andjur78 said:
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
forr said:
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
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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~~~~~~ Forr
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twelve years later...
... a thread hijack!
I've just obtained an 80's version of the P40, based on the same circuit I believe. Unfortunately the fuse ratings are not stated, either the power one or the two speaker protection ones. Someone has been inside this before, one speaker fuse fitted is 1A the other 5A! Does anyone know the correct ratings please?
Thank you in advance.
... a thread hijack!
I've just obtained an 80's version of the P40, based on the same circuit I believe. Unfortunately the fuse ratings are not stated, either the power one or the two speaker protection ones. Someone has been inside this before, one speaker fuse fitted is 1A the other 5A! Does anyone know the correct ratings please?
Thank you in advance.
this is the one
An externally hosted image should be here but it was not working when we last tested it.
If it is indeed the same P40 circuit, a single 55V power supply rail will be in use and the audio output will be via a 2,500uF capacitor. This was augmented with an unusual form of current limiting in the original too - see sch. and comments on Paul Kemble's site: A Paul Kemble web page - the Cambridge Audio 'P' series amplifiers.
An output capacitor makes the need for fuse protection less critical but from the attached image ( courtesy of the same site as your pic of the later P40 model), I don't think it is the same design. Perhaps it shares some elements but this version appears to have a dual rail power supply, 2 pairs of output devices and no output cap. If it looks the same internally and the rail voltages measure more like +/- 25V, I would treat it as a different design and use 2A fast blow fuses in lieu of the 2.5 amp rail fuses said to be used in P50.
An output capacitor makes the need for fuse protection less critical but from the attached image ( courtesy of the same site as your pic of the later P40 model), I don't think it is the same design. Perhaps it shares some elements but this version appears to have a dual rail power supply, 2 pairs of output devices and no output cap. If it looks the same internally and the rail voltages measure more like +/- 25V, I would treat it as a different design and use 2A fast blow fuses in lieu of the 2.5 amp rail fuses said to be used in P50.
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