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From: Larry Brasfield
Subject: Re: Cracks and pops on audio Op-Amp
Organization: Serendipitous Endeavors
X-Newsreader: MicroPlanet Gravity v2.50
Date: Sat, 7 Dec 2002 18:41:04 -0800
NNTP-Posting-Date: Sat, 07 Dec 2002 20:40:58 CST
In article ,
Adrian Black (email@example.com) says...
> 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?
Yes. Excess noise of that nature is one of the
ways that static damage can be manifested. Also,
manufacturing flaws can degrade a transistor in
the IC causing that sort of noise.
> Or would it more likely be one of the
> resitors, diodes or capacitors?
Less likely. If it was one of those, applying
some cold spray (gently!) will make it worse or
better, revealing the culprit. If cold spray
has no effect on the noise, I would absolve
> All of those visually seem ok. I
> resoldered all of their connections to make sure no cold joints were
> causing trouble.
> I thought the power supply might be noisy (+ and - 15v) but all three
> other channels including the other "half" of this particular IC work
I agree with your logic for absolving the supply.
> 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.
That is at least consistent with the opamp being
responsible, but it is redundant with your other
> Somehow audio
> was going all the way through the amplifier and conncting a speaker
> confirmed a nice steady tone.
The signal was probably going thru the feedback
elements to get around that opamp.
> Coneection the power again brought the
> gain up and all the noise right back. The noise happens wether a
> signal is present or not.
Yes, generated noise is usually additive.
> 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.)
Stage oscillation could also lead to noise, but
as you've described it, (being present whether
signal is or not), it does not appear to be an
oscillation problem. The oscillation (if that's
what it is) may be another manifestation of a
bad opamp. I would say don't worry about it
unless it survives replacement of the chip.
> 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.
It would be very odd for the two channels to
be implemented with different circuit designs.
To me, this mainly confirms that you are not
confused about which stage is which.
> 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 ...?
Yes. I think you've pretty much nailed it.
Replace it after the cold spray (or ice).
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