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120 Hz noise

A

Amanda Robin

Jan 1, 1970
0
Hi all--

I am having an interesting noise problem. I have an acoustic setup
whereby I output a signal from an MX31 DSP unit, take it through a
custom made lowpass filter PCB, send it through a power amplifier, then
out a speaker.

Actually I have two speakers, so I have two setups like this in
parallel, using the same DSP unit and different channels of the same
power amp.

I am doing acoustic work and normally send a 125 Hz tone to the
speaker. I've been using this setup for about a year now with no noise
problems to speak of. (I do actually get white noise now and then, but
I can make it stop by off/oning the circuit boards.)

Recently the system has gotten very sensitive to static electricity. If
I scuff my feet on the carpet, I can hear an additional tone going
through one of the speakers (not the other). It sounds like a
vibration, but it is an electrically-based noise.

I have taken the the SPL of the output of the speaker and the noise is
120 Hz. I've looked at the signal from the PCB and performed an FFT on
it, and the 120 Hz appears to be there.

I have traced my wires and made sure they aren't close to power cables.
I've checked my connections and grounds, etc. But here's the clincher:
the noise is not present when I don't play a tone through the system.
It stops as soon as I turn the tone off (even if I walk around and
scuff my feet).

Oh, and the noise is present when I send tones of different frequencies
through, down to about 70 Hz. I haven't tried finding the upper limit,
if there is one.

I am going to do some comparison of the signals since my other channel
is noise-free so far. I just wondered if anybody had any bright ideas
about this (besides waiting for wet weather....).

Thanks,
Amanda
 
J

John Fields

Jan 1, 1970
0
Hi all--

I am having an interesting noise problem. I have an acoustic setup
whereby I output a signal from an MX31 DSP unit, take it through a
custom made lowpass filter PCB, send it through a power amplifier, then
out a speaker.

Actually I have two speakers, so I have two setups like this in
parallel, using the same DSP unit and different channels of the same
power amp.

I am doing acoustic work and normally send a 125 Hz tone to the
speaker. I've been using this setup for about a year now with no noise
problems to speak of. (I do actually get white noise now and then, but
I can make it stop by off/oning the circuit boards.)

Recently the system has gotten very sensitive to static electricity. If
I scuff my feet on the carpet, I can hear an additional tone going
through one of the speakers (not the other). It sounds like a
vibration, but it is an electrically-based noise.

I have taken the the SPL of the output of the speaker and the noise is
120 Hz. I've looked at the signal from the PCB and performed an FFT on
it, and the 120 Hz appears to be there.

I have traced my wires and made sure they aren't close to power cables.
I've checked my connections and grounds, etc. But here's the clincher:
the noise is not present when I don't play a tone through the system.
It stops as soon as I turn the tone off (even if I walk around and
scuff my feet).

Oh, and the noise is present when I send tones of different frequencies
through, down to about 70 Hz. I haven't tried finding the upper limit,
if there is one.

I am going to do some comparison of the signals since my other channel
is noise-free so far. I just wondered if anybody had any bright ideas
about this (besides waiting for wet weather....).

---
Since it's 120Hz, I'd suspect that something has gone wrong in your (the
amp's ;^) power supply and that what you're hearing is ripple which is
occurring when the filter caps are being loaded and their voltage
valleys are falling below where they should be. More than likely it's
one or more rectifiers which have gone bad or one or more of the caps
themselves...
 
J

JeffM

Jan 1, 1970
0
the noise is 120 Hz.
Amanda Robin

Lets get the obvious out of the way:
How do your (full wave rectified?) supply rails look?
 
R

Ross Mac

Jan 1, 1970
0
Amanda Robin said:
Hi all--

I am having an interesting noise problem. I have an acoustic setup
whereby I output a signal from an MX31 DSP unit, take it through a
custom made lowpass filter PCB, send it through a power amplifier, then
out a speaker.

Actually I have two speakers, so I have two setups like this in
parallel, using the same DSP unit and different channels of the same
power amp.

I am doing acoustic work and normally send a 125 Hz tone to the
speaker. I've been using this setup for about a year now with no noise
problems to speak of. (I do actually get white noise now and then, but
I can make it stop by off/oning the circuit boards.)

Recently the system has gotten very sensitive to static electricity. If
I scuff my feet on the carpet, I can hear an additional tone going
through one of the speakers (not the other). It sounds like a
vibration, but it is an electrically-based noise.

I have taken the the SPL of the output of the speaker and the noise is
120 Hz. I've looked at the signal from the PCB and performed an FFT on
it, and the 120 Hz appears to be there.

I have traced my wires and made sure they aren't close to power cables.
I've checked my connections and grounds, etc. But here's the clincher:
the noise is not present when I don't play a tone through the system.
It stops as soon as I turn the tone off (even if I walk around and
scuff my feet).

Oh, and the noise is present when I send tones of different frequencies
through, down to about 70 Hz. I haven't tried finding the upper limit,
if there is one.

I am going to do some comparison of the signals since my other channel
is noise-free so far. I just wondered if anybody had any bright ideas
about this (besides waiting for wet weather....).

Thanks,
Amanda
Same question here...Put a DMM on those rails and what does the VAC look
like....should be almost nothing...
Or better yet...got a scope?....whats the waveform....
later....Ross
 
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