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Old 19th March 2006, 01:34 PM   #1
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Cool Cambridge A3i hiss

Help please! My much loved Cambridge A3i amp has started to hiss loudly on the right hand channel. This starts 2 seconds after powering up and is at a constant level irrespective of source or volume setting or ballance. The only control affecting it is the mute button which silences it but everything else, of course!
I have a very basic electronics knowledge, ie I know one componant from another, and do have a multimeter. I would love to hear from anyone who can give me any clues, if I can at least diagnose the fault I can get it repaired without fear of being ripped off!
Cheers everone.
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Old 19th March 2006, 04:03 PM   #2
anatech is offline anatech  Canada
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Hi justsomeguy56,
Is this a straight amplifier? I only have amp and supply / protection diagrams for this.

If it hisses with no preamp, I'd replace the two transistors that make up the diff. pair. with a matched set of the same type. A pair of BC550's with the same letter code as the original. Buy 10 and take the best matched pair if you can measure gain.

Bad coupling caps and Vas transistors normally make a rumbling sound as well, plus unstable DC offset (woofer moves in and out). The diff pair can cause the same thing as well.

-Chris
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Old 19th March 2006, 08:40 PM   #3
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Hi Chris,
This is an integrated amp and there would seem to be just hiss on the rh channel and no audible rumble. I have found the original manual but sadly there is no circuit diagram so any assistance in that area would be great. Thanks John.
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Old 19th March 2006, 08:51 PM   #4
x-pro is offline x-pro  United Kingdom
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Default Re: Cambridge A3i hiss

Quote:
Originally posted by justsomeguy56
The only control affecting it is the mute button which silences it but everything else, of course!
I have a very basic electronics knowledge, ie I know one componant from another, and do have a multimeter. I would love to hear from anyone who can give me any clues, if I can at least diagnose the fault I can get it repaired without fear of being ripped off!
I've designed A3i, and from what you've described the fault is in the power amplifier section. It is possible that the FET Q107 developed some leakage or one of the input differential pair transistors (Q101, Q102) became noisy. It would be useful to compare the voltages between channels on Q7 and Q107 drains (or on the R13/R113 resistors - it is the same thing). If you'll write me an email I can provide you with the schematics for A3i.

Cheers

x-pro
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Old 19th March 2006, 08:59 PM   #5
anatech is offline anatech  Canada
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Hi John,
Well you can't get any better assistance than x-pro! Especially since he designed the unit.

x-pro, if you wouldn't mind sending me if it's electronic? My large email box is "bhomester at gmail dot com " .

Thanks,
-Chris
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Old 19th March 2006, 09:11 PM   #6
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by anatech
x-pro, if you wouldn't mind sending me if it's electronic?
Done.

x-pro
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Old 19th March 2006, 10:34 PM   #7
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Thanks x-pro! Amazing, I only came on here with a vague hope that someone may help having sorted problems with my sat. nav. using a forum. I have clicked on the email link on your profile and got an administrators message that you don't want to be emailed! My address is jmw151149@yahoo.co.uk if you would be kind enough to contact me direct. By the way I also have a Creek 40/40!
Thank you again, John.
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Old 19th March 2006, 10:49 PM   #8
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by justsomeguy56
My address is jmw151149@yahoo.co.uk if you would be kind enough to contact me direct.
Hi John,

I've sent you the schematics by email. I'll try to advise you about your A3i as much as I can.

Cheers

Alex (x-pro)
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Old 20th March 2006, 12:25 AM   #9
Leolabs is offline Leolabs  Malaysia
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Possibly dry solder joints,try to resolder them first.
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Old 20th March 2006, 01:06 AM   #10
anatech is offline anatech  Canada
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Hi x-pro,
Thank you very kindly!

Let me know what the problem was, I am always curious.

-Chris
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