Autoformers, The Benefits in matching amp to speaker



There has been a great deal of conversation about Autotransformers in this forum. Many think they are similar to the output transformers we use on Tube Amplifiers. They are not for some very important reasons. They are not wound the same way, they have no High Voltage insulation, they are wound with heavy low resistance wire and all the winding is used all the time. In addidtion part of the signal current is direct and part is transformed. 

  • THE WINDING.  When we make a traditional output transformer we have to insulate the primary from the secondary for over 1000 volts. This insulation takes up space and winding space is most dear to the designer as we want as much copper in there as possible. We then have to section the windings and interleave them. An interleave of 5 is good and some think 7 or 9 or even 11 is better but that raises the capacitance of the transformer and is hard on the tubes at high frequencies. An autotransformer has no DC voltage in the windings and thus can be bifilar wound (taking 2 or 3 or more wires at once). This increases the coupling and extends the high frequency response by a factor of 2 or more. My ouput transformers are good to 65 KHz and the Autofomer is good to 140 KHz. 

  • THE CORE: As to the core, an EI core is preferable over a torroid as the torroid will have saturation problems if connected to an amplifier that has a DC offset. An offest as low as 20 mV can swing the core in one direction toward saturation. An EI core has a very small air gap that will allow it to ignore rather large offesets. 

  • IN THE AMPLIFIER: Here's where the difference is between a conventional output transformer and a Autoformer occurrs. This is why Wiggins at Electro Voice created the CIrclotron circuit. In a conventional tube amplifier. for most of the signal, only one half of the output transformer is active. It is very difficult to make the two halves of a push pull transformer identical above 20 KHz where the feedback really cares about phase shift. Even the taps on an Ultralinear transformer can go out of phase at high frequencies. This causes the amplifier to ring on one half of the square wave. Though not widely talked about, we who design amplifiers are very familair with this problem. Wiggins realized that if he put the transformer in a bridge circuit that the primary would act as a whole and this problem would go away. That is the essence of the WIggins Circlotron. Because he wanted to keep the ampifier efficient he did use a high ratio transformer with conventional taps. BTW, we do not put taps on an amplifier to "match" the impedance of the speaker as we know it varies. We put them on there to deliver the proper ratio of voltage and current to make the amplifier happy. You can always use a lower tap and enjoy lower distortion, better damping, lower noise and extended tube life. You also extend the class A region. The only reason to use a higher or matched tap is to get the most power out of the amplifier if you play it loud. In the RM-4 manual I suggest this strongly and have termed it "Light Loading"

Now, what is an Autoformer going to do for you? If you have an OTL amplifier you should know that the power is greatly reduced into low impedance loads. Even worse is that low impedance loads will overheat the tubes at high power levels as most of the power supply voltage is being dropped across the tube not the load. So low impedance loads are hard on the tubes and cause higher distortortion All of these ills can be solved by the use of a proper Autoformer.
  
For OTL amplifiers that have high output impedance and produce their best performance into 16-32 ohms one needs a 6 or 8 to one step down ratio. This will make the speaker and amplifier very happy and still preserve the qualities of the OTL. A 4 to 1 is not enough. This is no problem to make and I have been using mine for many years.

An Autoformer can also be used in reverse if one has a low voltage, high current amplifier like an ML-2 which is 25 watts into 8 ohms but 100 into 2. Again a 4 to one will get you 100 watts and and an 8 to one even more. Remember the impedance ratio is the turns squared. So an even a 9 to 1 impedance is only 3 to 1 turns and 1/3 of the signal is direct through the primary.

I hope this clears up the differences in these two very different types of transformers and we can stop considering them as the same. While some may consider a transformer a band-aid, I consider it a device that makes the problem go away.

Please feel free to ask your questions.
Fe8c8cab 4117 4c51 b1aa 6b134ad0dca6ramtubes
Much thanks for the information! 
Well I am certainly not literate in EE so a bit fuzzy on all of this but I did recently purchase some MC601 monos but have not heard them yet. Is your definition of the issue the same one as Mac has in their design?

I consider it a device that makes the problem go away.
When we introduced our Z-Music autoformer back about 1990, we always regarded it as a problem solver. The same is true of the ZEROs, which is a similar product.
Since we make OTLs, the problem to be solved is lower impedance speakers. We are in Magnaplanar's back yard and have a lot of local customers that wanted the the transparency of the Maggies combined with the transparency of OTLs. The autoformer was the answer, as its introduction didn't introduce the same loss of transparency that seems to be part of the conventional transformer route.
Paul Speltz, who makes the ZEROs, has a letter from Steve McCormick, a well-known manufacturer of solid state amps. In the letter, Steve says that his amps can drive 4 ohm loads easily, but he finds that when he uses the ZEROs to interface between the amp and 4 ohm load, it sounds better. Paul was pretty pleased to get that letter :)

Much thanks for the information!
Well I am certainly not literate in EE so a bit fuzzy on all of this but I did recently purchase some MC601 monos but have not heard them yet. Is your definition of the issue the same one as Mac has in their design?

I would be happy to comment on particular Mac designs using Autoformers. If anyone has a question and can provide a link to a schematic that would be a big help. Im not having much luck finding the SS amp schematics. 

I did find an early Mac SS amp that they admitted ran in class B, that was not such a good idea and I doubt they do that now.


Ralph,

I agree your amplifier is fine for the Maggies as the impedance is constant however the lowness of it is hard on the tubes. I look at low impedances are like driving a short. I did put a one ohm tap in the RM-200 and it will indeed output 100 watts into a 1 ohm load with good damping. So why did you stop making the Autoformers?

I am curious in reading you white paper and posts here that while you admit that an ESL can have a 10 to 1 impedance range you feel that it is appropriate to run such a speaker with high impedance drive. I was working for Beveridge when we did the model 2 and 3 speaker and was involved in the specification of the transformer for both(I was not winding yet). While we came up with something that kinda worked for the system 3 we found that system  2 was impossible due to the bass extension down to 30 Hz while the system 3 only required 200 Hz.

The original Beveridge direct drive amplifier produced 1500 VA (similar to watts). We could not make a transformer or hardly find a high enough current ampifier at the time to produce 1500 VA. This was in 1978. I did find a Mitsubishi amp that produced 60 amps but getting all those VA through the transformer was another challenge. When I questioned Bev as to why one needs 1500 VA he said. "go play some trumpet music and measure the current". I measured 1500 VA! This is why I make 5,000 volt direct drive amplifiers for ESLs.

Damn, this is great! For those in the San Francisco Bay Area, Roger Modjeski (RAM Tubes, Music Reference) teaches a course in amplifier design at The Berkeley Hi Fi School. You can build your own amp in the class, any kind you want. My home town is San Jose, and if I still lived there I’d do it myself. If you Google the school name, a website will be at the top of the list, and it contains all kinds of great amplifier design information.

If you truly want to "Walk the walk", this is the way to do it, not sticking pieces of wood and rubber in a 70’s Japanese receiver. Beware of false Prophets!

While some may consider a transformer a band-aid, I consider it a device that makes the problem go away.


I totally agree with Roger’s summation for autoformers, they are an "answer looking for a problem."
A bit like having rubber roads and looking for concrete tires.

It’s better not to have the problem to start with yes?

And have the right amp/combo yes?
And if you have this correct amp/speaker combo and introduce an autoformer, the sound will take a hit for the worse.

Cheers George
Note to people who love the sound of both magnetic-planar loudspeakers and tube amps: the Eminent Technology LFT-8b is a much better candidate for use with tube amps (especially OTL’s) than are Magneplanars. I love and have owned three pair of Maggies (currently one---the legendary Tympani-IVa), but they present a 3-4 ohm load to the power amp, while the LFT-8b an 8 ohm. And, if you bi-amp the LFT-8b (easy to do---it has separate binding posts for the m-p drivers and the dynamic woofer), the Linear Field Transducer itself is a mostly-resistive 11 ohm load---ideal for tubes! The Atma-Sphere M60 is known to be a great amp for the ET LFT-8b, as is the Music Reference RM-200.
So why did you stop making the Autoformers?
Paul Speltz came out with the ZEROs. I think our device was better in some ways- it was EI core rather than toroid, and it had 1 ohm taps! But the ZERO proved easier to explain and since that wasn't our main business we just went with it.
I am curious in reading you white paper and posts here that while you admit that an ESL can have a 10 to 1 impedance range you feel that it is appropriate to run such a speaker with high impedance drive
Like you, I didn't think it would work. It totally did. The reason is as I mentioned- the impedance curve of an ESL is unlike that of a driver in a box, where impedance peaks represent a resonance. ESLs OTOH don't have that behavior. Their curve is based on a capacitor (despite a matching transformer usually in use). So they don't respond all that well to the voltage rules. As a result, an amplifier that makes constant power rather than constant voltage can do quite well on them.
 While some may consider a transformer a band-aid, I consider it a device that makes the problem go away.


I totally agree with Roger’s summation for autoformers, they are an "answer looking for a problem."

I've examined these two statements for a while and they seem to be at odds. George, I think you missed Roger's point.

Note to people who love the sound of both magnetic-planar loudspeakers and tube amps: the Eminent Technology LFT-8b is a much better candidate for use with tube amps (especially OTL’s) than are Magneplanars. I love and have owned three pair of Maggies (currently one---the legendary Tympani-IVa), but they present a 3-4 ohm load to the power amp, while the LFT-8b an 8 ohm. And, if you bi-amp the LFT-8b (easy to do---it has separate binding posts for the m-p drivers and the dynamic woofer), the Linear Field Transducer itself is a mostly-resistive 11 ohm load---ideal for tubes! The Atma-Sphere M60 is known to be a great amp for the ET LFT-8b, as is the Music Reference RM-200.
The Maggies were also not push-pull drivers until more recently. ET was often quite gracious in the old days and sold individual panels to hobbiests. I have a friend who built up a system using multiple panels so that the overall impedance was 16 ohms. The speaker was not only wide bandwidth and revealing, it was also quite easy to drive full range- 100 watts was plenty!

remember input is not equal to output
with auto transformer,you have sacrifice the dynamics, and remember,,  an input is equal to output
in OTL design manufacturer don't mention about the specs about the tubes they used which is the most important in OTL designin,.transconductance of the tube is the sweetness of an OTL amplifier.they only built since the parts are cheap. 
Roger,The zero as you said the problem goes away with  your poor design.

There is a current thread here on AudiogoN from a guy who took two pair of LFT-8b's and "stacked" them. Each 8b speaker is comprised of a 1' w x 4' h panel containing two LFT "midrange" drivers (180Hz-10kHz) and one ribbon tweeter (10kHz up), and an 8" dynamic woofer (180Hz down) in a sealed enclosure. He made new metal frames for stacking, into which he installed two LFT drivers side-by-side, with two more above them, and one ribbon tweeter beside each pair of LFT drivers---two total. The two woofer enclosures sit below the frame, the stacked LFT-8b measuring about 5' tall by 2' wide. A pair of LFT-8b's retail for $2495, so for less than the price of a pair of Magneplanar MG3.7i you can have stacked LFT-8b's. Think about it!

A single pair of LFT-8b's are one of the best deals in all of hi-fi. Harry Weisfeld of VPI recently stated he considers the midrange reproduction afforded by the LFT-8b the finest he has ever heard. I disagree only to the extent that I consider the old Quad (which I own, in addition to LFT-8b's and Magneplanar Tympani T-IVa's) still the gold standard in that regard. But the ET's play a lot louder!

realdeal, you're embarrassing yourself.
I think he is and I'm not sure what he means by "your poor design"?    

realdaeal, I don't understand this either  

in OTL design manufacturer don't mention about the specs about the tubes they used which is the most important in OTL designin,.transconductance of the tube is the sweetness of an OTL amplifier.they only built since the parts are cheap.

The dynamics of transformers are very good. Just think how many transformers are in the recording chain from the one in the MIC to the final output.


bdp24     I consider the old Quad (which I own, in addition to LFT-8b's
and Magneplanar Tympani T-IVa's) still the gold standard in that regard.
But the ET's play a lot louder!  
I agree the 57s are the gold standard for midrange due to the virtually massless diapragm and excellent step up transformer. I used them to voice my ESL. 
Post removed 
I tried to understand the OP, but I'm not getting it or something. How does a transformer only act on 180 degrees of a wave form? That doesn't make sense. And if the transformers is making the load easier to drive, how is it not taking away the control the amplifier has over the load? I get it with the shrinking class A envelope, but all that really does is drive a class A amp into class AB with a little lump of distortion at the envelope edge. And isn't a transformer really just feedback in the form of degeneration?

Although at first read is appears that kosst_amojan is indeed confused but as I am considering his points his answer does beg some clarification on my part. Along with some assumptions and further questions. So here we go. Thanks for your reply and I hope this clarifices this for others.

The transformer acts on the entire signal not just 180 degrees. What I was saying is that in a push pull amplifier there is one half of the primary used for the positive going signal and the other half used for the negative going signal. Making those equal is difficult above 45 KHz where feedback can cause ringing on one half and not the other half of a square wave. In an Autoformer the entire primary is used all the time.

In the application of making a speaker easier to drive we use the transformer in a step down configuration, the step up would be a rare but still valid usage. Stepping down the voltage by a factor of two doubles the current and current is what is lacking in all OTL amps. There is plenty of voltage. Although this discussion is based around impedance I would like readers to consider voltage and current. Impedance is just a simple way to combine the two into one term. 

By presenting a higher impedance to the amplifier the amplifier has MORE control over the speaker, control=damping. 

The  Aurtoformer actually extends the class A region when used to increase the impedance as in this discussion. Similarly going to a lower tap on a tube amp extends the class A region and can reduce the distortion by a factor of 5-10.

A transformer is not a form of feedback, Perhaps you are talking about what ARC and some others do when they use the secondary for cathode degeneration. I tried that in the early RM-200 and later found it was not so useful. 


Roger, It’s great to have you contributing here on AudiogoN. I really miss your AudioCircle Forum! We met twice when you came to Brooks Berdan’s shop in Monrovia to give talks. The first time, I was the long-hair who asked you why you recommended a single RM-9 for stacked Quads rather than a pair of RM-10’s. You told me you considered the headroom afforded by the RM-9 to be of great significance and benefit. I asked if the load of paired quads on each channel of an RM-9---whether run in series or parallel---would not be of more concern than headroom? And if wouldn’t a single Quad on each of four RM-10 channels would not be preferable? The second time you told me you had been reconsidering my question, and had changed your mind. :-)

Side note: I know very few musicians who are audiophiles (most are far too poor, for one thing). The one I do is, as am I, a drummer (he’s currently working with guitar-great Albert Lee), and his speakers are---stacked Quads! Maggies are also great at reproducing drums, but they don’t have the liquid, see-through transparency of ESL’s. Roger, I’m dying to hear your direct-drive ESL’s; maybe on my next trip to the Bay Area.

ATENTION ALL TUBE AMP OWNERS! I just spent 90 minutes watching and listening to a video of Roger giving a lecture at the 2015 Burning Amp Festival. That 90 minutes is the best investment in time (something of increasing value to me as I approach the end of the line ;-) I have EVER made. Roger covers many aspects of tube amplifier design in deep detail, then takes questions from the attendees, some of them audiophiles, but many amateur and professional tube amp designers, one of whom appears (off screen) to be Mike Sanders of Quicksilver. If you own a tube amp, or are considering getting one, you NEED to watch and listen to the lecture too.

I’m one of those idiots who doesn’t know how to attach websites, so here’s how to see the video:

1- Go to "berekelyhifischool.com". Along the top of the website you will see "COURSES", "CLASS NOTES", etc. Go over to "RESOURCES" and a drop-down menu will appear. The first of three choices will be "VIDEOS". Click on it, and you will be taken to a new page. On the right hand side of the page will be "ARCHIVES". Find October 2015 and click on it. The video will start.

2- I suppose you could also find the video by doing a search on You Tube, but you may as well look at the rest of the Berkeley Hi Fi School site too!

I'm building an active system where the midrange will be driven by a 2A3 SET. SETs like constant, high impedance. And need high efficiency too. Many high end midrange drivers have relatively flat yet low impedance. Would an autoformer be a good solution?

If I place an autoformer between a 2A3 and a 4 ohm 90 dB/2.83V midrange such that the amp sees a 16 ohm load, would that be a good solution? How would efficiency behave? Any detrimental side effects to be expected?

09-01-2018 3:38pm

I'm building an active system where the midrange will be driven by a 2A3 SET. SETs like constant, high impedance. And need high efficiency too. Many high end midrange drivers have relatively flat yet low impedance. Would an autoformer be a good solution?

I would hope your 2A3 amplifier has a measured tap that is equal to the impedance of the driver you select. Low power ampifiers need this more than ever at 4 and 8 ohms. 

The popular Zero is just too much iron for a 2A3 to deal with. You would be hooking up a 200 watt transformer to a 2.5 watt amp. I do have a nice small autoformer that I made for my two tube OTL that would do the job nicely. Very low loss, easy to drive, 4 taps to match any driver.



To Lewinski and others thinking about multi amping please consider this.

The crossover in a typical two or three way speaker is entirely responsible for widely varying impedance, distortion and is just a bad idea in a high end system. Its fine if you need a speaker you can carry around and hook up anywhere, but we aren't doing that in our stationary home systems. We have the option of doing something much better.

Dynamic drivers have very flat impedance over their useful range. I know and hope I will get some disagreement on this, so go ahead.

For all the money people invest in expensive speakers and amplifiers there is a much better solution. If you don't care to make your own speakers then ask the maker whose speaker you like to "hold the crossover and give me direct terminals to the drivers".

Then get a good active crossover or have one made. Many of us offer them. Choose some amps approiate for those drivers like Lewinski is doing. You will have a great system, no impedance problems, no crossover saturation or the EQ tricks that are going on in most of them to obtain good frequency response at the expense of flat impedance.

I would hope that many of the readers here are tired of searching for the right amp for their speaker. I think that's a fools errand because the information is hard to find, some amplifier makers don't tell the truth, and most speaker makers assume you have a big SS amp with infinite damping, 60 amps of current just like they do. 

Here are a few examples of what you can do.

Drive your woofer with some inexpensive big SS amp, don't sweat the midrange, there wont be any here.

Drive the high efficiency midrange with a small tube of class A SS amp. Small amplifiers always sound better than big amps used at low power. 

Drive the tweeter similarly or use the midrange amp with a simple 6dB per octave passive crossover consisting of a choke and cap. Properly done these will not affect the drivers of the load. At these frequencies both the choke and cap are small, easy to obtain and have vanishingly small losses. Such a crossover will sum perfectly.

If you are into ESLs, DIrect drive is the way to go. 5,000 volts at 1/4 amp is 1250 VA and equivalent to a 1250  watt SS amp which is what many ESLs need.
Roger - 
Would your "light loading" recommendation apply to all transformer based tube amplifiers, or just to your designs? I have read many comments stating that the higher output impedance taps allow for better performance assuming compatibility with a given speaker. I have a pair of DeHavilland 50A monoblocks which have only one set of binding posts which are connected to the 8 ohm transformer winding. One must open the amp and rewire to access the 4 ohm winding. I assumed that was because the designer felt that the 8 ohm tap was preferable sonically (or perhaps it was because it allowed for the greatest power output as you pointed out).
Roger, thank you for joining this community! It’s exciting to have one of the preeminent HEA tube amplifier minds with us. I hope you find this venue interesting and engaging enough to stick around.

On a personal level, I spent an afternoon a while back with a friend who owns one of the better shops in the country. To provide a bit of acoustic wallpaper, he drove the Vandersteen 2 with your EL84 based amplifier. Dick Dale’s surf guitar sounded about as good as I’ve ever heard, which proved one not need to spend the crazy dollars so many believe HEA requires to get superlative sound and find bliss
georgehifi
I totally agree with Roger’s summation for autoformers, they are an "answer looking for a problem."


atmasphere
I’ve examined these two statements for a while and they seem to be at odds. George, I think you missed Roger’s point.


Your obviously not reading all his statements, eg: below.

ramtubes
"An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier."
Roger, your comments about speaker crossovers is very interesting.  Passive crossovers present another issue:  phase coherence problems.  I tried to deal with the issue of phase coherence caused by higher order crossovers with a DEQX device.  I inserted the DEQX between my linestage and amp.  While the tonality seemed better, my system lost a unit or two of blood (metaphorically speaking), … something went missing. The DEQX twisted the signal to be phase coherent but took something away.  I wound up taking the DEQX out of the signal path and decided to live with the phase issues but at least the music regained its former sparkle. 

That said, speaker phase coherence is just one problem that only a few manufacturers have attempted to tackle (e.g., Vandersteen).  As you said, the issues of speaker impedance and phase angle rock and roll curves are still there, … all courtesy of passive crossovers.  However, as a practical problem, I surmise that 95 plus percent of the speakers in so called hi-end systems (and the marketplace in general) use passive crossovers.  

BTW, I just switched the amp taps again.  Running with the 4 ohm taps now.  

BIF.  

Roger - echoing the appreciation for joining this community and hopefully you will see value and continue.
i worked atProgressive Audio and we carried the Bev 2, magic when they worked !!!!
Eric @bdp24 - thanks for the reference to the videos and the school...might be up my ally, always want to learn....
@trelja as you know RV tries to design for a benign load...but yes, magic can happen for not much $$$$
and finall RV makes a purpose built amp for the 7, he calls it cheating.....
I inserted the DEQX between my linestage and amp. While the tonality seemed better, my system lost a unit or two of blood (metaphorically speaking), … something went missing. The DEQX twisted the signal to be phase coherent but took something away. I wound up taking the DEQX out of the signal path and decided to live with the phase issues but at least the music regained its former sparkle.
When you insert a Deqx, your going from D to A the A to D then back to D to A, and you hear the final dac which is the Deqx dac and not your pride and joy you started with, no wonder we all say somethings not right, even though it should be, TO MUCH PROCESSING!.

Cheers George 
"An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier."
@georgehifi That quote is taken out of context but despite that it still appears to be that you have missed the point. I don't have any beef with this quote- I agree with it. What about an amplifier that is well designed? Does an 'autoformer or any transformer' help it with difficult loads? The answer is 'yes'.

bifwynne
2,044 posts
09-03-2018 5:43pm
Roger, your comments about speaker crossovers is very interesting. Passive crossovers present another issue: phase coherence problems. I tried to deal with the issue of phase coherence caused by higher order crossovers with a DEQX device. I inserted the DEQX between my linestage and amp. While the tonality seemed better, my system lost a unit or two of blood (metaphorically speaking), … something went missing. The DEQX twisted the signal to be phase coherent but took something away. I wound up taking the DEQX out of the signal path and decided to live with the phase issues but at least the music regained its former sparkle.

I have my doubts about digital crossovers and have no use for them. I was speaking of analog crossovers or simple RC filters that can easily be added inside the amp or in a little box. 

tomic6011,278 posts09-03-2018 7:46pm
Roger - echoing the appreciation for joining this community and hopefully you will see value and continue.
i worked atProgressive Audio and we carried the Bev 2, magic when they worked !!!!


The model 2 and 2sw were great speakers and  the beginning of High End audio. I fixed the reliabity problem in the amplifiers and did the electronics for the 2SW. However the woofer itself was rather poor quality, had mass added, leaky cabinet, not a great woofer for such a nice speaker.

I enjoyed being Chief Engineer under Bev and we had a lot of fun there.

"An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier." 

What I am saying here is that a difficult load stays difficult. The transformer just  puts less stress on the amplifier but at the expense of less power available. 

An analogy might be the transmission in a car. When going up a steep hill (difficult load) we downshift to take the load off the engine but cannot go as fast. 

As to stability we note if we don't downshift the car sometimes shutters, engine knocks. That is like instability, which down shifting also cures. 

A transformer is very much like a transmission in that the proper gear ratio gives the most power for the given situation and is kind to the engine.
An analogy might be the transmission in a car. When going up a steep hill (difficult load) we downshift to take the load off the engine but cannot go as fast.

As to stability we note if we don't downshift the car sometimes shutters, engine knocks. That is like instability, which down shifting also cures.

A transformer is very much like a transmission in that the proper gear ratio gives the most power for the given situation and is kind to the engine.

Should have just bought an automatic car to begin with. (The right amp to do the job)

Cheers George

The situation with component systems is that you have to buy the engine (the power amp) and the car (the loudspeaker) separately. The transformer taps allow the same engine (amp) to be used with different cars (speakers).

Audiophiles have historically resisted powered speakers, right? They should reconsider in the case of Music Reference, whose Roger Modjeski is offering his own ESL speaker with a dedicated, direct drive tube power amp. No ESL transformer, no power amp transformer! And no speaker cables!!

Roger Modjeski is offering his own ESL speaker with a dedicated, direct drive tube power amp. No ESL transformer, no power amp transformer! And no speaker cables!!

One of the nicest sounding speakers I owned, absolutely hated humid days.

http://www.audioservicemanuals.com/a/Acoustat/Acoustat-X/Acoustat-X-Brochure-1.jpg

Cheers George
Do they allow for aftermarket fuses?
George, Roger happened to mention the Acoustat speakers in his 90 minute presentation at the 2015 Burning Amp Festival, talking about why the Acoustat amp was discontinued in favour of the amplifier interface. Roger actually makes a direct-drive amp for the Acoustats, which can be used in place of their interfaces. The Acoustat speakers were fine, their electronics not so much.
Should have just bought an automatic car to begin with. (The right amp to do the job)

But automatic transmissions still have gears. If there is a right amp for every speaker then does one have to get a new one with every speaker change?

I've designed all my amplifiers to be as universal as possible to sound and measure well with a wide variety of speakers. Music Reference owners tend to keep their amps for a long time, you don't see them often and there are many cases of Sellers Remorse.

I do this by reasonable choices of damping factor around 10, distortion below 1% at full power and typically 0.1% at listening levels. The RM-9 and RM-200 provide ample current, and are easy on the tubes. A resaonable amount of feedback is used to achieve these things. I don't care to use so much as to get the distortion down to 0.001% or the damping up to 100. Those will result in an unstable amplifier.

I would like to say one important thing about damping. Paul Klipsch said many times "Who cares if the amplifier is 0.1 ohm or 0.001 ohm output impedance. The woofer is a 6 ohm resistor, so how much do you think you are going to damp that". This thought of the amplifier having a grip on the woofer is another misconception so lets look at another way 

The following is intended to dispel some of the misconceptions on how dynamic speakers work. You may find it hard to accept.

Certainly a low damping amplifier is going to produce boomy bass in many speakers though not in all. The extent to which it does this is determined by the impedance peak at the bottom end. One peak for sealed boxes and two peaks plus a rather deep dip between the peaks in a ported speaker. What happens at these peaks. To the extent that the impedance rises to 20 or 30 or 50 ohms the voltage of a low damping amplifier will rise as much at 6-10 dB, a very noticable amount and a very narrow peak. It is this rise in voltage and rise in efficency at resonance that produces the peak.not that the woofer is out of control. It is perfectly in control and resonates because the air load on the cone now equals the mass load of the cone. If one designs a woofer without a impedance peak then damping doesn't matter in the bass. 

Here is the impedace curve of a well designed driver from Fostex. Note how flat the impedance curve is; only 8 to 10 ohms over the usable range.Of course the bass peak has to be handled by the proper enclosure. These curves are open baffle. Also note the 90 dB efficiency!

https://www.madisoundspeakerstore.com/approx-4-fullrange/fostex-fe108ez-4-full-range-sigma-series/

Now what happens in the rest of the range? The other drivers, if operated in their range of comfort, will present constant impedance in that range.Once you accept the fact that all dynamic drivers are mass loaded devices you will understand that. The impedance peaks and dips in a mulitway speaker are entirely due to driver overlap, driver underlap, and components added to voice the speaker. 

Here is a nice tweeter  https://www.madisoundspeakerstore.com/soft-dome-tweeters-peerless-vifa/peerless-dx20bf00-08-3/4-text...

One might now see that using a woofer down to its resonance is not such a good idea as that is beyond its usable range. We don't do that with  midranges or tweeters.




But automatic transmissions still have gears.


Yes but at least with an automatic you’ll won't get: 
" When going up a steep hill (difficult load) the car sometimes shutters, engine knocks."

Cheers George
Roger has done a drop in board modification for the Acoustat direct drive amp. The main upgrade is the elimination of the solid state input (opamp) in favor of a tube input using 3 x 6DJ8/6922 tubes. I use am using the Acoustat direct drives on my Model 2's right now. The only thing the interface boxes are doing is holding up the speaker. I hope to either get the drop in board mod or a set of Roger's direct drive amps at some point.
Guess who getting off on music, to his Acoustat 3’s?

http://4.bp.blogspot.com/_NVQujPOPx2c/TSUhBAtkNeI/AAAAAAAATmg/RIIOAT-VuFc/s1600/steve-jobs-stereo.jp...

I went through nearly all the Acoustat’s, the best for me were the 2+2’s with Kef B1814 (fs18hz) bass drivers for 80hz down in 14cu ft boxes and my MP-02 Plasmas’s from 10kHz up all passive xovers driven by my 120w water cooled pure class-a S/S beast of an amp two man struggle to pick up. 24 Hirel EB or ED 203/4’s output Devices per channel.

Cheers George
clio09
The main upgrade is the elimination of the solid state input (opamp)
I can’t remember as I never fiddled with mine as I soon upgraded to the two+two's, but knowing the vintage, it was probably tlo72/74 about the only thing around at the time, not a very good sounding opamp.

Cheers George
I use a lot of opamps in test equipment I make but not in audio except for a servo. Too may transistors in there.

I have a pair of modified Acoustat amps available with an all tube driver stage. No transistors anywhere in the signal path. I am also making new amps from scratch of my own all tube design.
A friend of mine had powered Acoustats; he got frustrated with them and handed the amps off to me wherein I renovated it and installed a tube input stage. It worked pretty well- the original opamp circuit just didn't seem up to the task. Very nice speaker; the biggest problem was if you wanted more power or the like, you had to replace the whole thing.
I don't know a thing about autoformers. I only post to say Roger Modjeski/Music Reference is the real deal...I would tend to believe whatever he posts here, based on past experience.

In the last 1980s I had 2 X RM-9s. If memory serves, I bought each one gently used. I used one of them for the mids/upper sections of a pair of Vandersteen 4s. The subs were powered by a 200 wpc SS amp, and the crossover duties were handled by Vandersteens electronic crossover.

The RM-9 w/EL34s had a really lovely sound. 100wpc output was more than sufficient for my needs. That was an amazing sounding system. Still, I tried to make it better: I replaced the EL34s w/KT88s, resulting in a little more power and a tad more guts in the low mids (though perhaps a tad less refinement vs EL34s. 

Then I somehow learned that Roger would perform "triode mods" on the RM-9. This consisted on adding a switch for each output tube socket, permitting it to be switched to triode at the cost of 1/2 power. I had this done, switched all those sockets to triode, and went straight to heaven. That made a great-sounding amp into something really special, and the Vandy's fully conveyed that triode magic. 

I sent my 2nd RM-9 to Roger for the same mods. When I got it back, I planned to use both RM-9s, each bridged for mono output--but life intervened and it never happened. Both RM-9s have been in storage since 1991, along w/the Vandersteens and other things. My little "time capsule."

Roger makes terrific amplifiers and really knows his stuff...
Great post desktopguy. I was lucky in having a good hi-fi retailer in my area (Los Angeles) who was an enthusiastic Music Reference dealer. He also sold Jadis, VTL, and other much higher-priced amplifiers, for those who were as interested in bragging rights as in good sound. I need no such confirmation, and my RM-200 Mk.2 is the last amp I'll ever need. I might eventually pick up an RM-10 for my Quad ESL's, the best amp in the world for that speaker.
I might eventually pick up an RM-10 for my Quad ESL's, the best amp in the world for that speaker.

Thanks for the compliment. The RM-10 is perfect for the Quads and you don't need the voltage limiter boards because it puts out just the right voltage. The damping is good so you will get the true response of the speaker. Perhaps this combination is so popular is because the Quads were my living room speaker when I developed the amplifier. Though I didn't do any particular things for the Quad and it ran the Vandersteens well too. 
ramtubes
The RM-10 is perfect for the Quads. The damping is good


What is the output impedance Roger?
Have you ever toyed with the idea of a variable feedback dial, as I found it very good for dialing in the tightness for different speakers with the amps I made.
EG Linn Isobarics prefers virtually no feedback as they are a very over-damped design, and too much feedback and they had no bass!!! from  B139's!!. .

Cheers George
Then I somehow learned that Roger would perform "triode mods" on the RM-9. This consisted on adding a switch for each output tube socket, permitting it to be switched to triode at the cost of 1/2 power. I had this done, switched all those sockets to triode, and went straight to heaven. That made a great-sounding amp into something really special, and the Vandy's fully conveyed that triode magic.

Triodes have inherently low output impedance which allows one to use little or no feedback. DIstortion is also lower. The industry only went to beam tubes and pentodes to get the most power, and it appears people still buy power as evidenced by the rediculously high power SS amps out there. There is no benefit in using an amp at 1% of its power level and many people are. 

Many people are suprised how loud they can play their speakers with the dimunitive RM-10 which I routinely pick up with one hand by grabboing the transformer cover. 

I will make triode amps all day long when people come to realize that many of them don't need 100 Watts. One really needs to look into that with something as simple as an AC voltmeter.