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

@rego Where do I begin when considering an Amplifiers Characteristics?

The first thing people are hearing, whether they believe it or not, is the difference in amplifier output impedance that will affect some speakers more than others. An amplifier with an output impedance greater than 1/10 the tap impedance will start to modify the frequency response of a speaker. The frequency response modification will simply follow the impedance curve of the speaker. 

There are some tube amps out there, that people love, that have an output impedance greater than the tap impedance and those will make very obvious changes in frequency response. The bass will always have a peak, the mids may have a dip, the highs may be accentuated or reduced, all dependent on the impedance curve. 

I purposely did not use the term "damping" because that term implies controlling the woofer which is actually not what is going on. However 1/10 the tap impedance or 1/10 the speaker impedance is a damping factor of 10.  I firmly believe that we should retire the term damping factor or get a better understanding of what is really going on. One has to remember that an 8 ohms speaker has typically 6 ohms of DC wire resistance in the voice coil. This makes the output impedance rather unimportant.

I expect some flack on this. So just be kind and logical and I will blow your mind about damping.

@bdp24 

Roger, I assume the balanced/XLR-only input on your RM-200 is accomplished with a transformer. Do you wire it in accordance with AES File 48 (pin 1: ground, pin 2: non-inverted signal, pin 3: inverted signal)? Thanks---Eric.

Actually it does not though I do use th AES standard for the pins. 

The balanced input is a differential pair with feedback right to the input so that the characteristics of the amplfier are determined largely by 4 high quality resistors.. and of course the rest of tube magic.

The CMRR is very high at 90 db and the amps works and sounds the same if driven unbalanced (single ended)

@terry9  I build solid state amplifiers, and like to match output transistors. At first I matched for HFE, then VBE at constant current similar to operating spec. Only later did I realize how much this parameter drifts over the first hour of warmup.

However, my latest amps are Class A, and I suspect that a more realistic match is obtained by culling outliers by HFE, then match from VBE using the bias at constant potential and sufficient to generate the operating current. Finally, instead of using matched emitter resistors, I use emitter resistors tailored to the output devices, so that each emitter resistor sees the same potential drop.
Your thoughts? Any advice appreciated.

Kudos to you for building your own gear. I have often said that the best amplifier is the one someone builds for himself. It brings along with it the joy that it is your creation not someone elses.

Of course you match HFE at operating current?

What is the spread of emitter resistor values? That part concerns me a little.

How many transistors in parallel?

Yes transistors are very temperature sensitive. VBE varies at -2 mV per degree C. Thus a 40 degree rise will be 80 MV and that is a lot to deal with. Do you have a VBE multiplier in the bias circuit?

Mr Ramtubes, I'm looking for some information about a stereo receiver I found in my late dad's basement. It's a "Thomas America's sound of Music"  I can't find anything about it on the web. When I came across it, it was buried behind a bunch of stuff with other stereo equipment pilled on top of it. I was able to lean in close enough to get a couple of pics with my phone so, unfortunately I don't have any other info (i.e. model#, serial #) at this time. Any info would be much appreciated. 

@rocknss
 Curious on how you have trained your ears.

Time, attention to distortion, frequency response variations, lots of A/B testing... lots of that. I set up one today to use a QUAD 57 to compare a stock with a modified amplifier that has come to interest me and may become a product.

One experience I would share is a listening session with 5 rather proud golden eared audiophiles and myself. They were all lined up on the couch in pretty good position to hear the system. I was 90 degrees off axis having some wine and cheese. I heard a horrible rendition of a stand up bass.

I first said, thats the worst bass fiddle I have ever heard, is this some horrible recording, no its a well respected recording. Then I asked, "what do your guys hear" They said audiophile things like, poor imaging, no depth, yada yada yada. I said, What i hear is a lot of distortion, like 30%.  The source was a tube modified OPPO player. I said, hey got some more 6SN7s around? So new tubes made everything fine. The host, a good friend, quickly brought up his tube tester and we found the old tubes were down to 20% emission.

The point of this story is that audiophiles rarely hear distortion as distortion. That word is not popular. They were listening to and judging a system that was simply broken. So the first thing people need to know is when their system is broken. I think many systems are.

In closing I think time, confidence (which is hard to get depending on who you hang with), and the Harman link are all good things. I know that Harman has developed some very good listening procedures and selects their listeners carefully. The qualification process to become one of their listeners is long and many people are eliminated. What Harman wants to get out of all this is to have listeners tell them if they are going in the right direction with changes in speakers and other components. Of course they use measurements also. Any sane person would.