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

Thanks for all you do as it is greatly appreciated! As a DIY tube enthusiast I would love to hear your suggestions on a good tube amp kit to build. Looking for at least 20 - 30 watts per channel. Can you point me to a place where such a kit is available for sale or at least a parts list and general instructions? A kit you would approve of in terms of sound quality and design. It would be great if you offered such at kit? Perhaps you do, but I missed it? 

Thank you Roger. Had many techs look at my issue. Isolation transformer had a 3 prong plug. It was a Torus unit. I ran the system single ended and balanced, floated the ground on all and only some units and used a structured process of elimination, had them all connected to one Shunyata power bar therefore only one outlet used, connected them to a complete different circuit in the house etc. Interestingly, the hum sounded different than a ground loop, it was loud as in you could hear it 30 feet away. Like the noise when you plug an RCA cable into a powered up subwoofer. A loud buzz. My new amplifier is a stereo unit. When I connected the monoblock amplifiers to the speakers and powered them up they were silent, when I plugged a balanced or RCA cable to either one the hum/buzz started even if the preamp was off! Never have a had this problem in 30 years of high end audio.... the only thing that made me post here was how the VP of this company spoke down to me when I explained my issue... never will I buy from them again. 

Wow! What a great thread! I’ve been reading through this all morning. The comments of others, and your answers, have brought up questions that I have, but had no reliable resource for answers! So, allow me to start with this: I own Magneplanars. I keep reading that tube amplifiers are a poor choice for these speakers. I like tubes though, and I have a tube preamp, and a hybrid power amp. The amp is a CJ EV2000, which uses 5751 tubes with a mosfet output to make 200 w/ch. I like the sound of my system, but it could be better. It seems to me that I don’t really need so much power, and I don’t need so much gain from a preamp. I rarely listen to music above 85 dB in my 21’x14’ living room. I’m considering buying a Schiit Saga passive/tube buffer “preamp” to replace my current preamp, and buying a quality tube amplifier. It is so difficult to compare things these days due to the fact that audio shops have all but disappeared. What are your thoughts about my situation in terms of “how much power” is appropriate, and “how will a quality tube amp drive my speakers”? Thank you for your insight!

Hi Roger, and thanks for offering to answer some questions for us.

In an earlier post, you offered to comment on specific brands, and I have a few in my stable.
-PS Audio
-McCormack (you already gave a nod to Steve)
-Symphonic Line (German)

As a follow-up on class D amps (and PS Audio), I recently got the PSA new Stellar S300 power amp, and would be interested in your input based on some of this information from their website.  The input stage is handled by their "Analog Cell", described as "a proprietary, fully differential, zero feedback, discrete, Class A MOSFET circuit."  The output stage is a "dual mono design.  [E]ach channel has its own, separate power supply. Each of the two channels is a complete and independent power supply and power amplification stage, based on a modern Class D ICE module, designed in Denmark."  Darren Myers is PSA's designer for this.

And here are the technical specs https://www.psaudio.com/products/stellar-s300-power-amplifier/#tab-specs

I like the sound of it, although interesting with no discernible harmonics.  I had never noticed harmonics in SS class A or A/B until I compared to this class D amp.

Thanks again for taking the time here!

I'm still in the Polypropylene camp. Best thing for the money. Remember any part I buy is 5 x cost to the final buyer. So I cant buy too many $50 caps expensive resistors.

For a long time polystyrene was an alternative that performed better and was no more expensive. Things fell apart though after the German company that supplied polystyrene got out of the business. It was easy to hear how the polystyrene was better- and this seemed to verify the specs on paper.

The whole argument about 2nd harmonic distortion being "benign" totally ignores Intermodulation distortion which is far worse and always higher.

While I agree with your point here (and that of the rest of the post from which this is taken) this particular statement is false. It is possible to have low IM while THD is considerably higher, although clearly that isn't the case with the example you cited.

"Well, it’s the sound that matters". If a poor design, showing obvious performance weaknesses, sounds "good", something is very wrong somewhere.

The problem here is that the bench specifications used by the industry do not reflect the physiology behind how the ear/brain system perceives sound. The most egregious example is how we perceive sound pressure, which is done via the higher ordered harmonics- 5th and above. The reason this is a problem is that to obtain really 'good' specs on paper, a fair amount of loop negative feedback has to be employed to suppress distortion. As Norman Crowhurst pointed out 60 years ago, feedback introduces distortion of its own and its entirely higher ordered harmonics. Because our ears convert all distortions into a tonality, this causes circuits employing feedback to be brighter and harsher than the original signal.

None of the above is controversial- we've know this stuff for decades. But the industry continues to do nothing about it, which means that there has been little progress. IMO/IME the harmonics produced by a given circuit should be weighted so that higher orders are given a greater weighting than lower orders (for example small amounts of a 7th are considerably more audible than a 2nd at the twice the level). IMD of course should be kept low.

Question: Tube Phono preamp. Would replacing the 1N4007 diodes (in bridge config) in power supply with HEXFREDs or Schottkys improve it? The bridge feeds a CRC filter which then feeds a tube regulator. Thanks.

If the power transformer is not properly snubbed, then you will get less noise. But if its snubbed properly there won't be any difference.

I read a report by JA measuring a Prima Luna tube amp (some years ago). He found an output impedance of 8 ohms! This means a DF of 1 ohm or less! Combined with the typical varying impedance of most speakers this is way too high! How can supposedly competent engineers get away with something like this? Because the result is far from neutral, accurate SQ! No matter how pleasing the "golden ear" crowd claims!

Because the ear converts distortion into tonality, just because you have flat frequency response does not mean it will sound flat. A small amount of higher ordered harmonic distortion can introduce brightness. This is an additional reason of why two amps can have the same frequency response yet sound different. IOW, its not just the output impedance, its also the distortion signature.