Alchemist Forseti APD21A Pre Amplifier PSU

Phil50 · 25th November 2010, 01:27 AM

Hi
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

thanks
Phil

Phil50 · 30th November 2010, 12:46 AM

Well as there is no help whatsover available on this lamentable product its now in the skip and is being replaced with a cheaper imported alternative.

I wont post again.
goodbye

yairf · 23rd January 2013, 05:07 PM

Hi
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.
Yair

25th November 2010, 01:27 AM	#1
Phil50 diyAudio Member Join Date: Jun 2008	Alchemist Forseti APD21A Pre Amplifier PSU Hi 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 thanks Phil

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30th November 2010, 12:46 AM	#2
Phil50 diyAudio Member Join Date: Jun 2008	Well as there is no help whatsover available on this lamentable product its now in the skip and is being replaced with a cheaper imported alternative. I wont post again. goodbye

23rd January 2013, 05:07 PM	#3
yairf diyAudio Member Join Date: Nov 2003 Location: Israel	Hi 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. Yair