PS Audio 5.0 DC Offset

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
Hey all,

I have a PS Audio 5.0 Preamp that I have been playing with - I have 2.2vdc and 2.4vdc offsets on the main output, but I cannot hear any hum - thru amp - weird...

I have the preamp outs connected to a simple receiver cd input (so as to not destroy a good amp) and connot hear any noise - the music comes thru fine - the active / passive selector switch has some pop noise on switching (most likley due to the 2+ volt offset)

Anyway, could there be no noise as the receiver has caps on input? No noise at all, I would expect some noise.

I know there is NO POSSIBILITY of getting a service manual or schematic for this PS Audio premp - although I did find one for the PS Audio 4.6...

The six NPD5566 JFETS (two are for Phono) all have pots near them, but have not touched them. Main caps show +/-38volts (very close).

Some of the NPD5566 have different voltages - ie +22vdc and +24vdc on D1 D2 drains on same IC...thinking they need to match hence the offset...but do not know which pots would control DC Offset...but 2 volts seems a little high to correct with just the pots...

It looks like someone was really modding the hell out of this preamp...

Any ideas?
 
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It looks like someone was really modding the hell out of this preamp...
Any ideas?

Thank your lucky stars your test amp has input capacitors.

Why not call PS Audio and ask them for more information. They're pretty good with customer support questions. If it has been that severely modded, they might even want it to study what's been done to it...
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
The word is that they cannot fix / work on this pre...too old.

It is a Direct Couple Preamp - so no output caps.

So with dual voltage power supply - +/- 38V - I guess the design is to balance the bias to cancel the DC offset?

OK, Hold on - I see there is quite a bit more to this design - I guess the design of this pre was to connect to an amp that has an input cap - OK, I get it, but what company would stick its neck out to sell a preamp that REQUIRED the amp to have an input cap? How many amps were destroyed by this "feature"?

OK, so need to make sure any amp connected to this pre has input cap, or I may just install one...

Some pics
 

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john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
Bought this preamp to fix and connect to my Krell KSA-50 Clone, but now see no input caps ...

Does anyone know if an ICEPower ASP1000 or ASP500 have input caps?

There may not be anything "Broke" with this amp - maybe previous owner hooked up an amp that did not have input caps, and poof...
 
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Bought this preamp to fix and connect to my Krell KSA-50 Clone, but now see no input caps ...

Does anyone know if an ICEPower ASP1000 or ASP500 have input caps?

There may not be anything "Broke" with this amp - maybe previous owner hooked up an amp that did not have input caps, and poof...

Play it safe ... patch in some output caps in the pre-amp.
You can't count on power amp makers to have any better common sense than these guys.