The power transformer has arrived and I can now get into part 2 of the AC Calibrator project.
Aim: 0-20vRMS directly out of the power amp 'LV' module for frequencies from 50Hz up to around 50KHz.
This will require a little over 60Vpp or a minimum +/- 30v rails for the power amp. As I intend to regulate the power rails for additional stability, I chose a power transformer with two 25v secondary windings. Rectified and filtered these will give about +/-36v DC.
The voltage regulator kit I used had LM317/LM337 chips. I had to change the 5K pots as they did not allow enough adjustment. I set the regulated outputs to +/- 31v DC. This allowed about 5v (36-31) for the regulators to work.
At lower frequencies (<20KHz) there was no problem generating 20vRMS with this setup. However, as frequency was increased, the amp output capacitor to ground (220nF) has lower and lower reactance and presents a heavier load. At around 48KHz the reactance is ~15 ohm and this, along with the 15 ohm output resistor, puts a load of about 30 ohms on the amp output.
The resulting current of 670mA (20v/30 ohms) is enough to drop the 317/337 pair out of regulation with significant ripple breaking through.
Looking for feedback here. How much leeway do I have to fiddle with the 15 ohm resistor and 220nF cap? Looking to a least halve the amp output loading at 50KHz.