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Trouble with PIMETA: high DC offset

post #1 of 16
8/29/05 at 8:09pm
Thread Starter 
I am working on Tangent's PIMETA. I am doing everything by the book and got to the stage 9 of Tangent’s guide “Test the amp”.

I checked the voltages at the chip supply pins. They were about 8.5V (17v power supply). Fine.

Then I installed the chips: OPA637, OPA627, and 3x BUF634. Turned the amp on. Checked the chips. They stayed cool (may be just a bit warm).

Then I checked DC offset with no source connected. The results were scary:

OG = +0.1mV
OL = -4.07V (not mV)
OR = -4.17V (not mV)

I removed the flux. This did not help. I re-checked the results. The figures vary slightly every time I check but stay around -4V for OL and OR.

I double checked the guide and my soldering. I still cannot figure out what the problem is. Any help would be greatly appreciated.
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post #2 of 16
8/30/05 at 1:07am
For your L/R OpAmp, are you using two OPA637s (or 627s) with a single-to-dual channel adapter? This is necessary because they are single-channel, and the pimeta uses a dual-channel chip for its left and right channels.
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post #3 of 16
8/30/05 at 1:18am
Quote:
Originally Posted by Alf
Then I installed the chips: OPA637, OPA627, and 3x BUF634. Turned the amp on. Checked the chips. They stayed cool (may be just a bit warm).

Then I checked DC offset with no source connected. The results were scary:

OG = +0.1mV
OL = -4.07V (not mV)
OR = -4.17V (not mV)
Single buffers, or each one stacked with 3?

-Ed
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post #4 of 16
8/30/05 at 5:57am
Thread Starter 
Quote:
Originally Posted by JackH
For your L/R OpAmp, are you using two OPA637s (or 627s) with a single-to-dual channel adapter? This is necessary because they are single-channel, and the pimeta uses a dual-channel chip for its left and right channels.

You are right. I assumed that OPA637 is a dual-channel chip. What a blunder! I hope the chip is still OK after that.


Can someone suggest a part number for a single-to-dual adapter from Farnell or other UK suppliers? I did a quick search but I cannot find anything suitable.
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post #5 of 16
8/30/05 at 6:14am
Brown Dog adapters online (US company):

http://cimarrontechnology.com/

Notes on various op amps here:

http://tangentsoft.net/audio/opamps.html
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post #6 of 16
8/30/05 at 6:18am
There are also these pre-built versions with decoupling caps, but they're expensive:

http://www.referenceaudiomods.com/Me...ry_Code=OPAMPS
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post #7 of 16
8/30/05 at 9:19am
Quote:
Originally Posted by Alf
Can someone suggest a part number for a single-to-dual adapter from Farnell or other UK suppliers? I did a quick search but I cannot find anything suitable.
[edit]
I read that totally wrong, my bad. As others have mentioned you can get an adapter from Browndog, I'm not aware of any others.

For now it might be a good idea to pick up a cheap dual opamp like the OPA2227 to test and see if the amp is working properly before risking the more expensive 637s.

HTH,

Nate
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post #8 of 16
8/30/05 at 5:43pm
Thread Starter 
Quote:
Originally Posted by n_maher
For now it might be a good idea to pick up a cheap dual opamp like the OPA2227 to test and see if the amp is working properly before risking the more expensive 637s.
OK. I use OPA2132 for L/R and OPA627 for the ground channel. The amp works now!!!

Thanks a lot, guys! It would take MUCH longer without your help.
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post #9 of 16
8/31/05 at 12:49am
Quote:
Originally Posted by Alf
I hope the chip is still OK after that.
Installing a single into a dual socket shouldn't kill it. Also, 4mV is not "high" DC offset. I don't get worried until I see 20mV or so.
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post #10 of 16
8/31/05 at 12:59am
Quote:
Originally Posted by tangent
Installing a single into a dual socket shouldn't kill it. Also, 4mV is not "high" DC offset. I don't get worried until I see 20mV or so.
he said 4 V .. not 4mV, tangent
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post #11 of 16
8/31/05 at 1:09am
Quote:
Originally Posted by Jazper
he said 4 V .. not 4mV, tangent
Ah, what's a few zeroes between friends?
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post #12 of 16
8/31/05 at 1:48am
Quote:
Originally Posted by tangent
Ah, what's a few zeroes between friends?
Let me put 4v through your cans and we'll find out... :P

Rob.
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post #13 of 16
8/31/05 at 8:12pm
Thread Starter 
One more question on DC offset. Now I measure:

OL = 1.5mV
OR = 10.0mV
OG = 0.01mV

The difference between OL and OR is substantial. The source cannot be blamed because it is not connected. Where does the difference come from?
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post #14 of 16
9/1/05 at 4:45am
Quote:
Originally Posted by Alf
Where does the difference come from?
Manufacturing differences.
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post #15 of 16
9/2/05 at 3:57pm
how do you measure DC offset?
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