John Curl's Blowtorch preamplifier part III

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#13,362
simon7000 said:
10 mV would be -72 dB re 200 watts into a loudspeaker. Although it seems high that level would be lost when including the normal accuracy in voice coil centering. (Quick calculation predicts less than .001" of offset.)

But if the DC is trimmed to get that level, does it stay stable over temperature changes?

With normal professional amplifiers often having a voltage gain of 20 (26 dB.) that would require a preamp to be 500 microvolts or less. Probably much less as the audio power amplifier will have some contribution.

So where does that leave us, trimmer or servo? Which opamp?
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TL072 B grade is 3mV and over temp 5mV - admittedly with 50 Ohms source resistance. Nevertheless, its low enough to get good consistent results. I dial the offset out anyway because the servo opamp output feeds into the amp protection circuit. Measured on the output hot or cold the offset is never more than +- 1.5mV.
 
#13,366
Erno Borbely wrote up a project called DC 100 (i think) that would be good reading for folks interested in trying direct coupling

also, look for internal images of some Ayre power amp and preamp products to see some of the transistor coupling strategies that were helpful. probably have to spend a little time figuring out pieces of the schematic to really appreciate that however.
😉

mlloyd1

john curl said:
... The biggest problem of trying to direct couple amps or preamps is temperature drift. This can be a real engineering challenge ...
...
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#13,367
mlloyd1 said:
Erno Borbely wrote up a project called DC 100 (i think) that would be good reading for folks interested in trying direct coupling

also, look for internal images of some Ayre power amp and preamp products to see some of the transistor coupling strategies that were helpful. probably have to spend a little time figuring out pieces of the schematic to really appreciate that however.
😉

mlloyd1
Click to expand...

Erno's article was in Audio Amateur 2/84 part two the DC 100.
 
#13,371
simon7000 said:
As usual....

I am pretty sure audio predates controlled fire. Or did you mean the use of chopper amplifiers in instrumentation predates the use in audio circuitry? 😉

After using an external trim on an FET input opamp, any idea how much temperature will change things?
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Low enough to be of absolutely no concern in an audio application. On a TC or SG conditioner, a different story however.

You can safely count on 10uV/C Vos drift, but often much lower than this. Watch out for bias current in JFET input opamps with high source resistances though.
 
#13,376
john curl said:
Yes, Scott, I was designing DC servos when you were still in hi school. So what?
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Yes, so what, someone else was doing it before that. I'm confused though Mr. Marsh claims to have "invented " it in the 70's do you guys disagree?
I had at one time a mechanical chopper stabilized tube amp that predates both of you. I have no irons in the fire here, the "sound" of capacitors is unique to the audio industry, and has no not mattered much to anyone else (unless they could measure the result).
 
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