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Old 1st July 2005, 05:01 PM   #1
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Default How to test a preamp?

I finished my gainclone a while ago (dead silent, very good ) and I'm now testing the preamp I built (The Audio Link from Elektor), but the first try wasn't very succesfull (huge humm, more like someone scraping his throat at 100dB..). Is this what a ground loop sounds like?

Can anyone tell me what would be a good strategy to test the preamp, because my speakers weren't all that pleased with the humm Is there a way to measure the output levels of the preamp with just a simple voltmeter (and deduce something interesting from the values..)? The only other speakers I have are the ones I use with my computer, can I use these to test?

Thanks!
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Old 1st July 2005, 05:09 PM   #2
Nuuk is offline Nuuk  United Kingdom
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It's a very good idea to have an old speaker around to test anything that you add or change in your system!

You could try playing a continuous tone and measuring the signal on the input and output of the preamp but I think that the hum is more to do with grounding arrangements. Can you post a circuit diagram for us to see?
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Old 1st July 2005, 05:13 PM   #3
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Can you post the circuit or describe it?
If it uses opamps, check the voltage at the power supply pins of the opamp. The voltage should be symetrical. The voltage at the output pin of the opamp should be less than 100mv. For testing the circuit always power up using limiting resistors say 100ohms . If all this checks out fine, check the DC at the out put of the preamp.This should be a few mv.Hope this helps.
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Old 1st July 2005, 06:59 PM   #4
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Quote:
Originally posted by Nuuk
..Can you post a circuit diagram for us to see?

Well, that's pretty difficult. The gainclone is a brianGT lm3886 kit, which is star grounded to a central point on the chassis. The preamp is the PGA2311 preamp by Ben Hinrichs (link). I think the problem might be that I soldered the two incoming grounds from the cd-player together. This might form a loop all the way through the preamp into the gainclone's star ground. Is that possible?
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Old 1st July 2005, 07:07 PM   #5
Nuuk is offline Nuuk  United Kingdom
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Well, you are fairly sure that the power ap is OK. So check the DC offset on the output of the pre-amp (as Dine1967 suggested).

The hum will either be a grounding issue or you have something nasty on the signal out of the pre. So measure that first.
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Old 1st July 2005, 08:32 PM   #6
elco is offline elco  Canada
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Computer speakers are fine, and you can get a used pair for real cheap.

Did you connect the preamp outputs ground to the star ground? That's the usual mistake.

Both Input and ouput ground need to be connected to the preamp board ONLY, and must be isolated from the chassis.
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Old 2nd July 2005, 12:00 PM   #7
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Think I found the problem: without any load attached, the source selection module outputs about 20mV, which makes a lot of noise after my gainclone does its thing....

When I connect the input from my mp3 player directly to the volume control module, everything sounds pretty good (rather primitively connected to an old headphone that is). So my next test will be connecting the preamp to the gainclone again without the source selection and hope that solves the problem. Well, at least it will narrow the problem down to one module.

Thanks all!
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