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OTL amps and high impedance speakers.

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Posted on April 22, 2017 at 13:55:07
Cougar
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 I recently read a article on TubeCad that John said that if one could make a speaker of 32-72 ohms you could get more power out of OTL amps instead of using 4-16 Ohm speaker loads?

If this is true, is there a point where to use certain power output OTL and a high impedance range of speaker? I mean can you use a 15-20 watt output OTL with 4-16 ohm speaker which see all the time and then make a speaker with 32 or 56 or 72 ohm using the same amp and get more power output? Is there a limit of how high you can go with a high impedance speaker? Any drawbacks or danger in doing this?

I hope my question make sense and forgive me if this is a dumb question/s. I'm still learning this type of design. I have never heard of such high impedance speakers with tube gear or OTL's, the highest I ever heard was 16 ohms.



 

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The comment on Tubecad, posted on April 22, 2017 at 15:25:39
Ralph
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 was not entirely accurate if it was quoted correctly. There's a pretty big difference between four and eight ohms let alone sixteen!

As a generalization there are problems! A lot depends on the individual OTL. It would be a pretty low power OTL to put out more power into 72 ohms as opposed to 16 for example.

Our smallest amp is the S-30 which makes its highest power into about 25 ohms 50 watts) and it's actually less power into 72 ohms! So the comment doesn't really apply to our amps.

Generally speaking our smaller amps, the S-30 and M-60, are both quite happy on 16 ohms and both do well on eight ohms too. Even with a custom speaker I can't imagine why you would want to go over 32 ohms.

Our larger amps are fine with four and eight ohms; the biggest puts out its maximum power into three ohms (over 500 watts and as far as we know is the most powerful OTL made).

 

I figured 8-16 ohms was the best range for speaker ohms., posted on April 22, 2017 at 15:51:07
Cougar
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 Ralph, I was hoping you would answer this question. Thank you for the explanation and info, it answered a lot of question I had.

i saw that the Zero Autoformers were used with your amps. Are they as good as what I have seen written about them?

I have also been looking into your M-60 kit as well as the Transcendent. I currently have a Latino VTA-120 that I built from a kit and I'm happy with but I have been hearing good things about the OTL's for a while and now have the bug to build one. I was looking to do a Beast or the M-60 in kit form but this thing of proper speaker loads came up.

Thank you for the help.

 

Yes, the ZEROs really do work, posted on April 22, 2017 at 20:24:32
Ralph
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 They are a problem solver; when a person wants to use a low impedance load the ZEROs are often helpful for getting many amps to sound better on that load, even transistor amps.

 

For what it's worth...., posted on April 23, 2017 at 10:06:42
Lew
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 I have written about this before, maybe many times. I modified my Sound Labs 845PX speakers, which in stock configuration had a nasty impedance dip at critical midrange frequencies, down to below 4 ohms, such that the impedance after the modifications is a fairly flat 20-30 ohms, from around 100Hz to 5kHz. Impedance falls gradually above 5kHz to 2 ohms at 20kHz (nearly unavoidable with a large ESL like mine, due to the capacitative nature of the speaker itself). In any case, the improvement in both speaker efficiency and in the capacity of my Atma-sphere MA240 amplifiers to drive them, and hence the pleasure factor, was nothing short of phenomenal and continues to gratify me every time I fire them up.

(Efficiency went up as a result of the mods independent of the fact that impedance went up, because the low impedance of the OEM configuration is in large part due to the fact that the crossover places a rather low value resistor, anywhere from 5 ohms to 10 ohms depending upon the year and model of the SL speaker, in parallel with the output of the amplifier. The resistor inevitably sucks amplifier power and converts it to useless heat. That resistor is now in the trash.)

I feel certain that any 50W tube amp would drive the modified SL speakers to ear-splitting levels with no strain.

 

The power droop at very high impedances..., posted on April 27, 2017 at 07:24:32
Lew
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 As a non-techie person, this issue has caused me some consternation. Obviously, the power output of any amplifier has to go down, eventually, as impedance of the load rises. Otherwise, crossovers would not work as advertised. But it also occurred to me that tube amplifiers, and OTL tube amplifiers in particular, may drive crossovers more efficiently than do, say, solid state amplifiers with either very low output impedance, very limited voltage capability, or both. Thus, it could be said that the speaker crossover ought to be designed with the type of amplification in mind. An OTL might sound different from an SS amplifier on a given multi-way speaker, for that reason alone.

Ralph, can you comment? Thanks.

 

RE: The power droop at very high impedances..., posted on April 28, 2017 at 10:22:10
Ralph
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 The issue is that the output impedance of some amps is high if they don't run feedback, and low if they do.

This can affect how the crossover works.

More at the link below.

 

RE: OTL amps and high impedance speakers., posted on April 29, 2017 at 10:31:10
ozzyboy
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 What I can add is with my M-60's using the Zero's and the 16 ohm tap improved the sound quality, dynamics and power.

 

RE: OTL amps and high impedance speakers., posted on May 5, 2017 at 15:52:17
kablemetal@gmail.com
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 it depend on your OTL circuit topology,some topology can accept 4,8 or 16 ohms speaker.

 

RE: OTL amps and high impedance speakers., posted on May 5, 2017 at 16:01:42
kablemetal@gmail.com
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 Why do use Zero?It kills dynamics.

 

Can you please list the system...., posted on May 6, 2017 at 07:20:34
Cougar
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 You were using with the Zero's that you make the claim they kill dynamics?

Not looking to start a flaming war just would like to know for learning experience what combo may or may not work well with the Zero's.

 

RE: Can you please list the system...., posted on May 6, 2017 at 09:15:01
cpotl
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 "You were using with the Zero's that you make the claim they kill dynamics?"

While I can perfectly well believe that they sound very good, and indeed may even improve the performance with certain speakers, I have never quite understood the logic of deciding to go down the route of OTL technology, and then adding in a transformer that couples the amplifier to the loudspeaker. Doesn't it rather defeat the original aim of "eliminating the iron-cored components" in the signal path?

Chris

 

From what I understand the Zero's..., posted on May 6, 2017 at 11:09:20
Cougar
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 Are NOT a transformer in the sense of having primary and secondary windings. So this is not the same as an output transformer like regular tube amps have. The output signal comes off the same winding wire as the input signal goes into, so no degrading of original signal like a transformer with primary and secondary windings do. So it's like comparing Apples and Oranges.

 

RE: From what I understand the Zero's..., posted on May 6, 2017 at 11:46:44
cpotl
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 I agree that there are differences as between dual-winding transformers and autotransformers, but I think it is an overstatement to say "so no degrading of the original signal like a transformer with primary and secondary windings."

The functioning of the autotransformer still depends upon the audio signal inducing magnetisations of the iron core, and upon the way in which that magnetisation then induces emfs in the part of the winding that connects to the speaker. (The autotransformer would become almost completely non-functional if the iron core were removed.)

I think with the autotransformer, the amplifier setup cannot properly be called output-transformerless. It has become an amplifier that depends crucially upon magnetic induction in an iron core in order to function.

Chris

 

RE: From what I understand the Zero's..., posted on May 6, 2017 at 13:08:17
ozzyboy
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 Chris,

I don't understand your phase "Iron Core". There is no iron in the Zero's, just copper.

 

One way to find out is to check.... , posted on May 6, 2017 at 13:13:34
Cougar
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 And compare input to output signal on the Zero with a scope to confirm! Right?

If I do get a pair I will do this and try to post the pics. Now I'm talking about just the Zero autoformer and not just any autoformer.

 

Thank You for clearing that up...., posted on May 6, 2017 at 13:20:17
Cougar
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 About the Zero's.

 

RE: From what I understand the Zero's..., posted on May 6, 2017 at 15:18:30
cpotl
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 "I don't understand your phase "Iron Core". There is no iron in the Zero's, just copper."

Ah. I was assuming they were wound on iron cores. isn't that the case? I found a review somewhere, and it said

"...a pair of massive toroid transformers, that remind me of what you'd see as power trannies in a 250 wpc solid state bruiser. These have a biwired pair of black and white 14 gauge silver plated copper leads for your speaker connection, and on the other end you have have 4 wires, color coded gray, yellow, blue, and brown. These are your multiple impedance taps."

I was guessing the toroid cores were iron. Is that not the case?

Chris

BTW my phrase "iron-cored components" was a quote from one of the early articles presenting an OTL design, by Dickie and Mackovski in 1954.

 

You should go to their website...., posted on May 6, 2017 at 16:24:23
Cougar
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 And read up on the Zero's. This should help you understand what the Zero actually does with OTL amps and other amps as well.

 

RE: You should go to their website...., posted on May 6, 2017 at 16:34:40
cpotl
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 "And read up on the Zero's. This should help you understand what the Zero actually does with OTL amps and other amps as well."

Well I did look there, but I didn't see any surprises. They work in the standard way an autotransformer works, as far as I can see. It all depends upon the standard principles, with the input current causing magnetic fields in the iron (I assume iron?) core of the toroid, which then lead to an induced emf that is fed to the output. The mere fact that there is a shared portion of the winding for "primary" and "secondary" doesn't materially alter the essential principle of how a transformer operates.

Let me, again, try to make my point clear. I am not saying they may not sound good. I am just saying that by using them one is abandoning the notion of an output-transformerless amplifier. This is just a matter of basic physics. (In a similar sense, one could not claim an "output-transformerless amplifier" if one simply took a conventional tube amplifier and mounted the output transformer at the speaker end of the speaker lead rather than at the amplifier end!)

Chris

 

RE: From what I understand the Zero's..., posted on May 6, 2017 at 17:39:17
cpotl
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 "Are NOT a transformer in the sense of having primary and secondary windings. So this is not the same as an output transformer like regular tube amps have. The output signal comes off the same winding wire as the input signal goes into, so no degrading of original signal like a transformer with primary and secondary windings do. So it's like comparing Apples and Oranges."

OK, so let's take a conventional SET amplifier and replace the usual output transformer by an autotransformer. They probably don't make these as standard items (!), but we could easily engineer one by connecting the secondary (speaker) winding of a conventional output transformer in series with the primary, with the appropriate phase sense. We would then connect the "far" end of the speaker winding to B+, and the other end of the original primary winding to the anode of the output tube. The speaker would be connected, as before, across the original secondary. (The fact the speaker winding itself would now be at B+ voltage is a potential inconvenience, but the discussion here is about a point of principle.)

You would not, I think, claim that the amplifier has now become an OTL amplifier, just by virtue of the simple act of replacing the impedance-matching dual-winding output transformer by an impedance-matching autotransformer?

So why would one make the claim that a formerly-OTL amplifier is still operating as an OTL amplifier if the output is now being run through an impedance-matching autotransformer to the speaker?

Chris

 

RE: From what I understand the Zero's..., posted on May 8, 2017 at 20:46:50
kablemetal@gmail.com
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 Why do you have to put a transformer in an OTL??it's not OTL anymore!!

 

RE: From what I understand the Zero's..., posted on May 8, 2017 at 20:49:52
kablemetal@gmail.com
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 ZERO?input is not equal to output,It kills dynamics

 

RE: Yes, the ZEROs really do work, posted on May 8, 2017 at 20:54:46
kablemetal@gmail.com
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 ZERO?it will work & you can always transform the impedance but not the sound,input is not equal to output.simple physics.

 

RE: From what I understand the Zero's..., posted on May 10, 2017 at 05:42:05
cpotl
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 "Why do you have to put a transformer in an OTL??it's not OTL anymore!!"

As a general principle, I agree with you. To me, the term "OTL" has a very precise connotation, namely that there is no impedance-matching transformer between the tube output stage and the loudspeaker. This is a simple "yes" or "no" issue.

I find it to be a bit of a smokescreen when it is asserted that an autotransformer is allowable without invalidating the notion of OTL. As I pointed out before, if an impedance-matching autotransformer still allows one to call it OTL, then *any* tube amplifier could be easily made into an "OTL amplifier" by the mere contrivance of appropriately rewiring the output transformer so that the secondary was in series with the primary.

Of course, to use this argument would be rather absurd; I just presented it in order to demonstrate that one cannot take the autotransformer versus dual-winding transformer distinction as the determining factor in deciding whether an amplifier is configured as an OTL or not.

The only other line of argument would seem to be that if the primary-to-secondary turns ratio is "sufficiently small," then it can still be called OTL. Thus with two-to-one the term OTL would still be allowed, but for twenty-to-one it would not. This is really just a matter of sophistry, though. One would now have to declare some arbitrary ratio, lying somewhere between two-to-one and twenty-to-one, as the cutoff between OTL and not OTL. This would really make little sense.

To me, the simplest and cleanest definition is just the obvious one; OTL if there is no impedance-matching transformer between the tube output stage and the loudspeaker, and non-OTL if there is.

An entirely different issue is the question of whether a particular system sounds better with, or without, an impedance-matching transformer. But I don't believe that it is helpful for these more subtle questions to be blurred by what, to me, are unconvincing claims that a set-up is still operating in OTL mode even when there is an impedance-matching transformer between the output stage and the loudspeaker.

Chris

 

RE: From what I understand the Zero's..., posted on May 12, 2017 at 15:12:08
kablemetal@gmail.com
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 ZERO !!!!??? OTL with output transformer,are we going in circles?Who need ZERO for a well design OTL.

 

RE: Can you please list the system...., posted on May 12, 2017 at 15:22:46
kablemetal@gmail.com
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 Using ZERO transformer to an OTL?.We thought that an OTL don't have output transformer?So it's not an OTL.

 

RE: Can you please list the system...., posted on May 12, 2017 at 15:23:42
kablemetal@gmail.com
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 I don't use ZERO.

 

RE: Can you please list the system...., posted on May 13, 2017 at 09:14:10
kablemetal@gmail.com
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 Chris,I agree,we have the same idea about the ZERO.

 

RE: From what I understand the Zero's..., posted on May 15, 2017 at 13:05:16
Lew
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 (1) The impedance conversion afforded by the Zero autoformer is much lower than that of any conventional tube output coupling transformer, because the impedance matching can be as low as 2:1 and almost never more than 4:1. (The output Z of the OTL is nearly the same as that of the speaker, usually at least a bit less.) Whereas in a conventional transformer coupled (OTC) tube amplifier, you are facilitating an interaction with power tubes that have output Z in the thousands of ohms to a typical speaker with a Z of 4-16 ohms. This makes for wider bandwidth and lower distortion for the Zero vs the conventional output transformer. The job of the output transformer is much harder.
(2) Because it's an auto former and because of the reduced workload, you would not have hysteresis distortion, or at least much less of it.

I don't say the Zero is perfect, but I do say that to my ears the combo of the Zero with an Atma OTL is still superior to every OTC tube amp I have tried on the same set of speakers. Audio is full of trade-offs. If you want to stamp your feet and walk away in protest, that is your privilege. Or find a pair of speakers that don't need the Zeros.

 

RE: From what I understand the Zero's..., posted on May 15, 2017 at 13:33:16
cpotl
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 "I don't say the Zero is perfect, but I do say that to my ears the combo of the Zero with an Atma OTL is still superior to every OTC tube amp I have tried on the same set of speakers. Audio is full of trade-offs. If you want to stamp your feet and walk away in protest, that is your privilege. Or find a pair of speakers that don't need the Zeros."

I can absolutely believe that there are circumstances where a particular OTL amplifier may drive a particular set of speakers better if an impedance matching autotransformer is inserted between the amplifier an the speaker. And I can absolutely believe that the resulting system may sound spectacularly good.

My only gripe is if it is claimed that such a configuration is still adhering to the spirit, and the letter, of the concept of an OTL. I think OTL has a very clear and precise connotation: no impedance transformer between the output tubes and the loudspeaker.

As I said before, if you wanted to claim that with a turns ratio of 2-1 it is still an OTL system, but that with a turns ratio of 20-1 it isn't, then you would have to come up with some specific ratio somewhere between those two values, and assert that it is still OTL one side of the line, but not the other. This would be an entirely arbitrary cut-off, with no difference in qualitative behaviour between one side of the border and the other. Hence I simply go back to the (almost orignal) Dickie-Macovski notion, of the absence of any iron-cored components in the amplifier. (Actually they went one stage further and had no iron-cored components in the power supply either, but that is another story, and a side-issue for the present discussion.)

As with many things in audio, I think it is a shame when the image of a probably superb-sounding system risks being tarnished by virtue of claims that it is something that it really isn't. (And don't get me started on the subject of the ZOTL!!!)

Chris

 

RE: From what I understand the Zero's..., posted on May 15, 2017 at 16:14:39
Lew
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 Chris, You and a few others are the ones taking the purist stance. I have no desire to argue the point except to say what I said. Yes, the Zero is a kind of crutch to improve the match between an OTL and a typical low impedance speaker. That doesn't bother me. Most people who have employed zeros with their Atma and other OTL amplifiers report that they are happy with the pairing. I was too, until I found a way to modify my speakers to raise both their efficiency and impedance curve such that I no longer needed to use the Zeros. As you know, too, there are also ways to reduce the output Z of many OTLs, by adding NFB, that also might do the job and obviate the need for Zeros. It depends upon which is the lesser of two evils. It's all good, so far as I am concerned.

I would maintain though that a well designed autoformer, like the Zero, used at a 2:1 impedance ratio (I never went above 2:1; the Zeros permit up to 4:1, I think) is in fact likely to give wider bandwidth and lower distortions compared to a true audio output transformer as found on most OTC amplifiers, driving the same speaker, for reasons noted. If you want to say that an autoformer-coupled OTL no longer fits the strict definition of an OTL, I would not argue with that.

One other small point, the Zero is placed at the speaker end of the path from the amplifier outputs, not at the amplifier end, for whatever difference that makes. You might say it acts on the speaker. But I don't want to go there, either.

 

RE: From what I understand the Zero's..., posted on May 16, 2017 at 02:00:00
cpotl
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 "Chris, You and a few others are the ones taking the purist stance."

Well, I am a theoretical physicist in my day job, always questioning, seeking to extract general principles, universalities, and precise definitions. I suppose I tend to apply a similar approach in the rest of life; automatically questioning, looking for inconsistencies or ambiguities, and so on. (Doesn't always go down too well, I have to admit!)

So in this case too I just like to have a clear, precise and robust definition of what OTL means. For me, it means one thing and one thing only; no inductive impedance-matching device between the output tubes and the loudspeaker. As far as I can see, any attempt to broaden this definition so as to allow some impedance-matching transformers but not others is just opening a can of worms, and it will end up with arbitrary criteria being introduced in order to try to justify something that really cannot be logically defended.

In any case I don't see any need for such a broadening of the concept of OTL; better just to stick to the simple yes/no criterion of whether there is any impedance-matching transformer between the output tubes and the speaker or not.

In a case where a Zero autotransformer is being used, it can perfectly well be described as an OTL amplifier supplemented with an impedance-matching transformer that connects to the loudspeaker.

I guess the thing that prompted me to comment in the first place was that for myself, I would feel it to be a bit of an admission of defeat if I had set out to build an OTL system and then ended up resorting to using impedance-matching autotransformers in order to do the job satisfactorily. Now, I'm sure it is true that certain speakers with especially low impedance might be quite challenging loads for some OTL amplifiers. If I had such speakers, and was determined to use them with an OTL amplifier, I think I would direct my attention to beefing up the OTL so that it could handle the load, rather than using transformers.

In the end this comes down to a matter of personal taste, I suppose. I built my OTL amplifiers because I was fascinated by the idea that tubes could drive ordinary loudspeakers directly. I wouldn't want to give up on that motivation, and if I had very low impedance speakers that provided a difficult load for the OTL amplifier, I would view that as challenge to overcome by improving the OTL amplifier itself.

Chris

 

My thinking has a little more plasticity,...., posted on May 16, 2017 at 07:21:32
Lew
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 I am an MD/molecular biologist-virologist. That mixed identity is confusing enough during my day job. I also have a view of audio that is analogous to something Frank Sinatra is reputed to have said about religion: "Whatever gets you through the night". He went on to say that in his case that was a bottle of Jack Daniel's.

 

I have an idea..., posted on May 16, 2017 at 07:49:36
Lew
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 The operating instructions for a Zero suggest placing them at the speaker, not at the amplifier end of the speaker wires. So think of them as a modification to the speaker, not a conversion from OTL to OTC operation. Semantics can make anything feel better.

Otherwise, I think this is beating a dead horse. If one doesn't need the Zeros, then one should not use them. If one does need them, then one can either get a different pair of speakers more suitable to one's particular OTL, or use Zeros to mate the OTL to the speaker. (Or buy an OTL with more output tubes, more power, and lower output Z.) I don't think anyone ever said that the Zeros are needed as a matter of course, or as an upgrade, with an OTL. My experience was as stated: the combo of OTL + Zero sounded better than any of several OTC tube amps I tried, but I do prefer the sound now that I have eliminated the need for the Zeros. (On the other hand, the speakers are way different now that I modified them; I daresay they would sound better now even WITH the Zeros than they sounded prior to the speaker mods, with zeros.)

I've got a used pair that I would sell cheaply, by the way.

 

Paul Speltz, Zero maker, has done this...., posted on May 16, 2017 at 10:39:27
Lew
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 He compared sine waves, in and out of the Zero. Not sure that he made more extensive measurements than that, except he did or does publish bandwidth measurements as well.

 

RE: I have an idea..., posted on May 16, 2017 at 10:43:47
ozzyboy
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 Lew,
I am interested in your Zero's so I can Bi-wire.
Ken

 

I sent you an email, posted on May 16, 2017 at 11:58:10
Lew
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 nt

 

You might want to read..., posted on May 16, 2017 at 12:00:26
Lew
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 Spelt's explanation and analysis

 

RE: I have an idea..., posted on May 17, 2017 at 02:43:43
cpotl
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 "The operating instructions for a Zero suggest placing them at the speaker, not at the amplifier end of the speaker wires. So think of them as a modification to the speaker, not a conversion from OTL to OTC operation. Semantics can make anything feel better.

Otherwise, I think this is beating a dead horse..."

I agree that the talking points in this discussion are more or less played out. Reasonable people can agree to differ about fine nuances of interpretation!

However, my analytical side cannot let your first paragraph go by without a final comment. If one should think of the Zero as modifying the speaker, rather than the amplifier, since it is placed at the speaker end of the speaker cable, then what about an ordinary vacuum tube amplifier with a standard output transformer? Suppose now we remove the output transformer from the amplifier chassis and place it at the other end of the speaker cable, right up by the speaker. (So the speaker cable will now be connecting the B+ and output tube anode to the OPT primary.) Should we also now view the OPT instead as a "modification to the speaker," so that we would say the amplifier has become an OTL? I don't think that would satisfy anyone's reasonable criterion for calling the setup an OTL system. So I think the physical location of the matching transformer is not relevant to how one should describe the operation of the system.

Anyway, it sounds like you must have a great sound system!

Chris

 

RE: I have an idea..., posted on May 17, 2017 at 07:45:03
Lew
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 I think the reason that what you describe is never done with an OTC amplifier is probably because of the high voltages involved. The speaker wires would have to carry thousands of volts in most cases, which is both unsafe and unwise. But I think Dave Slagle has implemented something like this with Quad 57 ESLs and one of his amplifier designs, where he has eliminated the step-up transformer at the input to the Quad in favor of a single transformer that effectively mates the amplifier to the Quad stators, directly. This is not direct-drive; there IS a transformer at the amp/speaker interface, except where you would usually have two transformers, now there is one. I heard this set-up at the Capitol Audio Fest, using stacked pairs of ESL 57s. The speakers are owned by a local friend. At home, he uses three panels per side. Quite impressive.

 

RE: I have an idea..., posted on May 17, 2017 at 08:56:40
cpotl
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 "I think the reason that what you describe is never done with an OTC amplifier is probably because of the high voltages involved. The speaker wires would have to carry thousands of volts in most cases, which is both unsafe and unwise."

Yes, I agree that one would not wish to do that in practice, for the reasons you mention. But I was really just considering it as a thought-experiment, to make the point that it would not, in any case I think, change the interpretation of the setup as an OTC system.

Actually one thing one could do, which I don't think is often done, would be to make something like a totem-pole or circlotron OTL amplifier, but just with more common or garden tubes like EL34 or whatever, and then use something more like an ordinary output transformer to match the relatively high output impedance of the amplifier to that of the loudspeaker. Then, the primary of the OPT would be at close to zero potential, and there hazards you mentioned would not arise, even if one did want to mount the transformer close to the loudspeaker. Also, for what it's worth, there would be no quiescent DC current passing through the transformer primary. Probably it would require higher HT voltages than is normal in an OTL, to get any decent power.

Of course I would not call the setup an OTL system, for the same reason that I would not call the system with Zero autotransformers OTL, but just thinking of it as a possible type of amplifier one could construct.

Your comment about driving ESL speakers puts me in mind of a crazy thought I once had, namely that one could envisage an OTL amplifier that would directly drive the ESL speakers without any step-up transformer at all. It would be a horrifically fearsome thing, I think, requiring something like a totem-pole or circlotron but with transmitter tubes and HT voltages being of order thousands of volts. Definitely not something I would want to make or have in the house!

Chris

 

I never said it was...., posted on May 20, 2017 at 15:41:48
Cougar
Audiophile

Posts: 4593
Location: SoCal
Joined: June 25, 2001
 With the Zero's that the amp was still an OTL in the purist sense. I was just asking about the speaker impedance matching with OTL's and how critical it was after reading that article. I just made the comment that the Zero's were not a OPT like you were trying to say they were. So please keep the facts straight on that just like you are about the OTL design.

Some speakers give OTL amps a rough time due to their impedance or reactance and the result is bad sound. Then the OTL's get a bad rap for not sounding good or thin. Some of us do not have the money to burn to keeping buying amps or speakers until we get a perfect match, so the Zero's are maybe the better way to go to match OTL amps with our existing speaker systems IF there is an issue with the matching (yes, like a crutch).

As you know, just because a amp and speakers look good on paper....doesn't mean it will sound good in your system or home together. So the Zero's are the next best (cheapest) thing until you can either make or buy some speakers that match the OTL amp you have properly.

So instead of just arguing about the OTL purist design and the Zero's being an OPT (which they are not), why not bring some good advice or examples of what OTL/ Speakers you have or others that actually work well together so we can all learn.

Thanks

 

RE: I never said it was...., posted on May 21, 2017 at 02:29:16
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009
 Well, my own OTL amplifiers are home-built. Two of them use 6C33C tubes in the output stage, totem-pole style. (One pair of tubes in the output stage.) These give about 25W maximum into 8 ohms. One of the OTLs is based on a design by Hans Beijner, the other a design by Tim Mellow. I built another OTL that is a circlotron, based on a design by Alan Kimmel. This one allegedly gives up to 100W into 8 ohms, but I've never tried pushing it to measure that. Finally, I have one I really built just for fun, loosely based on the very early Dickie-Macovski design, with no iron-cored components whatsoever.

The speakers that I use are all high or fairly high efficiency. Lowther DX3 drivers in bass reflex cabinets; Klipsch Forte II; and Paradigm Monitor 7. With these, I've had no problems at all driving them from any of the OTLs. In fact the Lowther DX3 drivers have paralleled Audio Nirvana drivers too, so I suppose they are then nominally 4 ohm. But the sensitivity is so high that running out of headroom is never a problem.

I fully agree with you that if you have already an amplifier and speakers, and if the amplifier is not able to drive the speakers comfortably because of impedance and current-delivery issues, then Zero transformers may well be a reasonable solution to the problem.

Personally, I would be left feeling slightly unsatisfied if I were having to do that with an OTL amplifier, since it does, to some extent that different people can hold different views about, partially negate the whole OTL philosophy. For me, if I ran into such a situation it would be fairly straightforward for me to rebuild one of my OTL amplifiers in a beefed-up form, for example by doubling up on the output tubes. (In fact it would give me an excuse to build another one, which I have been wanting to do for a while, but currently find it hard to justify the need for a 5th one!) I can see, though, that if one is buying ready made amplifiers the financial considerations would be somewhat different.

Regarding whether Zero transformers should be called output transformers or not, I think that all that can be said on the subject has been said already. My main point in my observations on the topic is that one is going to be somewhat hard pressed to come up with a universal definition of what is, and what isn't, an output transformer if the definition is not simply going to be crafted for the express purpose of "allowing" Zero autotransformers while disallowing everything else.

Chris

 

Good info!, posted on May 21, 2017 at 07:08:21
Cougar
Audiophile

Posts: 4593
Location: SoCal
Joined: June 25, 2001
 Chris, Thank You for the info. Now this is what I was looking for in replies. :)

I totally agree with you about either building a new amp or speaker if there was an issue with proper matching OTL amp and speakers. I have built a tube amp from kit and have done a few DIY speakers but I am no expert in building tube amps from scratch let alone OTL amps.

What got me interested in OTL amp designs is how people who owned them said they were the best sounding amps the have ever had but speaker matching was a must to get good bass. I would really hate to add anything between the OTL amp and speaker that would take away from what the OTL amp brings in sound. So I do understand where you are coming from.

I hear those 6C33C tubes are pretty nice? Are these largely available?

I now have got some good info and direction/s on what to do which will help me out big time. I just wish I could build that many amps from scratch. There are some OTL kits I could try and hopefully with the DIY GPA driver (Altec) based 2 way with Zilch Z19 xovers should be a good match. If that doesn't work out I do have a Line Array design in the works with BG planars and Silver Flute 6.5 drivers I have already that I can arrange to get the proper matching impedance for the OTL amp/s.

I will look into some of those designs you mentioned

Thanks again for the info!

 

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