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Old 3rd December 2009, 11:11 PM   #1
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Default Threshold S300 offset issues

I have a Threshold S300 (original model from '82) that have been refurbing trying to eliminate 70mv (left) and 130mv (right) of DC offset on the speaker outputs. I've matched and replaced the input diff pair. I did the cap replacements suggested in threads here (thud and offset threads). But at this point, the easy stuff has not done the trick.

Following general practice I put in new matched differential pairs. The 70mv channel showed no difference, as the originals were still dead on. The 130mv channel dropped from an original 150mv to about 80mv. However adjusting the bias up to 50ma brought that 80mv up to 130mv. And I was so excited.....

Going by tips in other threads here, I replaced the 470uf 6.3v electrolyte cap and the 0.15uf tantalum. That made little to no difference.

The DC offset increases almost linearly while increasing the bias.

This amp had 2N6428As as the differential input transistors. I replaced them with MPS6571s based on the schematic I have.

Anything else to check? Could the CCS for the diff inputs be out of spec enough to cause problems? Hfe on the outputs too uneven?
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Old 4th December 2009, 02:17 AM   #2
eyoung is offline eyoung  United States
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Hola,

Did you read Mr.Pass' papers on matching transistors for current or voltage. Its on his(their) web site, build a simple jig and measure following his guidelines. Matching hfe is not a guarantee of a match. Its late and I do not have the link, but it is easy to find...

Regards, Elwood
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Old 4th December 2009, 02:29 AM   #3
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I think I've read the pdf you are referring to. I'll go hunt that down again. Thanks for the tip.
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Old 4th December 2009, 06:53 PM   #4
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Having done all the obvious things, the thing to do now is
to adjust either the resistor setting the bias current to the
input diff pair, or the resistor setting the load coming off the
drains of the diff pair - the resistor attached to the Base of
the VAS transistor and v+.

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Old 5th December 2009, 02:17 AM   #5
eyoung is offline eyoung  United States
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Thanks Papa for Bailing me out...


Elwood
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Old 15th December 2009, 12:58 AM   #6
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I ended up replacing the 82 ohm load resistor on the diff pair with a 200 ohm trim pot to find the right resistance. The DC offset seems to change about 40mv for every 10 ohms either direction. I used 93 ohm resistors on my S/150 as a test, and that dropped the DC offset down to less than 5mv. It was previously reading 40-45mv on both channels.

Thank you for the tip.

Out of ignorance induced curiosity, I was wondering why the emitter resistors on the input diff pair are different values? Every other schematic I recall has identical values.

Would making thermal contact between the diff pair transistors bring any tangible benefit? I'd use thermal grease and some heat shrink to bind them.
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