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From: firstname.lastname@example.org (Adrian Black)
Subject: Cracks and pops on audio Op-Amp
Date: 7 Dec 2002 18:20:50 -0800
NNTP-Posting-Date: 8 Dec 2002 02:20:50 GMT
My friend recently gave me a 4-channel Kenwood car amplifier with one
channel not working. It uses a decent design with a switching power
supply, two NEC uPC4574C quad bipolar op amps, a uPC4570C dual op amp
and 4 LM391's to run the output transistors. After looking at the PDF
of the quad op amp, it seem to be similar to the LM837N. Low noise,
high slew, etc.
Testing with my scope show three channels work well but one channels
is filled with noise. Not humming, but crackling and poping.
After poking around with my scope sending a 1khz signal through it, I
find that the noise appears at the output of the first op-amp the
channel hits. The signal then goes to "Line-Out" connectors on the
unit and back into the same IC to a second op-amp.
But I'm at a loss to what could be causing the noise. I desoldered the
connection to the "line-out" thinking it was somethng downstream
causing the noise, but that had no effect.
Could the chip itself be bad? Or would it more likely be one of the
resitors, diodes or capacitors? All of those visually seem ok. I
resoldered all of their connections to make sure no cold joints were
I thought the power supply might be noisy (+ and - 15v) but all three
other channels including the other "half" of this particular IC work
perfectly. I desoldered the +15v feed into the chip and when looking
at the power, it was totally clean and steady.
When I removed power from the chip, the noise went away. Somehow audio
was going all the way through the amplifier and conncting a speaker
confirmed a nice steady tone. Coneection the power again brought the
gain up and all the noise right back. The noise happens wether a
signal is present or not.
Looking at the input into the inverting input on the op-amps shows an
oscillator...? (it's what is looks like on thte scope and all the
other stages look the same.) The invering input is connected to the
output through a resistor as well as the non inverting input through a
cap and ground through a resistor and cap. It also looks like the
other channel going through the amp has the same capacitors, diode and
resistor as the bad channel.
Anyway, if you guys have any ideas for what I should try, I'de love to
hear them. I was thinking of ordering a LM837N and installing it as I
can't seem to find the NEC part. I just wonder if it's possible the
noise could be coming from the chip itself ...?
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