They just have come to learn that you have to listen to an audio product to really know if it’s for you. People realize there is no better way to select these products.
Yes- heaven forbid that audio companies actually publish spec sheets that told you how the gear sounds! As a result the only way to know if something works for you is to play it at home.
@alexberger
He used classic schematics. He runs his amplifiers without feedback and with feedback. The question is why you and my friend got so different results? What is the difference of your schematics to classical EL84 PP schematics?
Do you use balance input "+" for input and input "-" for feedback? Did you publish your schematic?
I think the problem comes in when you combine single-ended and PP circuitry at the same time! Thru algebraic summing you wind up with a more prominent 5th harmonic. Seems to me Norman Crowhurst wrote about this in some of his tomes back in the late 1950s- so this shouldn’t be a revelation (although I’ve come to suspect many designers are ignorant of his work). Back then the solution was to add feedback, which causes the circuit to be brighter due to added higher ordered harmonics (so to me does not seem like a solution). Crowhurst also wrote about this too, so none of this is new.
What strikes me as odd is, when these things have been known for so long, why do people persist in making what to me seems the same mistakes, over and over?
I am suggesting that your friend possibly made this mistake also. IME if you really want to hear what PP can do, you can’t combine it with single ended circuits or you’ll shoot the project down before it gets off the ground.
You do one or the other, but not both!
I didn’t publish my circuit, (which is very simple; two stages of gain including the output section which is a simpler signal path than most SETs), but I did publish an in-depth description over on audioasylum.com in the DIYtube section- search on ’EL95 amp’. The description allows anyone to build the circuit if they have the know how to build amps from scratch. The only tricky bit is I used some very effective constant current sources for the differential voltage amp which is something you don’t usually see in tube circuits (and so leaves performance on the table). To this end the Voltage amp also ran on + and - Voltages of the same value.
You are correct- the audio input is the + input and the feedback is on the - input. In that way there would be no intermodulations introduced at the feedback node. I got such good linearity out of the circuit I didn’t need the feedback at all; it was only used to reduce gain. IMO/IME if you are applying smaller amounts of feedback (like 15dB) that is really the only way to do it.
