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Old 12th October 2006, 07:23 PM   #1
keps is offline keps  Philippines
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Default DC offset reading

Just finished my GC, I tried to read DC offset from the output (speaker) channels.

started with the left channel...
1. no source connected.
2. hooked up one speaker on the left channel
3. powered on the amp
4. probed the output, red+ ; black- with multimeter
5. reading is -0.69V

now the right channel...
1. reading is -0.98V

What is this? I being a noob, am I doing things wrong?

what I read from this forum, ideally I should not get more than +0.5V. or should be near 0V.

what does these reading tell me?

I checked the polarities, OUT is + and OG is -.

please advise a newbie.

thanks
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Old 12th October 2006, 07:27 PM   #2
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What is your particular implementation? Do you have a schematic? Things like feedback resisitor values, input network etc. are of influence. Is it DC input coupled, if yes, what's the offset with shorted input?

Jan Didden
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Old 12th October 2006, 07:34 PM   #3
keps is offline keps  Philippines
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Quote:
Originally posted by janneman
What is your particular implementation? Do you have a schematic? Things like feedback resisitor values, input network etc. are of influence. Is it DC input coupled, if yes, what's the offset with shorted input?

Jan Didden
I actually got a rev3 LM3875 kit from Mad About Sound. and finished assembbling it.

22K NFB came with the kit. I did not connect any source yet.

I just tried to check the DC offset after finishing the kit. I don't know if I am doing the right thing in measuring the DC offset.

thanks for the reply.
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Old 12th October 2006, 07:48 PM   #4
keps is offline keps  Philippines
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How does DC offset nearing 0mV affect sound quality?
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Old 12th October 2006, 07:59 PM   #5
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I mean, one look at the schematic for some of the cognoscienti (sp) here will immediately spot the problem (might be measurement depending on input coupling), but most won't take the trouble to hunt up the circuit for you.

Jan Didden
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Old 12th October 2006, 08:04 PM   #6
keps is offline keps  Philippines
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Well, I don't have schematics, I just got the kit for a non-inverting GC.

Being a noob and just trying to get information through the forum made me wonder why this is so...

I just hope later on, after a lot of reading will tell me what these things are all about. I believe I will soon find out.
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Old 12th October 2006, 08:15 PM   #7
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Keps,
You say you have no source connected. I believe the way to measure the output DC offset with the input shorted to ground. That's the way I do it anyway...

Nice one,
David.
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Old 12th October 2006, 08:22 PM   #8
keps is offline keps  Philippines
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Quote:
Originally posted by daatkins
Keps,
You say you have no source connected. I believe the way to measure the output DC offset with the input shorted to ground. That's the way I do it anyway...

Nice one,
David.
Daatkins,

If the input is shorted to ground I get -0.13V.

the values looks promising but why is it negative -0.13V?
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Old 12th October 2006, 09:09 PM   #9
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The offset voltage is caused by imperfections in the chip: internal bias currents, offset voltages etc.

Your reading of -0.13V is more like what I would expect and is acceptable, IMHO. What is the reading for the other channel?

For reference, I built an eight channel non-inverting LM3886 amplifier of my own design and the DC offsets measure between -34mV and -79mV.
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Old 12th October 2006, 09:49 PM   #10
Bazukaz is offline Bazukaz  Lithuania
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Quote:
Originally posted by keps

Daatkins,

If the input is shorted to ground I get -0.13V.

the values looks promising but why is it negative -0.13V?

Try to measure DC voltage with audio source connected. DC offset of +/- 0.13V is OK. Even +/-1V shouldn't cause any problem with standard dynamic loadspeakers

Regards,
Lukas.
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