bass.

OTL amps have fewer high dollar parts, but in general the circuit design is considerably more complicated than a SET amplifier. There is also significantly more diversity in OTL amplifier circuits than single ended circuit. Though feedback isn't 100% required for operation, it is often times used to drop the output impedance without having to add an insane amount of extra output devices (so we could say it's 99% required).

How each design sounds to you will depend a lot on the speakers that you have. Critics of OTL amplifiers will call them harsh, brash, and solid state sounding. Critics of SET amplifiers will call them overly warm, colored, and thin in the bass.

 

RE: OTL vs SET, posted on January 13, 2015 at 22:16:04
cpotl
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 " Critics of OTL amplifiers will call them harsh, brash, and solid state sounding."

I love OTL amplifiers, and I have made quite a few. I would agree they sound more like solid state, certainly in comparison to SET amps. Frankly, I think that is because solid state amplifiers (good ones) are closer to an uncoloured and accurate reproduction. Amongst tube amplifiers, OTL are the ones that are most accurate in their reproduction, and thus they sound closest to solid state.

Chris

 

RE: The speaker's the thing..., posted on January 14, 2015 at 08:32:40
Catastrofe
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 I didn't care for the sound of the De Capos with the Autoformers. The sound was "bloated" and overly full. I love the way they sound fed directly out of the MA-1s.

I've not tried an SET, as I don't have easy access to a quality amp.

 

As you might imagine, I'm having a problem with this, posted on January 14, 2015 at 10:40:17
Ralph
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 OTL amps have fewer high dollar parts, but in general the circuit design is considerably more complicated than a SET amplifier. There is also significantly more diversity in OTL amplifier circuits than single ended circuit. Though feedback isn't 100% required for operation, it is often times used to drop the output impedance without having to add an insane amount of extra output devices (so we could say it's 99% required).

First, we have to pop the bubble about feedback. It does not reduce output impedance at all. If it did, a basic law of electricity, Kirchoff's Law, would be violated. The only way you can reduce output impedance is with more tubes, bigger output transformers, etc. Of course, what is meant by the term 'output impedance' must then be under question, but put simply you can't drive a lower impedance simply by adding feedback. Do do that requires the greater number of tubes etc. I just mentioned.

We don't use a whole lot of feedback- the M-60 employs 2 db (and some people have the amps built with zero feedback) and the S-30 uses 4 (and also works fine without it). Otherwise our amps have no feedback. The question is whether we have an 'insane' number of output devices... in terms of cost, probably not, as often it costs more to retube an SET than it does our amps.

Finally there is the issue of complexity. Our amps have one gain stage. An SET usually has two or three. Which is more complex? As far as the signal is concerned, probably the SET. Certainly our amps have more parts. So its my opinion of course but I just don't see this statement as reality.

 

RE: As you might imagine, I'm having a problem with this, posted on January 14, 2015 at 12:57:08
cpotl
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 "First, we have to pop the bubble about feedback. It does not reduce output impedance at all. If it did, a basic law of electricity, Kirchoff's Law, would be violated. The only way you can reduce output impedance is with more tubes, bigger output transformers, etc. Of course, what is meant by the term 'output impedance' must then be under question, but put simply you can't drive a lower impedance simply by adding feedback..."

It seems to me you are conflating two rather different issues here. The output impedance of a device can be defined in various different but related ways. Perhaps the cleanest would be to say that we take the two output terminals, drive an externally-sourced current (Delta I) through them, and measure the resulting voltage change (Delta V) across the terminals. The output impedance is then defined as
Z = (Delta V)/(Delta I).

The output impedance so defined will certainly be lowered if negative feedback is used. And this lowering of the output impedance will have all the usual concomitant features, such as increasing the damping factor for a loudspeaker connected to the output, and so on. This is an absolutely standard piece of basic circuit theory, which applies to OTLs, operational amplifiers, or whatever.

There is a caveat that must be included in the above. Namely, the discussion is applicable only provided that one is never calling on the device to supply, or source, more current than it is capable of handling. This, it seems to me, is the point you are really trying to get at; the amplifier will not be able to drive any more current through the loudspeaker simply because one has added feedback. To get it to handle more current, and thus produce more power into a given load, one would need to add more output tubes.

This same caveat would apply to any other example too. Thus the low output impedance of an op amp with lots of feedback would only be valid provided one did not try to get it to pass more current than it could handle.

But provided one never asks the amplifier to pass more output current than it is capable of, the output impedance, as defined in any standard way, is reduced by the use of feedback.

Chris

 

RE: more on output impedance, posted on January 14, 2015 at 13:30:09
Ralph
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 We're actually in agreement on this but the term 'output impedance' as you are using it (and for that matter, most of the audio industry) is not actually the output impedance of the amp. This is why I stated

Of course, what is meant by the term 'output impedance' must then be under question

-because in most cases its really a measure of the servo gain of the feedback loop, and not an actual measure of the impedance of the output section. However, I tend to work with the Power Paradigm rules (which predate the Voltage Paradigm rules, see link below) which measure the impedance of the output section rather than the 'output impedance'. A lot of people are confused by this topic, especially if the Voltage Paradigm is their only source of the terms.

My contention is simply that the term as it is usually seen is misleading, because so many people use the concept to suggest that if you add feedback to an amplifier, its output impedance goes down therefore it can drive lower impedance loads. As you correctly point out, there is a current limit involved which was what I was referring to with Kirchoff's Law. We just have different ways of expressing it is all :)

 

RE: more on output impedance, posted on January 14, 2015 at 19:53:44
cpotl
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 "the term 'output impedance' as you are using it (and for that matter, most of the audio industry) is not actually the output impedance of the amp."

Well, we could probably debate that endlessly. In my view it is better to stick with conventional usage of well-defined concepts such as impedance = (Delta V)/(Delta I), since it is meaningful and is understood in almost all of electronic circuit theory.

But as you say, we are probably in agreement about the actual underlying facts, and it is only the words used to describe it that differ.

Chris

 

RE: more on output impedance, posted on January 15, 2015 at 10:08:07
Ralph
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 That difference has a lot to do with the difference between the Voltage and Power paradigms.

I use the word 'paradigm' as such is defined as a platform of thought, outside of which nothing exists or else is blasphemy. IOW one schooled in the Voltage Paradigm will have trouble seeing how 'output impedance' can be expressed in any other terms- I have run into this a lot.

I like to use the black box approach similar to how one measures the impedance of a speaker. In this case, the amp has an unknown impedance but we can easily find out what it is by simply connecting it to a variable load. We then find out at what impedance it makes its maximum power and then continue to reduce the load until that value is cut in half. At that point, half of the power is being dissipated in the output circuit itself (causing it to heat up) and the other half in the load. At this point the load must be equal to that of the output circuit.

The interesting thing about this approach is that the value is predicted by formula and corresponds in practice.

Under the Voltage Paradigm, the output impedance is that where the circuit makes maximum power, which corresponds to the (Delta V)/(Delta I) method.

The Power technique is measuring the impedance of the output circuit, the Voltage method is measuring the dynamic response of the circuit. In the former, the measurement does not change regardless of feedback, in the latter, it does.

IOW, at this point we are simply talking semantics. I agree that most of the industry is using the Voltage Paradigm, and in lo- and mid-fi applications this is perfectly justified. In high end, it isn't so much on account of higher performance loudspeakers like ESLs, horns and others that don't conform to Voltage Paradigm rules.

In case you think I am making this up (some have in the past), Google the Fisher A-55 amplifier and one of the first hits you will see is a YouTube image of the damping control of the amp. It is marked 'Constant Voltage' at one extreme, 'Constant Current' at the other, and where they cancel (zero feedback) it reads 'Constant Power'.

 

RE: As you might imagine, I'm having a problem with this, posted on January 15, 2015 at 20:03:58
Caucasian Blackplate
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 The OP was asking about "averages", so I answered considering averages.

Bringing up feedback was not targeted at you at all, you have made your case clearly in the past about how little feedback you use in your designs, but again, it's averages.

(BTW - how is the M-60 a 2 stage amp? I see 3...)


 

RE: more on output impedance, posted on January 16, 2015 at 07:51:54
cpotl
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 "In this case, the amp has an unknown impedance but we can easily find out what it is by simply connecting it to a variable load. We then find out at what impedance it makes its maximum power and then continue to reduce the load until that value is cut in half. At that point, half of the power is being dissipated in the output circuit itself (causing it to heat up) and the other half in the load. At this point the load must be equal to that of the output circuit."

OK, I need to think about this a bit. May I check that I have your procedure correct? You first adjust the load impedance until the maximum power dissipation in the load is achieved. Let's say that the load impedance for this is Z1. Then, you reduce the load impedance further until you reach a point where the power dissipated in the load is one half of what it was at the maximum. This second, lower, load impedance is, say, Z2.

And you then define Z2 as the output impedance of the amplifier?

Thanks,
Chris

 

averages, posted on January 16, 2015 at 09:16:17
Ralph
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 At this point it might be that we have changed the averages when it comes to OTLs, being the oldest and largest manufacturer. You had used the 99% value, which I have to assume is just not right.

My girlfriend complains that I am very literal...

 

RE: more on output impedance, posted on January 16, 2015 at 09:21:42
Ralph
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 No, that's not quite it. You have to start out at a significantly higher value (i usually start at 100 ohms for an unknown box); then decrease it until the maximum power transfer point is found, then decrease from there to 1/2 power transfer.

 

RE: more on output impedance, posted on January 16, 2015 at 09:29:35
cpotl
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 "No, that's not quite it. You have to start out at a significantly higher value (i usually start at 100 ohms for an unknown box); then decrease it until the maximum power transfer point is found, then decrease from there to 1/2 power transfer."

I think that's what I was saying; it's what I meant to say at any rate. First find Z1 load impedance at which maximum power into load is reached, and then reduce further until the power into the load drops to one half that, and this defines your Z2.

Chris

 

Yes. nt, posted on January 16, 2015 at 09:38:55
Ralph
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 -

 

RE: averages, posted on January 16, 2015 at 10:12:52
Caucasian Blackplate
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 Oh, I wasn't intending averages by manufacturing, more like out of 50 OTL designs reviewed, I would expect nearly all of them to use more than 10dB of feedback.

I still stand by my assertion that properly designing on OTL amp is tougher than designing a good SET (but designing a good SE output transformer is harder than both put together) .

 

RE: averages, posted on January 16, 2015 at 11:29:53
Ralph
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 I think we might have more reviews that other OTL manufacturers too. I know there are more than are shown on our website.

But I do agree- especially that part about SET OPTs!

 

RE: more on output impedance, posted on January 17, 2015 at 05:15:27
cpotl
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 "I like to use the black box approach similar to how one measures the impedance of a speaker. In this case, the amp has an unknown impedance but we can easily find out what it is by simply connecting it to a variable load. We then find out at what impedance it makes its maximum power and then continue to reduce the load until that value is cut in half. At that point, half of the power is being dissipated in the output circuit itself (causing it to heat up) and the other half in the load. At this point the load must be equal to that of the output circuit."

The thing that worries me about your definition of output impedance is that it doesn't give a sensible answer if we test it on the classic example of an idealised device that is modelled as a perfect voltage source (i.e. zero impedance and voltage V) in series with a resistance R. This device unambiguously has an output impedance R.

If we now drive this into an external impedance Z, the power dissipated in the external load is P= V^2 Z/(R+Z)^2, and this is, of course, maximised at Z = R. Let's call this Z1, so we have Z1 = R. We also have Pmax = V^2/(4R). If we now follow your rule, and ask for the smaller of the two values of Z for which P = Pmax/2, we get Z = (3 - 2* sqrt2) R. Calling this value Z2, we have approximately Z2 = 0.17 R.

Thus in this case of an idealised output device, for which all conventional ways of calculating the output impedance will give the answer Zout = R, your way of defining the output impedance will give the answer Zout = 0.17 R, which is about one sixth of the correct answer.

And, by the way, the value of Z for which the power dissipated in the device equals the power dissipated in the external load is also given by Z = R. When Z = 0.17 R, which is your definition of the output impedance when applied to this idealised case, the power dissipated in the external load is about one sixth of the power dissipated in the device itself.

I suppose your argument must be that the idealisation of the output device as a perfect zero-impedance voltage source in series with a resistor R is not an accurate model of a real amplifier output stage. But it does seem to be troubling that your definition gives a result that is so much at odds with the actual answer, if applied to the usual idealisation.

Chris

 

There's gotta be a bug somewhere, posted on January 19, 2015 at 10:05:05
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 Thus in this case of an idealised output device, for which all conventional ways of calculating the output impedance will give the answer Zout = R, your way of defining the output impedance will give the answer Zout = 0.17 R, which is about one sixth of the correct answer.

In practice this is not borne out. I stated earlier what the differences are. However I should point out that its also impractical to assume that we are going for a perfect voltage source. In fact we are not, in our case we are going more for a power source. All amplifiers were power sources in the old days- SETs still are, as are many tube amps without loop feedback.

Obviously a perfect voltage source will have infinate power at zero ohms. It does not exist.

Its a different way of looking at it. What I recommend is that you play with the equipment and take measurements as they will reveal more than the math seen so far. Or you could trust that the numbers I suggest are right- I've done these measurements many times. Did you read the article at the link? There is a reason why someone would want to design an amplifier that has a higher output impedance like this- oddly, its because you can get less coloration- quite the opposite of the 'accepted wisdom'. This is because of how the human brain processes sound, which ultimately is the final arbiter.


 

RE: OTL vs SET, posted on January 20, 2015 at 15:41:33
blakey
Audiophile

Posts: 212
Joined: October 24, 2000
 Really depends more on the execution than the type, IMO. My GM70 based DHT amplifier does not really sound like a stereotypical SET amplifier in that "beautiful" or "lush" isn't the proper word to describe its sound. If anything, I would say it sounds bold and muscular. It only puts out 22 watts but drives speakers that some conventional 100 watt push-pull amps can not.

I've owned many OTL amps over the years - Counterpoint SA-4, Luxmax MQ-36, Graaf GM-20, Silvaweld OTL Reference, Futterman H3/OTL3/OTL4, Atma M60 in various versions, etc - so I know the OTL sound pretty well.

I would say good amplifiers sound very similar to each other regardless of the design.

 

RE: OTL vs SET, posted on January 21, 2015 at 04:45:17
berni
Audiophile

Posts: 184
Joined: May 30, 2011
 I am very interested to hear about the difference or specialities when comparing the Atma with Graaf . Ok , I am using the larger brother of the Graaf gm20, the gm200 but stiil interested.

 

RE: There's gotta be a bug somewhere, posted on January 21, 2015 at 05:33:40
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009
 "Did you read the article at the link? There is a reason why someone would want to design an amplifier that has a higher output impedance like this- oddly, its because you can get less coloration- quite the opposite of the 'accepted wisdom'. This is because of how the human brain processes sound, which ultimately is the final arbiter."

Yes, I read the article you linked. It seems to me there are several distinct questions here. Some of them, such as the voltage paradigm vs the power paradigm, can be discussed even within the conventional model of an amplifier output stage as a perfect voltage source in series with an internal impedance R.

What is certainly true is that if the output impedance R is tiny, such as with a typical solid state amplifier, then for a given signal level the power delivered into a typical loudspeaker load impedance Z falls approximately inversely with Z. On the other hand, if the output impedance R is comparable with the typical loudspeaker impedance Z, then one is "sitting on the top of the curve" in the plot of power dissipated into the load as a function of Z. Thus, to a good approximation, moderate variations in Z around the value Z=R will then not cause the power into the load to change by too much. This is illustrated in the first plot here, of power into the load Z, as a function of Z, for the case of output impedance R=8 Ohms. (x-axis is Z, y-axis is power into load Z.)



By contrast, in the case of a very low output impedance amplifier, the power into Z=16 Ohms would be one quarter of the power into Z=4 Ohms, as can be seen in this second plot (normalised to the same power at Z=8 ohms):



In other words, you don't need to invoke any non-standard definition of "output impedance" in order to make your point about the power paradigm vs the voltage paradigm. It is already visible, just with the usual definition, and the usual model of an output device. If the output impedance is tiny, then for a load impedance Z in the range between 4 and 16 ohms, the power lies in a large range, with Pmin being 25% of Pmax. On the other hand, if the output impedance is 8 ohms then for the same range of load impedances the power lies in a relatively small range, with Pmin almost 89% of Pmax.

Another rather different question is whether the usual model of an amplifier output stage as a perfect voltage source in series with an internal resistance R is a good one or not. I tried various experiments in the past with my OTL amplifiers, and it seemed to me that the model worked reasonably well. Now, in all of my amplifiers there is quite a lot of negative feedback. I am interested now to experiment a bit more, in particular with how well the idealised model works if I turn off the negative feedback. I suppose my expectation would be that the model would still be reasonable, but now the value of R would be a lot larger. But I will keep an open mind until I have done the experiments.

Concerning the non-achievability of the "perfect" zero impedance voltage source, yes, of course I agree with you it cannot literally be realised in practice. But that doesn't mean it isn't a useful concept in circuit analysis. As long as its "actual" output impedance is very small compared to that of the internal resistance R that one adds in the model of the output stage, then the imperfection of the "perfect" source is insignificant. I still feel that if a definition of "output impedance" gives a radically wrong answer when tested on the conventional model of an output device, then there is reason to question the validity of that definition. But then, as I said above, I don't think you even need your non-standard definition of output impedance in order to make your point about the power vs voltage paradigms.

Anyway, I shall try to carry out some further experiments soon.

Chris

 

Sounds/looks like we are on the same page here, posted on January 21, 2015 at 08:28:21
Ralph
Manufacturer

Posts: 4778
Location: Minnesota
Joined: April 24, 2002
 The non-standard output impedance method is actually based on the formula for calculating the output impedance of a cathode follower. But as you point out, the graphs really make the point.

The problem is that quite often the application of loop negative feedback contributes to higher ordered harmonic content (re.: Norman Crowhurst). Since the ear uses such harmonics to calculate sound pressure it is very sensitive to these harmonics; so much so that even trace amounts that are hard to measure are readily audible, which is why transistor amps tend to sound bright. The brightness is not measurable as a frequency response variation, in just the same way that the 'warmth' of a tube amp is not.

Quite literally the application of feedback causes a violation of one of the most fundamental rules of human hearing perception. The Power Paradigm is a means of getting around that problem. The idea is that while frequency response is likely to not be as accurate, colorations due to distortion (which are interpreted by the ear as tonality) will be reduced. Since no speaker is really flat with any source, the actual real-world result is that you can get what sounds like very uncolored sound.



 

RE: OTL vs SET, posted on January 21, 2015 at 14:32:31
blakey
Audiophile

Posts: 212
Joined: October 24, 2000
 Never having heard the GM-200, I can't comment on how it compares to the Atma, but I remember the GM-20 being more neutral with better highs and more defined lows, at least with the speakers I was using at the time.

 

RE: OTL vs SET, posted on January 22, 2015 at 23:46:12
berni
Audiophile

Posts: 184
Joined: May 30, 2011
 Thanks.
The gm200 is compared to the gm20 even wilder, to die for mids and with more control in low end but with no sacrifice at the top.

 

I will chime in here, posted on January 26, 2015 at 01:40:27
morricab
Distributor or Rep

Posts: 9181
Location: switzerland
Joined: April 1, 2005
 First, I have owned a couple of different OTL models: Silvaweld OTL refernce monoblocks (4 x 6C33C per mono) and a Transcendent Sound Beast. I have also owned a fair number of SETs: KR Audio VA350i, JJ 322, Cary CAD-572se, NAT Symbiosis SE and had friends with several others that I have heard at length.

A really good SET will have tight clean bass but it still won't sound like solid state bass, or even the bass of a single ended transistor output design (like my NAT). Harmonically, well within its power envelope, it will also be very transparent and sometimes even slightly lean...at least compared to those SETs with compromised iron. Highs will often be a bit soft(ish) as compared to just about any amp with feedback and/or push pull designs and this is regardless of how extended the bandwidth of the output iron is from my experience. If the amp is using a solid state power supply it will be a bit leaner and perhaps faster sounding than one with a tube power supply.

Palpability and 3d imaging as well as soundstage depth will be world class.

A really good OTL will have SHOCKING transparency...something that I have yet to hear with even the best SETs and of a wholly different quality from SS such that all SS sounds grainy and opaque by comparison. Dynamics also are quite lively. Imaging and soundstaging is right up there with the top SETs. Where they fall down, IMO is often in tonal balance and overall coherence. They tend to sound leaner than most SETs and, IMO, than real life. I think this has to do in many cases with the use of negative feedback (My OTLs used around 10db of negative feedback). Not using output iron probably also impacts this.

Those OTLs that don't use negative feedback are somewhat warmer in character but still have that amazing transparency.

Bass (for a circlotron design) is somewhere between SS and SET bass. It is drier and tighter but still not SS sounding. For other types the bass on normal speakers has issues and could contribute to the lean sound.

Where my OTLs fell down ultimately was in coherence...nothing beats a really good SET, IMO, for seamlessness and "continuousness" (to use a classic HP descriptor). A single ended OTL might do it but it seems that they need too much negative feedback or dozens of tubes to be a viable solution.

There is one company, a Cyprus one called Aries Cerat, that makes a zero feedback, single ended OTL, that is massive and with a large number of output tubes...I am dying to hear it.

My NAT is the closest so far that I have heard to marrying the concept of SET and OTL because it is a direct coupled single ended hybrid with ONE big MOSFET on the output per channel. It has NEARLY the transparency of an OTL and NEARLY the tonal character of a top SET but with a SS like bass. It does not quite get there as the ultimate amplifier. After a couple hours of playing it does get pretty hallucinogenic and very close to the ideal, IMO, but not quite. A MOSFET just has not got the same character as a power triode and you can hear that.

 

RE: I will chime in here, posted on January 26, 2015 at 05:58:55
throwback
Audiophile

Posts: 762
Location: Colorado
Joined: December 8, 2003
 Very nice post, Morricab. Thanks.

I have found that to properly benefit from the wonderful transparency of the OTL—in my case, a pair of Atma-Sphere Silver Edition MA-1’s—I must pay attention to the smallest parts of my overall system. The amps are at the penultimate end of a long chain of segments and interfaces, any one of which—or in combination—can affect the sound markedly. Thus I would no longer be sure enough of my ground to say that an OTL or SE amp is thus or so per se.

When I first hooked up my amps to my current speakers (GR Research LS-9’s with servo subs) I was mildly disappointed. I had heard the 9’s at RMAF driven by conventional amps (Dodds) in a carefully set up system, and they sounded terrific. I believe I have gone well beyond that sound now, but here’s what it took:

1. New internal wiring (Marigo)
2. Removing a sleeve I had put on a segment of the Marigo’s and separating the wires
3. Different series/parallel wiring scheme to optimize impedance
4. Different solder on the new wiring (Stop that! I heard someone giggling.)
5. A change to the crossover (because of the new wiring scheme)
6. Different IC’s (my silver ones were now too bright)
7. Replacing the 6SN-7 input tubes

I and several experienced observer friends heard a difference with every one of these changes. Whereas before the changes an especially picky (annoying, really) audio buddy politely declined my offers for a listening session, now he comes over often and brings others.

Placebo effect? Maybe. I did believe going in that the Marigo would make a substantial difference, and it did. But the difference was immediately obvious to other observers as well. As to the predicted effects of some of the other changes, well skepticism reigned supreme until the changes were made.

I suspect that the same attention to detail is necessary to make SE’s really sing in a real world system.

What’s next? Maybe some further tweaks: fuses, room mods, vibration control? The point is, the OTL’s let me hear what’s going on to an unprecedented degree compared to what I had before. Benefit or curse? You tell me.

 

RE: I will chime in here, posted on January 26, 2015 at 20:44:28
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 Hi, Good to have you aboard, but any comment on how an amplifier sounds, particularly an SET or OTL should be qualified by speaker-specificity, and the subjectivity of the opinion as well. That said, there's nothing particularly jarring about what you wrote, except huge amounts of NFB in a SET OTL? I am not familiar with any SET OTL amplifier, personally, but if such a design "needs" large amounts of NFB to drive a speaker properly, then the design itself would be questionable in my mind. Like maybe it's not a good idea at all to marry an SET with an OTL. A shotgun wedding at best.

 

RE: I will chime in here, posted on January 27, 2015 at 01:33:30
berni
Audiophile

Posts: 184
Joined: May 30, 2011
 When do we talk about too much negative feedback?

 

Morricab mentioned it, posted on January 27, 2015 at 06:43:31
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 In his post he says that SET OTLs "need" large amounts of NFB. I am not sure why that would be the case, except to reduce apparent output Z, but it is not a desirable property of an amplifier design, IMO.

 

RE: I will chime in here, posted on January 27, 2015 at 20:00:42
c1ferrari
Audiophile

Posts: 640
Location: Southern California
Joined: March 16, 2001
 I'm using four Silver Edition MA-1's to drive my Dali MegaLines. Ralph's gear/circuits will readily respond to variants of 6SN7. I'm also using an MP-1. I wish all my sources were 1/2" master tapes :-)

Vbr,
Sam

 

RE: I will chime in here, posted on January 27, 2015 at 20:22:38
throwback
Audiophile

Posts: 762
Location: Colorado
Joined: December 8, 2003
 Oh boy, me too! I had a great collection of tapes, but nearly all deteriorated to the point that they were unusable (Ampex Grand Master back-coated). Sure was great while it lasted. I love vinyl, but a great tape setup was truly magic. Sigh!

 

RE: I will chime in here, posted on January 27, 2015 at 20:53:57
c1ferrari
Audiophile

Posts: 640
Location: Southern California
Joined: March 16, 2001
 Oh, what a bummer! :-(
Baking couldn't resurrect them?

Vbr,
Sam

 

RE: I will chime in here, posted on January 28, 2015 at 06:12:19
throwback
Audiophile

Posts: 762
Location: Colorado
Joined: December 8, 2003
 Might've, but you get to a certain point of disgust . . .

 

Marigo wire, posted on January 28, 2015 at 07:21:34
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 What is it? Copper or silver or silver-plated copper? Stranded or solid core? LItz or not? Round or ribbon in cross-section?
Thanks.

 

RE: Marigo wire, posted on January 28, 2015 at 19:55:51
throwback
Audiophile

Posts: 762
Location: Colorado
Joined: December 8, 2003
 Lew,
Pure copper, 18ga, round in cross-section,600 individual (lacquered, I believe)strands.

Ron Hedrich is easy to talk to; why not give him a call?


Contact Us


Call Ron Hedrich at (360) 835-9239

Our Address :
Marigo Audio Lab
32711 SE 16th St., Washougal, WA 98671 USA

map
Tel: (360) 835 – 9239
Fax: (360) 835 – 9249

 

RE: Marigo wire, posted on January 29, 2015 at 00:01:21
20202tom
Audiophile

Posts: 8
Joined: December 5, 2014
 Sound like the Symo wire that Krells KSA 100 class A Amp was wired with. I used in the 90s to run my Apogee Duettas, the late Jason Bloom used on his Apogees.
Vary,Vary hard to tin!






http://www.6moons.com/audioreviews/gma4/sidebar.jpg

 

RE: Marigo wire, posted on January 29, 2015 at 05:43:49
throwback
Audiophile

Posts: 762
Location: Colorado
Joined: December 8, 2003
 Not so hard to tin if you use a solder pot. It takes several steps: 1) debraid the sleeve to about one inch; 2)cut and cauterize the end of the braid; 3) dip the end into the solder pot. Rewiring my speakers required 200 such operations. As I said, a royal PITA--well worth it, though.

 

RE: Marigo wire, posted on January 29, 2015 at 07:22:49
20202tom
Audiophile

Posts: 8
Joined: December 5, 2014
 Most don't have a Pot laying around,I never like the sound of the Symo!
Only difference I see is the cloth type covering that can make all the difference in the sound.
Wire is tuff,you like Vanilla I like chocolate.
But thanks for the info on Marigo Audio Lab.
Looks like Mr Bloom at Apogge was on to some thing with the Symo wire in the 1990s.

 

RE: Marigo wire, posted on January 29, 2015 at 07:26:46
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 Thanks, Chuck.
I use a Marigo power cord on my preamplifier, but that wire would be very difficult to use as chassis wire (for me, anyway). It's murder to strip off all the enamel insulation in order to make a good solder joint. I have a solder pot for that, but it's not really a cure for the aggravation. Anyway, I am happy with narrow gauge solid core silver (for chassis wire). In my speakers I use 6 or 8 parallel insulated strands of 19ga Cardas silver wire on the treble and parallel strands of enameled copper transformer wire on the bass. (Only because the silver is so darned expensive.)

 

RE: Marigo wire, posted on March 6, 2015 at 06:00:02
horn kid
Audiophile

Posts: 128
Joined: November 2, 2014
 Having owned double digit OTL and SET amps, from low power to high power versions of each, I can assure you that the sound of neither type of amp can be stereotyped. There are some lean, some neutral, and some fat versions of each. There are dynamic and less than dynamic versions of each. Some of each are very sensitive to speaker load and some of each seem relatively insensitive to speaker load. I've heard a 2 watt OTL sound more powerful into an easy load than a 100 watt OTL. I've heard 20 watt versions of 2 different SET amps using the same tubes into the same speaker with one sounding very powerful, tight, and dynamic and the other sounding weak and strained.

No specs will allow you to figure this out without trying, and no manufacturers' claims will allow you to figure this out. You need to hear the combinations somewhere or try them yourself.

 

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