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Old 24th February 2012, 10:29 AM   #1
eccdbb is offline eccdbb  United States
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Default noise when cable is unplugged

I've been playing around with BUF634's, but something weird is happening. I am getting noise whenever:
1) 3.5mm cable is connected to the input, but not connected to a source
2) Headamp pico turned off, connected to the input

I am using a SSM2143 to receive the signal, in a 2x gain. It is connected to a LME49710 with BUF634's inside the loop. When I had 1 buffer installed, the noise was small, and after I put in the 2nd buffer, the noise became louder. Does anyone know why the noise happens?
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Old 24th February 2012, 11:21 AM   #2
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Hi

can you post a schematic.
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Old 24th February 2012, 12:03 PM   #3
eccdbb is offline eccdbb  United States
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It's simple enough to describe:

SSM2143 in 2x gain -> unity gain LME49710 with 2 BUF634's inside the loop -> 2.5 ohm and 1nf zobel in parallel with the load. Each chip has either .1uf, .47uf, or 1uf capacitors from power supply rails to ground. There's also 100uf caps from rails to ground.

Power supply is +/-15v SMPS -> +/-12v linear regulators.
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Old 24th February 2012, 12:18 PM   #4
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Does the noise go the instant the SMPS is switched off.

Excuse my ignorance why do you want 2 BUF634s in the loop.

Last edited by epicyclic; 24th February 2012 at 12:27 PM.
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Old 24th February 2012, 11:35 PM   #5
Minion is offline Minion  Canada
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Doesn"t having a cable not connected to a source just act like an antenna that picks up noise ?? I know with guitar amps not having your guitar plugged into the cord causes an evil noise , I believe that is why most guitar amps inputs have jacks that short the input to ground when a cable isn"t plugged in .......
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Old 24th February 2012, 11:36 PM   #6
eccdbb is offline eccdbb  United States
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The noise fades away.

I'm in the process of figuring out Iout vs. required loudness. Apparently 1 buffer is good for close range listening, but I want more.
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Old 25th February 2012, 11:40 AM   #7
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Im inclined to agree with Minion....... however you have got a lot of gain available but your not using it , just 6dB, so on paper youd think it would be quiet . The thought that i do have is the very wide bandwith your setup has the potential for . The wider the bandwith the more noise you will have . The balanced i/p screen is earthed at the i/p connector ?
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Old 25th February 2012, 04:17 PM   #8
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I assume there isn't a volume pot on the input? If not, try putting 100k resistors from the inputs to ground.
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Old 25th February 2012, 04:28 PM   #9
GloBug is offline GloBug  Canada
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Yes I agree with Minion, its normal.
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Old 28th February 2012, 03:36 AM   #10
eccdbb is offline eccdbb  United States
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There's no volume pot at the input.

I tried connecting the inputs to ground via 100k resistors, but that didn't help. What I did find out is that connecting the inputs together (by shorting or using a resistor) decreases the noise. Shorting and small resistors kill the noise; large resistors only decrease the noise a little. Also, using a transformer to couple the signal to the inputs gets rid of the noise completely. Any more suggestions?
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