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

@prof

What a terrific thread! Thank you Roger!

I hope you can answer a couple questions that have long baffled me about the sound of some amplifiers - in this case, using my own as example.

Earlier in the thread, you mentioned how the impedance interaction with a speaker is a significant factor in how one amp might sound different than another, depending on the design.

I’ve always understood (or not understood?) this to mean that if the amps performance varies with impedance swings presented by the speaker, you will start to get contoured sound around those impedance swings - e.g. more or less bass here, more or less highs there etc. Like a tone control.

And if I have that right, and one is using an amp that is very susceptible to altering the signal based on the impedance character of a speaker, then the "tone control" variations ought to vary as you switch between speakers. So one speaker may sound fairly neutral (if it doesn’t present a tough impedance load with said amp), another may sound rolled off, another boosted in the bass, etc.

The thing is, in my subjective experience, the character of an amplifier (at least the ones I own) tends to seem more constant than that.


So take my Conrad Johnson Premier 12 tube monoblocks - for reference, measurements here:

https://www.stereophile.com/content/conrad-johnson-premier-twelve-monoblock-amplifier-measurements


I have had a ton of speakers of varying designs - Quad 63s, big Thiel speakers, transmission line bass, reflex, sealed, tough impedance curves, easy ones....

And I’ve tried numerous amps, including some solid state (Harmon Kardon, Bryston 4BST, and others), and the thing is, the "character" I hear when I implement the CJ amps seems to remain constant: a slightly more rounded sound, that classic bit of "golden glow" in the upper mids/high frequencies, rounded but taught bass, etc. I just don’t seem to hear the tonal variation I should think may happen.

An even worse example is my Eico HF81 14W integrated. I’m sure you know this infamous little "classic" amp. It sure didn’t seem to measure well:

https://www.stereophile.com/content/eico-hf-81-integrated-amplifier-measurements

And, if I understand the implications of those measurements at all, it would suggest the amp would be susceptible to sonic variation, as speaker loads vary.

And yet, the little guy just seems to "sound the same" no matter what speaker they drive: from easy to drive monitors I own, all the way to MBL radialstrahler 121 omnis (a brutal load), to even my big Thiel 3.7s. They always have that same sound - a sort of lush, thickened, sparkly not "dark" sound, with a swollen bottom end that makes any speaker sound "bigger." (My hunch had always been this was some "low damping factor" in the performance of the amp that gives it this character across speakers, but now you’ll slap me upside the head for that).

So my questions are:

1. Why might I be perceiving that the amps have the same essential character even when used across a wide array of varying speaker loads?

2. Even more confusing to me: We are often told that the distortion signature of an amp tends to give it a sound. But for the most part, this arises only when power is under high demand, clipping etc, correct? If so, why do the amps seem to retain their specific character no matter what volume I play them at?


I don’t tend to listen loud. Often topping at around 75 dB or so. As I understand it, at least when I’m not listening to music with dramatic dynamics, most amps should essentially be cruising in terms of distortions. So if they are not going in to the type of distortion that starts to distinguish amp distortion profiles, why do they still sound different even at lower volume levels? All those "tubey" characteristics seem to be there no matter the volume.

Now, of course a confounding variable here is listener bias. But, if I’m hearing what I think I hear, I’d appreciate your unraveling my confusion.
Thanks!

First I noticed that the conrad and the Eico both have similar damping factors. As I have said before that is the first thing we hear. Therefore they will modify the speaker frequency response in the same way. If you can get a low impedance amp then you can hear what the speaker designer intended you to hear.

Since you listen at such low levels both amps are playing no power at all and thus both have very little distortion.

Are there some yes/no questions I could answer. It hard for me to see exactly what you are getting at.

The Eico has the added problem of the tone controls which are hard to make flat.

One has to remember, the world of hi fi when the Eico was built is nothing like what we are dealing with today with your Conrad. Pre war the frequency range for good music was 80 Hz-8Khz. Post war it became 20-20K, thanks to DNT Williamson. 78’s were replaced with vinyl, Prerecorded tapes had a fair market and were excellent. Some record companies got serious about RIAA and getting things right.

The Eico is just on the cusp of all that. Heath, Fisher, Lafayette, Knight and Scott all became very hot. The progress made then far surpasses the progress in amplifiers we are making now, if we are making any at all. Now its alll about a pretty face isnt it.

I am blessed that my dad built a Heathkit preamp, amp and tuner when I was 4 years old. Mom says I watched all of it,though I cannot recall the images. The W2 is an excellent amp. Very open chassis, not expensive on eBay. Rather unknown. 

I mean what could be better than a Williamson.  :)

Speaking of member names, I particularly like wolf_garcia's, as Howlin' Wolf is big in my house. Ain't that a man?! Mine comes from my preferred drum finish and bass drum (kick drum, if you insist) size; bdp for black diamond pearl (not the current lame version, but the old stuff, which became unavailable after the 1973/4 oil crisis), 24 for 24" diameter (also Buddy Rich's preferred size).

Thanks Roger for offering to answer questions.
     I´m using a balanced configured Pass SS XP20 pre with output impedence stated at 1K/leg, balanced  and 200 ohm, single ended (according to manuel) driving  tube monoblocks with rated input resistance at 47K. 
     Amps have both RCA & XLR (only + phase) inputs. So my question is what input is preferable and why ?
     Your response will be greatly valued and appreciated.

Roger - What brand of speaker wires and interconnects do you use ?

If I am a good guesser I would have to say that since wolf_garcia plays guitar and he has mentioned the Grateful Dead in a few of his posts the reference would be to the Doug Irwin Wolf guitar (known for the Dire Wolf inlay) made for Jerry Garcia. I’ve seen Garcia play it a number of times and the current owner of the guitar has made it available to be played by others, most notably Steve Kimock who Garcia really respected as a guitarist. You can see it here:

https://liveforlivemusic.com/features/a-brief-history-of-jerry-garcias-five-most-well-known-guitars/

[Side Note: Upon Garcia’s death some of the Irwin guitars made for him were auctioned off to help pay for Doug Irwin’s medical expenses, and it got a little gnarly with the Grateful Dead asking for a piece of the action since they contended none of the equipment ever belonged to Garcia, but was purchased under the Grateful Dead corporate umbrella. In essence the guitars weren’t Garcia’s to give away. So the band got to keep a couple of the guitars in the legal settlement. So much for that community thing.]

As for the Dennis Had Inspire amp, I have a lot of respect for Dennis having owned and enjoyed very early Cary equipment, but knowing someone that measured one of these amps I couldn’t ever bring myself to buy one. If anyone wants to have their amp measured Roger is game to do it (even brought his equipment to Burning Amp one year to measure amps). He keeps notes on all the amps (and some preamps) he has measured. I think Roger has measured most of my equipment and even I was surprised at some of the results.