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In Reply to: AKSA output/gain stage - power supply separation posted by harvardian on November 6, 2001 at 07:43:26:
Hi Dale, Wim, Russell,Difficult question for me because I strive so hard for simplicity. If you choose to completely isolate the diff pair and voltage amp supply from the output stage - a GOOD thing - you blow out the supporting infrastructure hugely. However, let's explore the options.
The voltage amplifier is bootstrapped, and able to generate large voltages, momentarily, around 25% more than the rail itself. As the output goes positive, it routinely pulls the junction of R5/R6 beyond the rail, in fact at anything beyond 18V peak. Thus running these two stages at three or four volts lower than the rail is feasible; but you must choose a voltage reference for the regulator which is just below the minimum voltage of the rail at full output, less the dropout voltage plus half a volt for insurance. This is very constraining. Of course, the diff pair runs at very low line levels; so cutting its voltage supply merely dictates we re-dimension the feed resistor (R3).
If the output drops from say 37V to 32V at full output, the dropout voltage of a LM317 is around 2.5V, and we make it 3 for good measure, the chosen regulator voltage is 29V, a long way off 36V.
I suspect a better way would be to purchase a 10VA EI transformer rated at 15-0-15, series connect the secondaries, rectify (with ultrafast rectifiers, of course), and pass the resulting 42 volts to a LM317 set to 36 volts - essentially the maximum rating. Have it pass around 10mA into a dummy load, so that peturbations in the voltage amp current draw have less effect. Use 470uF smoothing after the rectifier, 47uF after the regulator, with a film cap bypass.
After all this, I would suggest the sonic improvements would be marginal at best. I admit haven't tried it, but the existing regime works extremely well (I have tried everything here short of attaching the prototype to my car and driving main street for a couple hours!), and is simple. You can try a BAT46 Schottky for D1, but the difference is very subtle. A Black Gate for C3 is helpful, too, but once again, marginal.
Hope this helps!
Cheers,
Hugh
www.printedelectronics.com
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Follow Ups
- Re: AKSA output/gain stage - power supply separation - AKSA 13:42:47 11/06/01 (0)
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