This is the first time in a long career about writing about audio gear that I have to, with such vigor, announce that I do not recommend buying the M-60 amplifiers. As you know I have very well defined and refined taste, and there many forms of audio technology and, tube gear in particular, that I do not like, because I have connoisseurs taste, because I have bat ears.

A number of years ago I liked the sound the M-60s…. while I was still in my hormonal stage of love, but as the relationship grew my dissatisfaction grew. New amplifiers came into my life, and when I compared them to the M-60s, the M-60s lost out. A battle then began between me and Ralph Karstin.

The reason I am reacting so strongly to this situation is that Ralph continued to use my favorable review for at least one year against my expressed wishes as part of his marketing campaign, and I am sure music lovers bought these amplifiers believing that I endorsed them, when I didn't, and would have recommend other amplifiers.

It took a phone call from my lawyer to convince Ralph to remove my reviews from his web site.

I feel a Karmic responsibility to all who mistakenly took my advice, and in previous articles that appeared on this site I was, according to Ralph, stepping over the line, so I have amended this article so that it solely reflects my personal artistic judgements, and not my personal feelings about Ralph.

In all fairness to Ralph, there are others audio writers, including Harry Pearson and Jonathan Valin who praise Atma-Sphere amplifiers, and that is what makes horse races.

I speculate that Ralph may be especially sensitive to my comments because I am the only audio writer who is an expert on OTL circuits, and my comments have more weight than audio writers who are looking for the golden amplifier of the month.

After my romance with the M-60s ended I began searching for ways to improve this circuit’s sound. I spent a great deal of time experimenting and discovered the reasons that I was personally dissatisfied, made some improvements to the circuit and shared that information with Ralph first and then the public. I will also point out that I have been "tweaking" audio circuits for over three decades, so this is a normal and well known gizmological activity for me. You will note this service has become a very valuable contribution I make to manufacturers. I will also point out that when I first heard an early version of the M-60s and strongly suggested that Ralph use a different mechanical/wiring layout, and a driver stage that used 6SN7s, that in due time, he adopted both of these suggestions.

About one year later my extended marriage to M-60s I had an affair with the Berning ZOTL circuits, and my marriage to the M-60s was over, end of story. I also preferred the music quality of the Sun Audio Amps and an $1,800 Golden Tube 300B amp. I reported that I preferred the sound of the Berning circuits in Positive Feedback Magazine. Ralph response was to personally attack me.

I have also been very aggressive in my writing, as you know, about the need for optimal amplifier/speaker interface. This is especially critical with low powered single-ended amplifiers, but it is even more critical with OTL amplifiers because they are unique among all tube amplifiers in that they are have both a high voltage and high current tube output stages. When a OTL amplifier is connected to a "normal" American speaker, which are usually around 4 ohms but then dips down to 1 ohm at low frequencies…or is 6 ohms and dips down to 2 ohms at low frequency, this burns up the OTL's output tubes because these tubes can not produce enough current. The other negative, which is basic is that, after twenty years experience with listening to OTL amplifiers, when I listen to them on "normal" speakers I wonder why anyone would want to listen to that mediocre sound. On the other hand listen to OTL amplifiers on the right speaker, which is rare in America, and you are in for a completely unique thrill, if you are using a refined circuit. This information obviously upset certain OTL manufacturers because my writing is getting in the way of their sales campaign

You will note that over the last two decades there have been a number companies that have created very high powered OTL amplifiers and almost none of them have survived, and that is because it is impossible to design a high powered and reliable OTL amplifier..... because the intention is to connect such an amplifier to an inefficient/low impedance speaker so that these necrophilic speakers will sound alive. I think that one hundred watts is the limit for a reliable OTL amplfieir.

The results of using a complex high powered OTL amplifier are usually disastrous. When the NYAL service department received a Futterman amplifier with a set of dead output tubes, and I called the customer and asked what kind of speakers they were using..."Acoustat Monitors"...which are 2 ohms speakers...my response was..." I told you not to use those speakers", and the voice at the other end always pleaded..." But the Futterman sounded so much better than everything else". Don’t use a Porsche to plow a field.


Please remember that I have been tweaking tube amplifiers for over three decades and I the nose of a tube hound dog so I have a good sense when I am unhappy, where the tweaking must be applied.

I approached these amps as if they were the Dynaco 70s of OTLs, which is understandable given their low price point. Because I was listening to high quality single-ended and push/pull directly heated triode coupled tube amplifiers to me, it was obvious, that in spite of the benefits of OTL topology, there was a big price being paid in terms of the musical aesthetics that mattered to me: the tone was crude and hard, and it had the midrange discombobularity I associated with poorly designed push/pull amplifiers...and there was a ragged quality to the sound.

And I must point out that I was using these amplifiers with an absolutely optimum pair of loudspeakers..my Tannoy Churchills, very high impedance/high efficiency .and I was using them with a tube crossover, so the masking effect of the passive crossover was completely eliminated. I ask for an give no mercy in my appraisal of audio circuits.

It is important to clarify these points:

Ralph’s claims that the M-60 is really two parallel single-ended amplifiers which, I believe is an attempt to convince the public that this circuit has the aural characteristics of single-ended circuits. Doesn’t sound that way to me.

I have a completely different point of view of tube circuits than Ralph, and personally believe that OTL amplifiers are the tools of ultimate refinement, and in their low/cost implementation they are not competition with transformer coupled amplifiers.


The 6AS7 Tube: The benefit in using this tube is that it is cheap, available from Russia, and because it is a regulator tube it has a low output impedance. That is the good news. The bad new is that it is a dual triode and it is a rare case that both triodes in each tube match. Measure one hundred and you will understand what I mean. This inherent mismatch is an impediment to sound quality. I strongly suggest you search the web for tube sites that deal with OTL amplifiers, even the Transcendent, because you will find almost universal criticism of this tube for use in OTL amplifiers.

Pause for a moment and consider this: when you are looking at an M-60 with its eight output tubes, you are really looking at a push pull amplifier with sixteen output tubes. When you look at an Atma-Sphere amplifier with 12 or 24 output tubes you are looking at either 24 or 48 output tubes, and this would be, in my opinion, the first time in the history of the audio arts that anyone has gotten that degree of complexity in output stage to work properly....for more than a few days or weeks.

Here’s the problem: in spite of the fact that the two triodes in each tube don’t match, there is no way you can’t adjust them to match....like you can in a conventional push/pull pentode amplifier. In the 1950s Julius rejected these tubes for that reason, and at New York Audio Labs, after testing this tube and realizing that this was an inherent problem we also rejected the tube. But there is no better down and dirty tube for an OTL output stage....I know...in the course of six months I went through about a dozen...because they were so out of match.

The Japanese response to this tube’s inherent mis-match problem is to have a separate bias control on each of the two triodes so that each triode can be made to match the other....which explains why they only use this tube in very simple circuits. This means that you would have to have two separate bias controls on each tube, and to achieve optimal symmetry in the M-60 there would be sixteen individual bias controls. You notice that all high quality pentode amplifiers have separate bias adjustment for each tube. In the M-60s there is one bias control...to match the aggregate of the left and right tubes..nothing could be easier, and less frightening to the consumer...in spite of the penalty in terms of sound quality.

Why do I mention this?...because it is, in my opinion, a key factors contributing to the aural discombobularity I hear in this circuit, and why it never approaches the subtly and refinement of simple one tube single-ended circuits, or the better transformer coupled amplifiers.

You will also note that I commented that the M-60 sounded better with less tubes, though its output impedance went up...it sounded less confused...because there were less out of balance tubes.

THE TONE OF 6AS7 TUBES: These tubes again are a great bargain, but do no have a good reputation in terms of tonal quality, for a good reason. They were designed as regulator tubes and they are great doing that job. If these tubes sounded great, because they are so cheap, other tube amplifier designers would use them in transformer coupled amplifiers. In my experience listening to the Berning ZOTL circuit in its pentode/push/pull and single-ended DHT models it was clear that DHTs have the tonal advantage and push/pull pentodes have the power advantage...but because David’s pentode/push/pull ZOTL only uses four output tubes as opposed to the sixteen triodes of the M-60s it had a much clearer and more open midrange. Because TONE IS KING I started to become annoyed by the M-60s. But again its tubes are cheap and available, and high tonal quality is not demanded by every music lover.


Yes, the good news is that OTL amplifiers can have the widest bandwidth and fastest rise time which means that they can be the most revealing type of tube amp circuit. The bad news is because of that ability they also reveal, more than any other circuit, the faults or crudity of the circuit’s implementation. High resolution demands refinement in circuit implementation or the opposite effect will be achieved....they will sound worse than transformer coupled amplifiers...and you are hearing that from the man who started the whole commercial OTL trend in 1980.

Ralph’s core circuit concept is a brilliant idea, but the implementation is done to create an amplifier at popular price point and the results to my ears is a crudeness, which is the core paradox of OTL circuits: they are so transparent that they immediately reveal any compromises. In other words, compromises that you can make and get away with in tube amplifiers with transformers don't make it with OTLs. This is his Ralph's choice and he is clearly looking to offer the most OTL power for the buck, and the price paid for this is a lack of harmonic and tonal refinement.

My writing about this OTL paradox is very upsetting to OTL manufacturers because their job is to sell as many OTL amps as possible, and they didn’t like me making their job harder. Let’s face it every manufacturer wants a great review and doesn’t want a truly critical review because...business is business...and let’s not confuse the public with facts.

My dissatisfaction began a process of modifying his amplifiers to eliminate the unacceptable tonal crudity which was due to:

LACK OF POWER SUPPLY FILTRATION: You have read numerous articles on my web site about this subject. I am not singling out the M-60s because this applies to any amplifier that has no power supply filtration. The combination of solid state diodes, computer grade capacitors, and absolutely no form of filtration makes the "grunge" we associate with these parts extremely apparent...more apparent than in transformer coupled circuit. The first thing I did was to buy a bunch of chokes for the output stage and input stage. Because this is a balanced circuit I needed two for the output and two for the input stage, and I also added oil caps to the four different power supplies. This caused a dramatic improvement in sound and tonal quality, and test confirmed what we all know...an OTL amplifier needs power supply filtration or all of the "power supply grunge" will infect the sound.

After my comments started to appear in print, Ralph started to use better quality hexfred rectifier diodes and even offered an input stage power supply mod that included chokes. This is an improvement in the right direction, but I suggest that you also need, at the very least Pi filters for the output stage too.

Because of the power supply compromises I decided to go all the way and connect regulated power supplies to the four M-60s I was using. The picture of this configuration can be found in the picture of Triodzilla on this web site. This was obviously an extremely serious attempt to discover the "better" soul of the M-60 circuit.

At this point I had moved all of my audio equipment into Ambient Recording Studio because I wanted to use only the finest quality input...my own master recordings to compare the sound of amplifiers, which brings me back to the beginning of this drama.

In our comparative listening tests that included me, Mark Conese, a wide variety of recording engineers and musicians, all prefer the sound of the two Berning ZOTL circuits, to the maxed out and best sounding M-60s with tube regulated power supplies.

It should be noted that stock Berning ZOTL are almost twice the price of the M-60s, but I was not comparing the Berning circuit to the stock M-60s which were much cruder sounding to my ears than the M-60s..even with the attached regulated power supplies, so I was giving the M-60s a big positive boost...and still it suffered in comparison.

At the beginning of this drama I suggested that Ralph should be debating with David Berning about circuit topology.

In closing let me comment on the rave review that Ralph got for his $25,000 amplifiers in the Absolute Sound. For 20 years I have asserted that big high powered transformer couple push/pull pentode amplifiers are a wrong headed solution to a bad idea...but perhaps the only choice for those who use dysfunctional speakers designed for transistor amplifiers, which are low impedance/efficiency. These " tube tanks" certainly sound better than "transistor tanks". And it doesn’t surprise me that HP prefers the sound of Ralph’s mega-OTLs to his once favorite "tube tanks". In the world of wretched excess, a wretched excessive OTL should be preferred to a wretchedly excessive transformer coupled amplifier.

This is America, and nothing excites most men more than wretched excess, but this is not the path of The Triode Guild, and why Ralph’s work no longer impresses me.


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