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Old 28th February 2005, 01:34 AM   #1
akunec is offline akunec  Canada
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Default buzz noise on outputs with no input attached

Im building a 4 channel parelleled 4780 and i notice a fairly loud buzz coming out of my speakers when attached to one of the channels when there aren't any inputs (RCAs) attached to that channel. The noise is gone immediately when i attach an RCA to it. I used nationals design except I didnt use an input capacitor (1uF) on it. Could this be causing the problem?


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Old 28th February 2005, 04:35 AM   #2
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Do you have a ~22k resistor to ground, similar to this picture:

http://www.diyaudio.com/forums/attac...amp=1109310964 ?

Input cap would be used to block DC, and shouldnt be necessary unless you have a large offset. Or use cheap equipment as inputs (some cheap radios give 2v dc on the output for example).
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Old 28th February 2005, 04:47 AM   #3
akunec is offline akunec  Canada
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I have a 47kohm resistor across the inputs.
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Old 28th February 2005, 05:01 AM   #4
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It's likely an oscillation. With nothing at the input, the input lead becomes an antennae. It's picking up some EMI/RFI which is modulating back down to where you can hear it buzz.

I wouldn't worry too much about it. When do you need to run the thing with no input signal? For testing, you should ground the input to keep this from happening.
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Old 28th February 2005, 05:41 AM   #5
AR2 is offline AR2  United States
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Interesting,
I have a quite oposite thing happening on my Brian's GT LM3875 boards. Without input cable the amp is dead silent. With input cable plugged in, all that I am missing is tunning knob
This is audible ether with no music playing, or with low level signal present. I tried many things, and I was able just to attenuate it some by implementing CarlosFM's snubber circuit. I didn't try zobel network yet but I am not so sure that that has to do anything with this problem.
Any suggestions???
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Old 28th February 2005, 06:06 AM   #6
ROVSING is offline ROVSING  Denmark
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This might sound like a stupid question to you , but have you soldered all the pins on the chip to your PCB or only the pins used ? I've just looked at a clone with the same problem turned out that not used pins wasn't soldered this caused a noise like your description.
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Old 28th February 2005, 06:13 AM   #7
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Quote:
Originally posted by ROVSING
This might sound like a stupid question to you , but have you soldered all the pins on the chip to your PCB or only the pins used ?
That's weird. I don't think any of those pins are meant to be grounded. Why would attaching them to the board make a difference? On my p2p chip amp, the NC pins are all waving in the air.

Maybe by soldering the NC pins, you heated up a bad joint on one of the other, more important pins?
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Old 28th February 2005, 06:17 AM   #8
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Quote:
Originally posted by AR2
Interesting,
I have a quite oposite thing happening on my Brian's GT LM3875 boards. Without input cable the amp is dead silent. With input cable plugged in, all that I am missing is tunning knob
In your case, your preamp or source probably creates a ground loop when you plug it in to your amp.
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Old 28th February 2005, 06:27 AM   #9
ROVSING is offline ROVSING  Denmark
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I agree that it sounds weird but i tried to use a pencil (good old wood pencil) to make better connection between the pad and the not soldered pin and the noise went weaker.
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Old 28th February 2005, 12:18 PM   #10
AR2 is offline AR2  United States
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I built two stereo amps, and they both behave similar. In one system, I have CD player, pasive preamp and GC, and in the main system DA converter, digital crossover and GC.
They are all grounded in the same plug.
When I connect different amp, the problem is gone, so the problem is in GC. Ground loops - would be more like a humming sound I think, and what I have is radio noise.
I will try to ground one unused pin, but on Brian's board that one is left unconnected.
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