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Old 12th June 2001, 12:18 AM   #1
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Join Date: Jun 2001
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I working on finishing up my first DIY amplifier. I am building Nelson Pass's A75. It in nicly documented and I bought the PCB's from old colony.

I'm wondering what is DC offset voltage? I am adjusting the bias on the input stage per Nelson's article and he mentions the dc offset voltage should be almost zero. I thought that he ment the DC in the output but when I hook a VOM in dc mode to the output it tells me I have about 6.5v which I think is the RMS voltage of the ac that is coming out (I have a 200mV 1kHz signal on the input). The scope that I have on the output is giving just the amplified 1kHz signal. Am I on the right track here?

Any insight would be appreciated,

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Old 12th June 2001, 12:31 AM   #2
jam is offline jam  United States
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Location: Auburn, CA, USA
Default DC Offset Voltage


DC offset voltage is the DC voltage between the output terminal(+)and the (-)output terminal or ground. This voltage should not be more than approx. + or - 100mV or so.
No signal should be applied to the amplifier when taking this measurement and allow the amplifier to run for 15min. or more before making adjustments.


PS. I think you will be pleasently surprised how good this amplifier sounds.

[Edited by jam on 06-11-2001 at 07:34 PM]
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Old 13th June 2001, 06:51 PM   #3
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Smile Thanks


Thanks, my stupid error, I should have known! I shut off the inoput signal and got about 500mv of DC so after I I get my tranny and build the power supply, and stop using the bench power. I should be able to get it a bit lower.

BTW has anyone documented the the errors in the stuffing diagram for the Pass A75?

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