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Coytee

Jeffrey D. Medwin... OTL question

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First question is, what do you think of them & why?

 

I'm not technical in this aspect so have just my ears & what I've read to go on.....  but the little SE-OTL amp (1 1/2 watts /stereo 4 watts bridged....and I used a pair of them bridged)

 

Anyway, that little thing was the nicest most 'real' sounding amp I had, when I had tube amps.

 

(McIntosh MC-2102, MC-30's, Viva 300B SET, Wright 2A3, Quad II's, and a Jolida ??)

 

None of them had the relaxed "presence" that the OTL had, even my wife and independent of her, her sister both, made comments while system was playing about how "real" it sounded.

Curious as to your general thoughts though for me, I've moved the tubes on and now use SS.

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I understand what your listening experience was, and given the list of amps you mentioned owning,  precisely WHY you wrote and asked me such a question.  I am fully understanding, Coytee, of what you heard. 

 

I do NOT like OTLs at all, because the coupling device between the amp and the speaker is a capacitor. 

 

A capacitor is possibly the worse sounding part we have in audio, to couple through.   Sure, you got rid of an output transformer, but NOW, you are listening through something far worse, a CAP !!!!

 

The OTL's basic topology is flawed, not the best possible, when you have to couple to the loudspeaker with a capacitor !!! 

 

Also, they have to use negative feedback , a huge NO NO in quality amplifier design.

 

OTL amps ( eg , the Ro****lit 2 Watt SE-OTL ) are EASY - to thoroughly and totally " trounce" - in performance.  I did this in 2018, and can post the Listener's evaluation for anyone to read, ( his email back to me ), if, there is any sincere interest.

 

Jeff 

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Richard, I am not the least surprised that you enjoyed an OTL amp as many audiophiles consider them the best.  Having a capacitor in the circuit does not in any way degrade the sound.  Remember, interconnect cables are capacitors as are the tubes themselves!  I am sure that Ralph Karsten and Bruce Rozenblit, both of whom sell some excellent OTL designs, would have a great deal to say about this.  All amps are flawed in some respect,  and OTL designs are certainly no more flawed than any other.  
 

 

Maynard

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Yep Maynard.

 

I'm "independent minded enough" (my wife would prefer to call it stubborn!) to like what I like....and not worry pro/con what anyone else has to say.

 

I found the OTL's to have a different enough 'flavor' to their sound, it got me wondering if some of the 'benefits' of an OTL (no OPT) might indeed be worthy.... While playing within a similar range, none of the other amps had as comfortable of sound (Khorns, ES crossovers).

 

Admitting my ignorance, it had me wondering if there was merit to losing the OPT if possible.

 

If I got another tube amp, I'd keep that type in consideration.

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There’s no question in my mind that the OPT is the weakest link in a conventional tube amp design.  Anyone who is willing to deal with a larger number of tubes should certainly consider OTL.  If I weren’t so cheap, and did not spend my time with similarly minded audio folks, I would do a few OTL designs myself.

 

 

Maynard

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Coytee,

 

As I said, I understand what you heard, and think its good that you trust your ears.  You asked me, and I gave you my opinion, from me having experiences, and hearing amps, the likes of which, you have never heard. 

 

If all you have been exposed to, and owned, is tube amps that score a Grade " C ",  ........any Grade  " B " amp,  (in this case, hearing your SE-OTL ), will be the best you heard.   But a Grade " B " score, is NOT the best score possible, is it ????

 

I have a different set of listening experiences.  I have DIY built for my own use, ( and have heard manufactured ), Grade " A ", amps.   World-class amps.  This is all that me, and many of my closest audio buddies, ever consider using .  

 

I find the world's BEST possible sounding amps always have the same topology,  ( and...... it is never OTL ). 

 

The output transformer is not any big problem,  IF......the basic topology has STARTED OUT right,  ( the proven topology ......... two-stages, directly-coupled, zero negative feedback. )

 

A three stage amp will never do what a two stage amp will do.

 

An amp using a coupling cap, or an interstage transformer, will never do what a directly-coupled amp will do. 

 

Simple and  UNBEATABLE, when done well :

 

       1) 2 inch Silver wire, RCA Input jack to GRID of DriverTube

       2) Silver wire, PLATE of Driver Tube to GRID of Output  Tube

       3) Output transformer, from Plate of Output Tube, to power the Loudspeaker 

       4) No negative feedback

 

There is one other aspect that I failed to mention.  OTLs often require paralleling of tubes.  

 

As soon as you parallel tubes,  you have a degrade in the best possible performance, a " choir" effect. 

 

This is easily noticeable  (  IF you are used to using  Grade "A" scored amps with well-set-up, highest efficiency speakers. )

 

So, add to our list " Any amp that is an OTL, will not be able to do what a two-stage direct-coupled amp zero negative feedback amp can do. "  Why, ....... because of the OTL's typical use of paralleled tubes.

 

If all of a person's experience and lifetime has been mired listening to Grade "C" score amps, (  because you have insufficient funds, or don't know any better ), you will never ever know about the sonic differences when using one tube, versus paralleled tubes, and, hearing a zero negative feedback amplifier play music. 

 

Nor will anyone know about the differences in using two tightly-matched highest-quality plate resistors, VS. using one.   No, one will not know this,  if one has only been exclusively experiencing Grade "C" and "B" audio amplification.  

 

So Coytee, you asked me,........... and here you have it, from me, ... my friend .

 

Jeff Medwin 

 

Thank you, Moderators, for allowing me to answer !!!!!

 

 

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Having heard what I heard....  I'd have to conclude that it's more out of group ignorance.  Most might not have heard them themselves.

 

I'd bet you lunch (at a slightly higher grade than McDonalds.....  hmmm.....did I just call myself cheap??!!)

 

Anyway, I'd bet you lunch that if you built one and someone nabbed it.....  it would get good reviews and a second, third might follow.

 

What surprised me was not my wife, not my sister in law, but BOTH of them just stopped cold turkey while eating dinner (which was NOT turkey) and said paraphrased "OMG, what are you listening to in the living room......it sounds SO REAL like the singer is in there with us"

 

The ONLY time I ever (ever ever ever) heard any comments like that is when the OTL's were playing.

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Appropriate immensely to this thread and discussion,  a Klipsch Forum post I wrote in April 2019 :

 

 

 

 

 Most people have little or zero experience, direct listening wise, hearing a well executed amp with LSES topology

 

Erik, is Transcendent Sound  the same company run by Bruce Rosenblit??  If so, we are in luck.

 

From 7-3-18 to 12-31-18, I prototyped a SE triode-connected 6005 DC amp, with a 159ZA LSES supply..  Only seven people have ever heard it.  One of them, in February 2019,  was a DJ and astute audiophile that had a low powered ... name....?? Transcendent Sound ?? amp on his reference Lowther system, it was a maybe 2 Watt  OTL amp.   Let me go to my email directory, and try to find his words to me, upon hearing the 2018 DC 6005 SE prototype.  I'll post this entry now, and add his email comments below in this post, later.

 

                                                                                              * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 

 

Here we are, yes, he did use and A-B my 6005 SE  2018 prototype amp to his existing Transcendent Audio OTL:

 

737056081_InitialListeningReport2-9-19.thumb.jpg.c88fbc2944047e1b940eb64f143d2c2c.jpg

 

 

This 2018 SE DC triode-connected 6005 amp is only 0.25 Watts clean, 0.33 Watts maximum, and he was using a T.A. 2 Watt amp.  He still wanted to hear mine, and he is reporting back to me, and to my audio friend / mentor  Dennis.

 

                                                 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

 

The person who wrote that email to me (above) is NO FOOL.  He at that time, was the President of an Energy Company, owned by Bill Gates, and was using special Lowther speakers. 

 

Since my amp, at only 1/4 of a Watt ( 6005 triode connected ) was not sufficient to power his speakers, he found a best solution.  He contacted Dennis Fraker, my Mentor, and bought reasonably,  a used pair of monoblocks, customer-trade in of 2A3 two-stage DC amps, which Dennis had refurbished.  This person has been delighted, listening-wise, since then.   When you move, as he did,  from only hearing a Class " B" scoring amps ( Transcendent SE OTL ) , to Class " A " scoring amps, there is NO going back !!

 

Jeff 

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My understanding is that the output tubes are NOT necessarily capacitor-coupled to the speaker in an OTL.  That apparently was the case with very old designs such as Futterman, but not more modern designs such as my Joule Electra.  There, my repair guy is emphatic that there are no capacitors in the Joule output signal path.  He thinks the speaker is directly  connected to the plate of a 6C33B.

 

Jud Barber, who designed and made Joules, felt strongly that music detail is lost in an output transformer.  That may well account for why Joules are extraordinarily transparent, and very linear to my ear.  However, it is very difficult and costly to design an amp without an OPT.  Otherwise there would be more of them!  Disadvantages are high current consumption and heat output.  My heat output with only 12 OP tubes is tolerable but does need A/C.

 

Joule 100-watt OTLs have a rather high 10 ohms output impedance, so that the sound is a little thin and light in the bass.  I try to compensate some with component selection including speaker wire, but by and large my system is very well balanced IMO.  My Joule VZN-100's have six 6C33B's on each side.  Like Atma-Sphere, the issue is addressed by having more tubes to reduce impedance than you would need in a P-P. -- I just don't have as many as Ralph K. does.

 

I would put my amps up against any that I have heard.

 

 -- Larry

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3 hours ago, LarryC said:

My understanding is that the output tubes are NOT necessarily capacitor-coupled to the speaker in an OTL.  That apparently was the case with very old designs such as Futterman, but not more modern designs such as my Joule Electra.  There, my repair guy is emphatic that there are no capacitors in the Joule output signal path.  He thinks the speaker is directly  connected to the plate of a 6C33B.

 

 -- Larry

 

Yes, I think you are right; most OTL amps these days do not have a capacitor between the output and the speaker.  I have four different home-built OTL amps myself, and none of them has an output capacitor as such.  Same is true, I believe, of commercial designs like Atmasphere or Bruce Rozenblit's Transcendant Audio amps.

 

There are still capacitors in the output path, because the audio signal passes through the power supplies.   The same is also true, of course, for an SET or push-pull amplifier.

 

It is not true that OTLs require paralleled output tubes.   Two of my OTLs, for example, each use just a pair of 6C33C tubes in the output stage (totem pole), so no paralleling of tubes at all. They both give 25W into 8 ohms.

 

In any case the alleged "choir effect" of paralleled tubes is a non-existent phenomenon, in my opinion, that is not recognised by any reliable authority. 

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Thanks Backfire.  Mine have been described as very esoteric (they probably all are!).  Jud called them  "class A SET" (they may not be SETs).  I'll dig out Jud's description and post it some day.  They do sound like class A's as well as OTLs.

 

Where are you located?  I'd be glad to have someone stop by for a look-see if desired.

 

 -- Larry  

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56 minutes ago, Backfire said:

There are still capacitors in the output path, because the audio signal passes through the power supplies.   The same is also true, of course, for an SET or push-pull amplifier.


For those concerned about capacitors in amplifiers many SET amplifiers using a bias resistor in series with the filament also have capacitor/capacitors bypassing the resistor which the audio signals also pass through.
 

miketn

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5 hours ago, mikebse2a3 said:


For those concerned about capacitors in amplifiers many SET amplifiers using a bias resistor in series with the filament also have capacitor/capacitors bypassing the resistor which the audio signals also pass through.
 

miketn

 

 

Are you talking about a bypass capacitor across the cathode resistor???   If so, most SET amps I see, maybe, just guessing  98%, regularly use electrolytic caps across the Rk, as a bypass.  IT is IN the circuit !!!!   Would these same people use a high quality electrolytic as a coupling cap. Why...NO !!   But they unconsciously use electrolytics across the Rk, of a 2A3, in its cathode return to ground.   That seems thoughtless to me !!! 

 

When I bypass the Rk of a DHT FInals tube, I use films, with as "S", because it takes about five high-quality film caps, of different types and uF values, in parallel, to get the Rk bypassing musically correct.  Also, it takes a lot of money, to do it right. 

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Here’s what Rozenblit had to say about OTL output capacitors on the Transcendent forum:

 

 

Maynard 

 
Bruce Rozenblit03/29/04
There seems to be a lot of confusion about what an OTL is on other message boards. OTL stands for output transformerless. The idea is to eliminate the output transformer from the signal path. That's what I do. It is a very difficult task because of the impedance mismatch between tubes and speakers.  

There also seems to be a great deal of resistance to the use of an output capacitor. Many people for some unexplained reason, have gotten the idea that an output capacitor will greatly injure the signal. That is baloney. A transformer is about 1000 times more injurious to a signal than a capacitor. Most tube amps have output transformers but that is OK. Most speakers have internal coupling capacitors but that is OK. When an amp has an output capacitor, its the end of the world. I don't get it. 

Many people have spent a couple of hundred dollars to replace the stock caps in the SE OTL with black gates and all have reported back that there was no improvement. All have wasted their time and money.  

There is a big difference between conceptual idealistic notions and what works in the physical world. I concern my designs with what works in the physical world. The T16 is direct coupled and the SE OTL sounds even better but it has an output capacitor. Its the results that matter, not the concept. 
 
 
 

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I have no problem with a cap in the signal path or either a transformer as long as the amplifier sounds good. The sound will depend on how both are implemented. Mcintosh uses iron in both it's tube and SS amplifiers. One an OPT and one an autotransformer. They have a loyal following and have had a lot of praise for a long time. Holds their value really good as well for a reason. The sound fantastic. 

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On 9/12/2020 at 6:19 AM, tube fanatic said:

Here’s what Rozenblit had to say about OTL output capacitors on the Transcendent forum:

 

 

Maynard 

 
 
Bruce Rozenblit03/29/04
There seems to be a lot of confusion about what an OTL is on other message boards. OTL stands for output transformerless. The idea is to eliminate the output transformer from the signal path. That's what I do. It is a very difficult task because of the impedance mismatch between tubes and speakers.  

There also seems to be a great deal of resistance to the use of an output capacitor. Many people for some unexplained reason, have gotten the idea that an output capacitor will greatly injure the signal. That is baloney. A transformer is about 1000 times more injurious to a signal than a capacitor. Most tube amps have output transformers but that is OK. Most speakers have internal coupling capacitors but that is OK. When an amp has an output capacitor, its the end of the world. I don't get it. 

Many people have spent a couple of hundred dollars to replace the stock caps in the SE OTL with black gates and all have reported back that there was no improvement. All have wasted their time and money.  

There is a big difference between conceptual idealistic notions and what works in the physical world. I concern my designs with what works in the physical world. The T16 is direct coupled and the SE OTL sounds even better but it has an output capacitor. Its the results that matter, not the concept. 
 
 
 

 

 

My take away from Rosenblit's SALES blog :

 

1)  Bruce certainly seems to tell us above, that his SE OTL amps have some sort of Output Coupling CAP, unlike what has been posted earlier, above.  These were words of the Manufacturer, who should know his own circuit topology. 

 

Any OTL that is direct coupled to the speaker, ( has no cap couple, ie: his T16 ) is a push-pull design.  This requires a phase inversion,  which is typically, an extra stage, and always ADDED/ extra circuit manipulation, ...which the SE amp totally avoids !!! 

 

2) Of course Black Gate WKZs didn't help,  if the overall amp itself is built to a popular price point,  and likely has enough  price-sub-optimizations throughout, to be beyond help. 

 

3) A OTL amplifier begins its mediocrity with the basic OTL topology - pointed out in 1) above,  as being imperfect, too complex, ( an output cap or a phase inversion added , and the use of negative feedback loops ) ................this is problem Numero Uno !!!

 

4) No discussion of the OTL Transcendent amp means anything ( to me ) when my  two stage DIY 12AX7-6005 Direct-Coupled Single Ended zero negative feedback amp far far outplays the Transcendent OTL,  (except for power - 1/4 Watt VS. 2 Watts), in actual listener testing.   

 

The proof of it all, is how it sounds.  Due to it's OTL topology,  as mentioned in 1) and 3) an OTL amplifier can become a " Grade B " amplifier, at best.   

 

Nothing can ever compete with " Grade A " amplifier topology.  ( two tube stages, directly coupled to each other ) .  " Grade A " topology,  is a take -off of the 1929 Loftin-White topology.  It is the simpler.   

 

An example, from 1995, is nicely written up in different / current thread, entitled " A Simple Triode Amplifier". 

 

4) Bruce ends his sales pitch........ " Its the results that matter, not the concept".  ...........True !!

 

For results of the Transcendent OTL :  read how the amp Bruce designed and sells compared, to a simpler two stage DC amp .   Simply read the Feb. 9th, 2019 email report, embedded in this post :

 

https://community.klipsch.com/index.php?/topic/194394-jeffrey-d-medwin-otl-question/&do=findComment&comment=2534656

 

That email says it all !!!   WHY Coytee should I - on this Forum, or anywhere, endorse " Grade B  " solutions?   No sir. 

 

Realize this   KNOW that " Grade A " solutions exist ( as I claim, and as this ( now "ex" ) Transcendent OTL amp owner reports to us ).  A simpler circuit, when well executed,  can EASILY be markedly superior .

 

The output transformer is not the problem.    Eliminating it is a wrong solution, as it serves a good function in matching impedances.   

 

Results matter !! 

 

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11 hours ago, Jeffrey D. Medwin said:

 

 

Are you talking about a bypass capacitor across the cathode resistor???   If so, most SET amps I see, maybe, just guessing  98%, regularly use electrolytic caps across the Rk, as a bypass.  IT is IN the circuit !!!!   Would these same people use a high quality electrolytic as a coupling cap. Why...NO !!   But they unconsciously use electrolytics across the Rk, of a 2A3, in its cathode return to ground.   That seems thoughtless to me !!! 

 

When I bypass the Rk of a DHT FInals tube, I use films, with as "S", because it takes about five high-quality film caps, of different types and uF values, in parallel, to get the Rk bypassing musically correct.  Also, it takes a lot of money, to do it right. 


 

Here is the perspective of an EE from the Steve Hoffman forum.  I agree with him.  I believe that using multiple caps in parallel, regardless of type or price, craps up the sound.  My ENT doc friend showed me some frequency response plots of typical males in their mid to late 70s which have convinced me that some of what we believe we hear is wishful thinking.  Also, the nonlinearity of the human auditory system is so great, and so varied from one person to the next, that universal recommendations about audio components are worthless.

 

 

Maynard

 

 

John Dyson

John DysonForum Resident

Location:
Fishers, Indiana
R. Totale said: 
OK, but once it's doing its job of showing a short circuit to ground for audio frequencies, how linearly it grounds these things you will never hear or be able to measure with any tool pales in importance. If it can be done (which it can) with a 25 cent electrolytic the point of inserting a 10 dollar film cap eludes me.



Again OK, but I try not to do any work or spend money based on feels.
If I was making the decision for myself, I wouldn't change the cap unless I had a concrete reason to do so. Some people like to change things -- and that is fine also, perhaps part of the enjoyment.

Put my EE hat on (I am an EE RF/Audio/Systems, SW Operating Systems/DSP -- and generally pretty good at all of them :)), so here is a quick (back of scrap pad of paper) analysis of the situation:

Lets see -- cathode resistor/capacitor (I dont know the tube, so I'll guess a gm of about 2mmhos or 2mS.) That would imply an effective resistance of about 500ohms at the cathode (without bypass.) With an electrolytic with perhaps 0.1ohms of ESR, lets guess at 10% of that is nonlinearity. That would mean a delta of 0.01ohms with a 500ohm impedance at the node... The additional amount of distortion caused by the electrolytic changing by the 0.01ohms would be less (much less than) 0.002% additional distortion, and I am guessing VERY, VERY HIGH. In reality, the amount of distortion caused by the nonlinearity of the resistance would be pretty small. Now, we have an issue about the nonlinearity of the capacitance of the capacitor, and the best success there is to make sure that the capacitance has a reactance that is very small (relative to the lowest audio frequency) in relation to 500ohms. Also, unless the tube/valve is in a negative feedback circuit, the changing gm will totally swamp the nonlinearity of the capacitor (either the resistive or the capacitive reactance nonlinearity.)

If I was making a design decision, I would simply try to choose a reasonably high quality capacitor with fairly low ESR -- the rest will take care of itself. The difference in distortion of a reasonably good electrolytic vs a perfect capacitor will be vanishingly small, and probably not even measurable.

Of all of the things that might need to be improved (other than as a failed/degraded parts replacement) I don't think that changing the capacitor would be all that helpful. Audio source material and transducers (speakers, headphones, microphones, etc) are much more important.
 
john
 

 

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On 9/11/2020 at 12:50 PM, Jeffrey D. Medwin said:

As I said, I understand what you heard, and think its good that you trust your ears.  You asked me, and I gave you my opinion, from me having experiences, and hearing amps, the likes of which, you have never heard. 

Doesn't get more arrogant or condescending than that.

 

Richard, you are a neophyte apparently.

 

Additionally, what you are hearing is on inferior speakers and thus even if you had listened to good amps, you couldn't tell they are good.

 

 

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I completely agree with those saying that capacitors are not a big problem. 

 

And another point is that if one has any doubts as to whether a capacitor might have any deficiencies, one can easily check by using an oscilloscope to look at the signal across it. The ideal would be that a capacitor in the audio path (e.g. a cathode decoupling capacitor,  or the final capacitor in a power supply) would have zero audio signal across it.  Of course in practice, being of only finite capacitance, it will have some non-zero AC reactance, and so there will be a small audio signal across it (less and less as the audio frequency increases).  The magnitude of the expected audio signal as a function of frequency is of course easily calculable.

 

If the capacitor were causing any non-linear distortion of the audio signal, then this would show up in the signal one would see across the capacitor using the oscilloscope.  If the distortion were enough to be audible, then it would easily be visible and measurable.  By measuring the distortion, if any, in the signal across the capacitor one could easily estimate the distortion it would cause in the audio output from the amplifier.  Almost certainly, unless one has made a really unsuitable choice of capacitor or it has some serious fault, the distortion it would cause would be completely negligible.

 

But in any case, if the capacitor is causing any problems of this kind it is easily measurable and understandable using basic physical principles.

 

I would be much more inclined to believe what the instruments were saying than the anecdotal accounts of a human who probably hears what he wants to hear.

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