Alchemist Forseti APD21A Pre Amplifier PSU


2008-06-03 3:00 pm
does anyone know the output voltages of the transformer in the
Alchemist Forseti APD21A Pre Amplifier's Power supply unit?

Its a small torroid with two secondary outputs each with a 0v centre tap so its six wires in all. The output on the DC plug end is two +v and two -v with one 0v.

The manufacturer no longer has data on this transformer and its dead short so no volts out!

I think its about 25v 0v 25 each winding but need to be sure. The colour codes on the PSU are
2 reds 1 black winding one

2 whites 1 green winding two

I await your replies



2003-11-04 11:25 pm
Just found this thread after searching on Google for info on that preamp.
The design is weird, with dual-mono external power supply providing +/-32V, which is then regulated internally to +/- 18V, which is then regulated again to +/-12V by an adjustable regulator.
The actual circuit has 2 op-amps for the adjustable regulator, 1 op-amp for signal path, and another op-amp acting as DC-Servo. Output stage is using TIP-31C/32C biased into Class-A by the adjustable regulator.
Sounds confusing? it is !
Don't bother with balanced inputs/outputs, as they are converted into unbalanced by a crappy op-amp.


2008-06-03 3:00 pm
I have calmed down and since had a few of these overpriced and not really that good amplifier actually.

I did manage to make notes of all relevant voltages on a working one.

But I would never say no to a schematic of any amplifier.

Happy New Year All.