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Old 17th March 2007, 01:16 AM   #1
eris is offline eris  United States
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Default Need help with LM3875 Noise

Hi!

I just finished my LM3875 Chip Amp using the PCBs from chipamp.com. For the most part its running okay and DC offset is around 20mV on both channels. Here are some problems I'm having:

1. When the amp inputs are shorted, its dead silent. When I play a CD into the amp, i can hear low level hiss that sounds like tape hiss only slightly louder. The CD I'm listening to is digitally recorded so I know its not tape hiss. The hiss seems most obvious if I turn the volume down to about 1/4. I listened to the CD player using Sennheiser HD600's and can just barely hear hiss (to the point where I think I'm making it up). My son, who has much better hearing, says he can hear hiss even with headphones. My high-end CD player decided this was a good time to stop working, so I don't have a reference.

Do you think this is likely to be coming from the CD player and its more noticable because of an impedence mismatch?

2. When I disconnect the source CD player or turn it off the amplifier outputs a rather loud 60 Hz hum. As long as a source is connected and even a marginal signal is running into the amp there is no hum whatsoever.

Any ideas?

..eris..
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Old 17th March 2007, 09:28 AM   #2
Netlist is offline Netlist  Belgium
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I'm thinking bad ground, ground loop(s) or/and too long (unshielded) wires in proximity of power rails.

Any pictures?

/Hugo
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Old 18th March 2007, 05:19 AM   #3
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I also at first experienced this problem, and a little bit annoyed at times with it but I found that All I needed was a good power strip that I could plug in all devices that were in one way or another connected to each others ground into. OK, for your test you could just simply plug in your dvd player and amp into the exact same power strip.
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Old 19th March 2007, 03:05 AM   #4
gofar99 is offline gofar99  United States
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Greetings, The gound loop could be a problem. Even though chip amps are pretty immune to PS noise, it can get through. I put snubbers on mine and it is extremely quiet. Two other things come to mind though. Is the chip amp direct coupled (no caps in the signal path) If so you might be getting some dc component from the player ouput and since chips can go down to dc, it could be amplifying it. Second and sometimes a problem with wide band amps is oscillation. The fact that the volume control changes it in a non-linear way, suggests to me that something is feeding back at a high frequency (above audio). I recently had a tube amp do just that and it baffled me until I put it on a scope. It was singing away at 180 KHZ when the volume control was in certain positions (just like an old time phase shift oscillator). A scope BTW is an easy way to tell if you have noise or other stuff going on. If you don't have one, the problem can be tedious to find. Maybe some one out there can help you diagnose the amp without a scope.

Good listening
gofar99
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Old 19th March 2007, 03:14 AM   #5
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Hi. Sounds like a ground loop. Break it by isolating your input ground from the main ground with a low value resistor ( 50-80 ohms ).
Goodluck
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