Anyone got schematics for the Cambridge Audio model P40?
 

Not bad sounding Amp if I must say!
This one has a scratchy vol pot, intermittent select switch...a good old raod map would be nice

Thanks in advance, Andjur78

Yes, I have the schematics of the Cambridge P40 but without the tone control. I can scan them. Just send an email to me.

~~~~~~ Forr

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Re: Anyone got schematics for the Cambridge Audio model P40?
 

It's always good to have the schematics of one's amp.

But from what you say, diagnostic seems quite easy: clean or change the volume pot.

For the switch things may be more difficult. If it's not a rotary switch (I think the Cambridge had button-type selectors), then you will have to open the switch to clean it.

If it's rotary, go for a Grayhill switch which is a great deal.


Carlos

The volume control of the Cambridge is very special as it uses a virtual earth. Even before changing the 1 MOhm pot, change C1 and C2 (10 µF) which may be leaky.

~~~~~~~~ Forr

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As far as I know there are no pots that use virtual earth per se. The designer is using virtual earth on the circuit, and the pot is connected to it. Any pot will do that.

But you are right that if there is a cap in the middle there shouldn't be any problem.

That cap may be a compromise that should be corrected before anything else. Different caps or no cap there should improve things a lot.


Carlos

I'd like a schematic too!

Some volume controls use virtual earth with a pot, either used as a feedback resistor (Cambridge, from what I see from the schematics; I think it has a log variation) or both as input and feedback resistors in a circuit invented by Peter Baxandall and used by Douglas Self in his preamp. The great advantage of this last circuit relies in the use of linear pot, whose tracks, in stereo, match better than for the log type, the output being quite linear in dB.

~~~~~~ Forr

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Hi forr,
can you send me schematic to?
My e-mail:
janevski_69@yahoo.com

Schematics of the Cambridge P40 preamp.

Schematics of the Cambridge P40 amp.