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Old 2nd July 2009, 03:59 AM   #1
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Default cmoy help (DC offset)

Hi guys, I need a little bit of help with my cmoy amp that I've just built.

I've built the amp based on the "tight layout" drawings from the tangentsoft website, and the amp seems to be working fine (it sounds GREAT). The left side of the amp (the one with the L in and L out) has 8.05v going through it, and the right side has 8.06v, so it's split quite evenly (yes I'm using two 9v batteries because I had a Radioshack op amp before I just got the OPA2227).

Everything sounds great--for a gain of 11 (I just used the default not knowing any better at the time) there is very little hiss. But I was reading about DC offset and decided to check it. I am getting 40mV on the right channel and 22mV on the left.

And, I just now kind of moved some things to get better access (moved the jacks away from the board so they're sticking up), and now I just registered 3mV on the left side but 279mV on the right! (Two hundred seventy-nine--not a typo). My DMM is on 200m (but I had to switch it to 2000m to get the reading for the right channel the second time). The ground is still split evenly.

I can post pictures of the top and bottom of the board in a few minutes if it would help.

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Old 2nd July 2009, 08:48 PM   #2
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Definitely post pics. I built one as well using the tangentsoft diagram and it works great.
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Old 2nd July 2009, 11:18 PM   #3
Minion is offline Minion  Canada
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Location: Cowican Bay , vancouver island
Are you useing the Batteries as a dual rail PSU ?? If you are not and are useing a single rail PSU with a Virtual ground then you need to put output DC Blocking caps in the curcuit as Half the supply voltage will be on the output ......

If you are useing the Batteries as a Dual Rail PSU and are getting close to 9v at the output then look for a short somewere or a loose connection on the ground wire ....

A schematic would let us know for sure ...

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Old 3rd July 2009, 04:48 AM   #4
wwenze is offline wwenze  Singapore
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A DC offset at the output means a DC offset (or rather, wrong voltage or value) somewhere else in the circuit.
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Old 3rd July 2009, 05:50 AM   #5
lgreen is offline lgreen  United States
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Location: San Diego, USA
Default LEDs

What's up with your batteries and LEDs?

Do you have an LED drawing from one battery only which would drain that battery faster?

Pics and schematic would help.
Retired from DIY (2010) but still lurking now and again. My DIY audio projects-
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Old 3rd July 2009, 07:15 AM   #6
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Instructional diagrams:
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Old 3rd July 2009, 08:32 AM   #7
wwenze is offline wwenze  Singapore
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Default Re: cmoy help (DC offset)

Originally posted by erictooth
The left side of the amp (the one with the L in and L out) has 8.05v going through it, and the right side has 8.06v, so it's split quite evenly
Are you using a single or dual-channel op-amp? Seems to me you have separate op-amps per channel. The schematics is for dual-channel op-amp though.

If you have separate op-amps for each channel, you can use the working channel to help check which part of the non-working channel has gone wrong, compare all the voltages.

If you're using just one dual-channel op-amp, then can rule out any problem with the power supply, look at the input/output signal paths instead. Again with the help of the working channel.
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Old 3rd July 2009, 12:32 PM   #8
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
running the indicator LED wastes battery life.

The opamp with 10k and 1k on the inverting input gives a gain of 11times (+20.8dB).
The 0.1uF with 100k for Rin rolls off the bass very early for a pre-amp.
-3dB @ 16Hz and -1dB @~30Hz.
For a pre-amp I would increase the DC blocking cap to 2u2F or 1u5F.

The opamp sees different DC resistance on it's two inputs.
The inverting input sees 10k//1k0 = ~909r
The non-inverting input sees 100k.
This difference in source resistance guarantees that the input offset voltages will be different and with the DC gain of 11times the output offset is guaranteed to be much higher than necessary. It will also vary with temperature much more than necessary.

It might sound great, but it could be a bit better for very few changes.

the 227 spec shows input offset voltage <=+-200uV and input offset current<=+-10nA over the temperature range.

The 200uV gives rise to an output offset of <=+-2.2mV
and the 10nV gives rise to an output offset of <=+-11mV
These in worst case condition add together to give output offset of <=+-13.2mV for the Cmoy layout in post6.
That is far too much offset to send to a Power amp that may not have DC blocking on it's input.
Consider adding a 10uF polypropylene film/foil output cap in parallel to the DC output for the power amps that may need it, i.e. two RCA outputs, one AC coupled and one DC coupled.

Consider also adding a low value resistor (R5b) after the R5 optional to R4 connection. Maybe 51r?
regards Andrew T.
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