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Old 18th July 2012, 12:07 PM   #351
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Default Help DC offset

I wonder if anyone can help. I've had a ezdac built for a while now and it's output goes to a blanaced output board. Problem is the opamps on the balanced output board keep failing. After replacing them a few times (always the left channel) i thought I try to locate the source of the problem. I checked to dc offset on the two outputs of the ezdac. The right on is 2mV but the left is -1.4V (problem found). The only trouble is I can see nothing wrong with the board. Have replaced the opamp and checked fo bad joints but everything looks OK. Anyone got any ideas?
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Old 18th July 2012, 03:47 PM   #352
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Hi,

You should check the resistors at the outputs of the DAC and around the opamps. Looks like there's a value way off, or some 25R not connected to GND properly. The offset coming out of the DAC is equal at both outputs, and by summing it with the opamp this results in (almost) 0V. So 1.4V is way too high and indicates there's something wrong with the balance between the two.

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Ray
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Old 19th July 2012, 11:49 AM   #353
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For the right channel the voltage at the DAC output is 155mV for both outputs. For the left channel one output is 266mV and the other is 269mV. Might try and replace the DAC and see what happens. I have a spare chip but desoldering the existing chip might be a bit of a problem. I have checked all of the resistors and they all seem ok.
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Old 19th July 2012, 12:20 PM   #354
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Can you measure the voltage at pins 2 and 3 of the opamps?

The voltage for the left channel is not right. The idle current is the same for both, so if all resistors are equal, the voltage should also be equal.

Lift R25/26 and R31/32 off the board on one side. This way, only the I/V resistors are connected to the DAC. All voltages at the DAC should now be equal.

If it's still not o.k. and you're sure the 25 ohm resistors are correct, then the DAC is fried :-(

Ray
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Last edited by 6h5c; 19th July 2012 at 12:28 PM.
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Old 31st July 2012, 10:19 AM   #355
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It was the opamp. One of the connections on the inputs must have been dodgy. Anyway working now. Lifting the resistors confirmed it was not the DAC so I'm glad I didn't replace it.

Thanks for your help
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Old 31st July 2012, 02:11 PM   #356
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Good to hear that it's working now. Having to replace an opamp is better than working on the fine-pitched DAC package

Ray
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Old 20th September 2012, 10:29 AM   #357
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Just thought I'd post a few photos now it's finished.
Attached Images
File Type: jpg DSC_0384.JPG (171.0 KB, 139 views)
File Type: jpg DSC_0387.JPG (454.5 KB, 131 views)
File Type: jpg DSC_0389.jpg (572.0 KB, 130 views)

Last edited by stonedowner; 20th September 2012 at 10:42 AM.
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Old 20th September 2012, 06:33 PM   #358
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Nice, work! Funny to see that the majority of the space is occupied by power-supplies
But that's not uncommon in audio. How does it sound?

Regards,

Ray
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Old 21st September 2012, 08:58 AM   #359
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Yes it is mostly power supply but the result is worth it.

How does it sound? - I find that a very difficult question to answer:-
About 6 years ago I had a dCS Delius DAC and Purcell Upsampler. Didn't buy them new - couldn't afford that - but got them reconditioned from dCS for a resonable price. I was usind a squeezebox receiver as source and using 4 WNA MOS125 power amps to bi-amp various speakers (Mission 757, 782 - I change speakers quite often or I did 'till I aquired a pair of Pink Triangle Ventricals) Anyway I was really happy with that sound - it was very musical, warm and detailed. Never tired of listening to the system. My ezDAC is very similar to that system although I'm now using a Squeezebox touch and am listening to 24/96 music (Same amps and Pink Triangle speakers). I've also recently purchased a Audio GD NFB 3.1 (for when the ezDAC wasn't working) and have been compairing the 2. Keep coming back to the ezDAC as it's so easy to listen to, althought I'm nowhere near the 200 hours that Audio GD recommend for burn in of thei DAC.
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Old 22nd September 2012, 07:26 AM   #360
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Sounds good. You can come to a point in audio when things are not just sounding 'good' or 'bad' but more like different. Musical reproduction is fine but there are subtle differences in sound which makes you like one piece of kit more than the other. It takes a lot of time to finally settle for one kind of sound, although the other sound may have some things you like as well. You then wish you could have the best of both

Ray
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