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Old 27th May 2012, 10:26 PM   #1
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Default DC OFFSET AT ROTEL

Good for all, I am a follower of this site, but is the first time I need your help.
I have an integrated amplifier Rotel RA-970BX, in immaculate condition and in apparent good performance. But out of curiosity I measured with a multimeter at the terminals of speakers and I find that has left channel DC offset at least 45mv and more 68mv right channel.
Measurements were performed with the amplifier with no load, with zero volume and after 10 minutes of ignition.
I have understood that an amplifier should not exceed the 15mV offset. I'd like to target the solution. I am told that the transistors 2sa1016 and 2sc2362 have been old and must be replaced. I should also replace the electrolytic capacitors in the power stage. thank to help me. Greetings Jose Alpuy (From Uruguay. Here there is no technical service Rotel).
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Old 28th May 2012, 03:02 AM   #2
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There is nothing in the user manual for maximum offset and 68mV is still acceptable. Transistors don't get "old", the electrolytics may be ageing and could need a refresh.
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Old 28th May 2012, 09:35 AM   #3
alayn91 is offline alayn91  France
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Hello,

Output DC offset is mainly caused by differential input transistors.
You can try to change C607 & 608 and C601 & 602.
Non polarized electrolytics capacitors would be better.

Regards.
Alain, from France.
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Old 28th May 2012, 10:52 AM   #4
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Somebody else on here recently had the same problem with the same amp I think. The conclusion was that it is inherent to the design and not a problem.

The thread starter had been alarmed by a website saying that the offset was a problem and caused by drifting transistors, which should be changed. Both of these statements are completely untrue.
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Old 28th May 2012, 07:57 PM   #5
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I appreciate your opinions. I've bought on eBay matched transistors and capacitors Nichicon audio type (I'm still waiting for them). My intention is to change, but would not if any of you tell me that would be detrimental.
I am very pleased with the quick response I've had in this forum.
I would like to display photos and comments from a project I've done, taking the cabinet of a Technics SA-5551 where I placed LM3886 amps, preamps and toroidal transformer and phono line of Rod Elliott circuits using OPA2134, but still not as post . Regards, Jose Alpuy.
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Old 28th May 2012, 09:30 PM   #6
tvrgeek is offline tvrgeek  United States
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None of my Rotels are quite that high, more like 35. Caps would be my guess.
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Old 28th May 2012, 10:15 PM   #7
jaycee is offline jaycee  United Kingdom
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renew the caps first, if that doesnt bring it down enough then look at installing a matched pair of transistors for the input LTP. Or add a DC offset control. Or just live with it - under 100mV is fine
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Old 29th May 2012, 11:44 AM   #8
AndrewT is offline AndrewT  Scotland
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I would not expect changing of capacitors to change the output offset.

Have you shorted the input and left the output with out a load to measure the output offset? If not then do it again, properly.

If the NFB capacitor is leaking badly, this will change the output offset. I think the only way to check this is to remove the NFB cap and reform it and find out what leakage it is passing. If it fails to meet specification then replace it with a freshly reformed capacitor that has been fully discharged.
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Old 29th May 2012, 03:52 PM   #9
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Some Rotels have unequal dc resistance in the input and the feedback, causing a dc offset even if the LTP transistors are perfectly matched.
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Old 29th May 2012, 06:39 PM   #10
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Dear Jaycee. could tell me how to add a dc offset control?. Greetings Jose.
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