Does anyone care to ask an amplifier designer a technical question? My door is open.

 Preamps are much easier and to me not so interesting as power amps.

From the heart, I don't want to tease you out and set you up for some type of audio sparring, a form of website f-erism I despise. I respect you and am willing to do my best to listen/read with an open mind. But that said, you can't possibly mean that preamps are easier in general and I am unaware of any accolades for your preamps (whereas you are lauded for your amps). Obviously you mean that for your tastes and purposes, preamps are easier than amps. Few would argue that the audio world is full of great amps and that there are a dearth of great preamps. I have an opinion that is not subject to proof-that the easier a loudspeaker is to drive (in terms of both impedance curve and sensitivity) the more a great preamp's sonic attributes are appreciated. A great preamp does more than attenuate a signal. It breathes life into music. A great preamp is quietly powerful, while a great amp simply provides grunt. Once again, purely resorting to engineering and lab measurements is a sure way to fail when it comes to preamps. A great preamp needs a great PSU section, preferably outboard. Lab measurements don't explain why a great PSU is so important to the performance of a preamp. 

@fsonicsmith

Your own decision to go hybrid with your higher powered amp and to go true balanced with your higher powered amp but not your lowered power amp is a head-scratcher. If-as you state-monos are more susceptible to hum than stereo amps, why do you implement RCA-only vs. offering true balanced in the opposite direction? And while a solid state input stage may very well offer better measurements, where is the proof that it sounds better? At the end of the day, isn’t it indisputable that it is cheaper to produce and less complicated? You on the one hand have little good to say about Rogue and yet when it comes to hybrid tube amps, I think of Rogue (and Musical Fidelity though I don’t count their "tubes" as tubes).

I do not understand the first sentence at all. Where is the head-scratcher?

Sound, reliability, completeness of design, ease of service are all pretty equal in my mind. Sound is subjective. The other are objective and will bite you in the butt.

I think you misunderstand the RM-200 and RM-10 so lets get this straight. I dont like the term hybrid at all. In a good design we use the parts and circuits that do the job best. I wanted the RM-200 to have good CMRR (hum rejection in simple terms). One cannot do that with a tube at the input. One needs two active devices that are critically matched and stay matched. So I used a "supermatched pair". That is not a made up term but a technical term for a pair of transistors where each transistor is 100 transistors in parallel. I can trim the CMRR to 90 dB or better and it will stay that good for a very long time.

The RM-200 has many other special characteristics such as, fully balanced input to output, taps down to 1 ohm, reasonable damping, good power bandwidth. It also has something few amps do not have which is the abilty to drive a dipping load with increased power rather than decreased power. Neither CJ, Rogue, ARC or anyone else I can think of has done that.

The RM-10 is a sweet little EL84 amp that appears to delight QUAD owners, can be switched to mono or stereo with a unique driver circuit that does not require gain matching or summing resistors like the Stereo 70 and others do. You can look this up as to how stereo tube amps are converted to mono. BTW they are not bridged.

For a small amp like the RM-10 I saw now need and have not had even one request for balanced. The RM-10 is unique in that is produces 35 watt from one pair of EL-34s where most produce 17.5 watts. I spoke about this application at Burning Amp 2018. Perhaps you might watch the video. http://berkeleyhifischool.com/having-fun-at-burning-amp-2018/

Monos are generally suseptible to hum because of power cord grounds. Not much else. I dont understand your point of "going in the opposite direction" The RM-10 is a very affordable amp. It is one of a very few tube amps that has no PCB. Its hand wired start to finish.

It seems to me that JA has a tendency to disagree with his own reviewer’s perceptions as to which output tap is best rather than criticizing the design of this amp and it’s output tap measurements per se.

As far as JA is concerned. The reviewer writes what he writes, JA measures and comments and sometimes wonders why the reviewer didn’t hear distortion or damping or whatever. He is trying to make sense of sometimes disparate opinions as I would. The reviewer does not see JAs measurements or comments till publication.

Read MIke Fremers lead in to the RM-200. To paraphrase, "I know I wont get caught with my pants down when JA measures this amp" In other words, he knows it will measure well. I am still amused at manufacturers like Cary where the marketing guy really got caught with his pants down.

I am far more interested in JA’s measurements than the fluff about this record and that CD sounded. The reviewers rarely tell us anyting about how much power they are using, what the speaker and amp interaction might be. IMO most of them listen around a watt. Should JA only measure up to a watt on a 100 watt amp where someone might really need 100 watts?

This may dismay you quite a bit from an engineering standpoint, but I take measurements of tube amps to be very analogous to measurements of DAC’s; the best measuring DAC’s don’t often sound the best.

This dismays me not at all. How does this analogy apply? A DAC drives no load at all. A speaker drives a host of loads and will sound different into each of them, with output impedance being the most obvious factor and distortion the next. How can you make such a statement?

Now if you think speakers should be matched with a particular amp then your are faced with the problem of changing both at the same time or hunting for another speaker that likes your amp or amp that likes your speaker.

I am proud to say that my amps sound damn good on a wide variety of speakers and that is what good engineering is all about.

Just to lay your mind at ease. Your ARC is not a bad amp at all. Im glad you like it. Its just not up to my standards.... or John’s as he warned in the end of his measurements.

What is intereresting is that current ARC power amps are not up to Bill Johnson’s standards. All of Bill’s amps were very low distortion, high damping. I dont particularly like the way he went about it but it is crystal clear that he valued those two parameters in all his amplifiers. The current designer has an entirely different set of values. I appreciate that he has gone to simplicity. Now all he needs to do is get the performance up a bit. What do we know about his background?

While I agree that not all things can be measured I am writing about things that can be measured and their effect can be understood.

May I ask since the RM-200 had both excellent sound according to Mike Fremmer and excellent measurements at half the price 33% more power, why did you not buy and RM-200? 

@prof 

I still love to "visit" electrostatics (the Quad ESL 57s being my favorite), for their unique qualities. But every time I listen to an electrostatic, of any make, I come away happy to have moved on to cone speakers.That includes every hybrid I’ve ever heard: The cones seem to add some body, but only within their frequency range. As the frequencies climb up to where they are handled by the panel, the sound character changes to my ears to that ghostly quality, so I am always aware of this discontinuity.

The term "visit electrostats", the 57 in particular is apt as I "visited" mine today. I am in the process of modifying a OTL amp to improve its "specs", cure offset drift and increase reliability and life of the tubes. I am using the 57 in that test because of its widely variant impedance and revealing nature. The differences are palpable. 

I do not like all ESLs. Beveridge made a great one but it is very hard to place. The 57 as it was called. is truly "Walker's little wonder".
I owned 63's and do not miss them. I like line sources. I like the wide dispersion and center fill that line sources produce. So perhaps its the line source I like the most. I have heard some large cone line sources and I would say they approach ESLs. But the drivers will never do what ESL drivers do. 

I have been around ESLs all my life and sometimes when I hear cone speakers I just want to run out of the room. As far as moving air a 12 inch cone has to go a lot farther than a 4-8 sq ft piece of 1/2 mill mylar that has virtually no mass. Be aware, a cone speaker is a "mass loaded driver" by its vary nature. This cannot be disputed. An ESL is an "air loaded driver" Id rather have air than mass to move around.

In closing, and not to be a tall poppy, but it takes knowledge of both acoustics and electronics to make an ESL. One has to make his own drivers, step up transformers, polarizing supplies and at voltages that scare most people. Cone drivers are bought off the shelf, the good ones have already published curves that you can count on, no high voltage and not much knowledge to hook them up. Red wire, Black wire, cap, choke nothing dangerous, nothing EXCITING. 

@mapman 

I figured it out by listening and trial and error over time and based essentially on those parameters as a guide. However specifications alone seldom tell the whole story. The devil is always in the details much of which is never specified. Detailed measurements like those in Stereophile help a lot but I have found there may still be surprises playing real music even with very comprehensive test measurements at ones finger tips, though if done correctly, measurements certainly help with the decision making process of what to try next or not.

Today I compared two identical amplifiers, level matched, A/B switch in hand. I played Willy Nelson (raspy male voices are very telling), I played Bach Organ works, piano, lots of things. Sometimes listeners bring in music to these tests that they like, but will not expose differences. 

All I had done is add 12 dB of feedback to one amp and left the other stock. i didnt change any caps, didnt replace resistors with naked ones, didnt put in premium wire or fuses.

Both had been measured and were in proper operating condition. 12 dB of feedback will reduce distortion by a factor of 4 and increase damping by a factor of 4. It was not hard to hear the clairty, dynamics, and defined (rather than one note) bass. 

This is the story that the specs told and the listening confirmed. Come on over and have a listen. Lets not worry about comprehensive measurements when even the basic ones arent even good. The stock amp has a damping factor of less than 1 and 4% distortion at full power and I played it at full power. 

These tests are not easy to do even for me. It can take a whole day to set it up, and ive only been doing it for 40 years. Very few people set up these tests.

Why do audiophiles try so hard to ignore, object to, claim that measurements are irrelevent? Why is one reviewers impression more important? Many readers of Stereophile dislike measurements so much they say its a waste of space?

I don't design by measurement but I know what minimum performance is required to make a good amplifier. Of course some designers loose sight of balance in design and drive one spec to the limit. That often results in a bad sounding amplifier. 

What are you going to say to the poor lad who buys the Cary SLI-100 hooks it up to a pair of Maggies, turns it up loud and smiles. If you have a good sense of clean sound you will not enjoy 10% distortion, compression, muddy bass. Without JA's measurements there would be no warning. His reviewer did not use more than a few watts of a 100 watt amplifier, what happens when someone does?

@unreceivedogma

 By “more power” I’m just going by where I set the volume control before and after. It was a surprise, as I thought there would be less, and I was pleased because in my layman’s view it meant less stress on the tubes and circuitry.

This is a popular thought, however where you set the volume control has nothing to do with power or stress on the tubes and circuitry. A volume control controls gain, not power. You an be at full power anywhere on the control setting depending on the gain of other parts of the system. 

Also, I’m not sure what you mean by better technology than cone speakers. First off, the Altec is a horn/cone hybrid. That aside, while electrostatics have their virtues, they don’t move air the way a cone does, as someone here also observed. That’s what I mean by the dynamic punch of the Altecs. I’ve heard some great horn speakers, with prices in the stratosphere.

The Altecs were designed to get a lot of SPL with not a lot of power. When they were designed 20 watts was a lot of power, now its not. I have not heard any good horn speakers, though one of my students has the Avantgarde 3 way. We are having a demo this coming week. Horn speakers tend to have peaks in the response. They are very efficient, however I am not running a movie theater.

Actually ESls couple better to the are and are virtually massless. A cone speaker is a "mass loaded driver" whose motion is totally controlled by the mass. The problem with mass, as we well know is how to start and stop it. 

Rather than defend cone speakers, lets open our thoughts to why they are so popular and do they belong in the home? Fine for cars and portable radios, but that is not HiFi.

There was a time when the best and most expensive speakers were ESLs. But then came along Wilson and the world changed. We made a $4,000 ESL with DD amplifier at Beveridge when wilson was still working in his garage. He won, we lost. What happened?