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Old 9th July 2010, 09:36 AM   #31
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Join Date: Jun 2010
Finally had success with my offset issue - with the help of some excellent advise - I replaced Q601/2 & Q605/6 with some high gain matched transistors (Toshiba 2SA970BL) - Zetex ZTX795A where recommended as superior but I could not get them in NZ without paying huge shipping.

I found that R610 was also way off spec and replacing that brought both channels to within 1mV of each other. I replaced all the elect caps and also the bias trim pots with multi turn sealed versions to make adjusting easier and more accurate compared to the single tun original.

Final results come to 31 & 32 mV at the binding posts down from 98 and 70ish at the beginning.

I have a very slight hum sounding like it is coming from the transformer and I can also hear it at the speakers and so I am hoping that the ground re routing that abraxalito is suggesting might help.....any thoughts before I start hacking up the track?
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Old 15th May 2011, 07:36 PM   #32
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Join Date: Oct 2004
Location: Cymru/Wales
Originally Posted by abraxalito View Post
The other suggestion I have is bypassing the tone controls. This is quite a simple job - just a few cut links and two added wires.

For the left channel : cut J62 and J69. Link from the north end of J69 to the north end of J62.

For the right : cut J66 and J37. Link from J66 E to J37 N.

Since the tone controls are passive and have an insertion loss around 10dB, this gain is at present made up in IC501. With the controls out of circuit, the buffer's gain should be reduced by 10dB, so change both R507 and R508 to 4k7s.
hello I have this amp and have REMOVED both tone controls, should I change R507 & R508? sounds Ok!
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