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Old 12th February 2013, 07:17 AM   #1
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Default HK 330A channel imbalance

So I've finished recapping my garage amp, a Harman Kardon 330A. I probably wouldn't have noticed previously, due to the speakers I was using, but there is an imbalance between the channels, and it seems to be mostly in the bass. I first noticed it visually when I was testing it with some KLH model 5's that I also just overhauled. The woofer excursion on one was about 4 or 5 times that of the other.

I can see that this happens before the power amp, by switching which preamp goes to which power amp. I just got an oscilloscope, so I started poking around with that and I think, not sure, that it's happening in the tone board. I re-cleaned the bass pot, made no difference and it seems really clean on the scope.

Tonight I got a second probe for the scope and looked at both speaker outputs side by side, and confirmed that the difference is greatest in the lower midrange and bass. Then I tried adjusting the bass per channel, since it has split tone controls. Two things were interesting. One, when I adjusted the bass tone control away from center, it seemed to shift the phase of the sine wave I was feeding it. That was a surprise. Secondly, it seemed like a non-linear thing, i.e. I'd set it to equalize the curves at one frequency and it would be different at another. By observing this on the scope while I swept the freq., it was implied to me that the bass control centers on around 400 Hz or so, if that seems right.

Anyway, my question is should I assume it's a problem with the pot, and should I just replace it? Or keep diggin? Or any other ideas??
thanks
-Mark M.
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Old 12th February 2013, 07:35 AM   #2
Mooly is offline Mooly  United Kingdom
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Tone controls do alter phase shift, that's normal.

Best test (no speakers connected) is a low level squarewave as that will instantly show problems. With the bass and treble centered the squarewave should be unaltered. Any distortion shows a problem.

It should be easy to trace with a scope and working from input to output.
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Old 12th February 2013, 02:36 PM   #3
djk is offline djk
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Just replace all the small electrolytic caps and it will probably work OK, they're all dried out after 40 years now.
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Old 12th February 2013, 04:44 PM   #4
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Just replace all the small electrolytic caps and it will probably work OK, they're all dried out after 40 years now.
Just replaced every electrolytic in the unit
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Old 12th February 2013, 04:57 PM   #5
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Quote:
Originally Posted by Mooly View Post
Tone controls do alter phase shift, that's normal.
Surprised, I guess that's one good reason folks like to eliminate tone controls...
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Originally Posted by Mooly View Post
Best test (no speakers connected) is a low level squarewave as that will instantly show problems. With the bass and treble centered the squarewave should be unaltered. Any distortion shows a problem.

It should be easy to trace with a scope and working from input to output.
I'm using a smartphone app for a signal generator, will see how clean the square wave is and if it's decent, I'll try following the signal path. I know it's somewhere between the input jacks and the preamp output. Prime suspect right now is the bass tone pot, although it really doesn't generate any noise when turning it, even on the scope.

The other wildcard is I picked up this old Philips scope for $30. and while it's straightforward enough to figure out, it could be contributing its own distortion as well...

Will update after next excursion, thanks. Oh, last question - is it a given that the channel with lower bass is the one with the problem, or could it equally be that one channel has an overemphasized bass response?
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Old 12th February 2013, 05:00 PM   #6
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Originally Posted by Mooly View Post
Best test (no speakers connected) is a low level squarewave as that will instantly show problems.
p.s. I'm imagining my workflow would be feeding my signal generator into one trace and then using the other probe to poke around and look for distortion, yes?

I went through it once trying to figure out where the level was dropping but that was a bit too cumbersome for me to do comparatively between channels.
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Old 12th February 2013, 06:09 PM   #7
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If the bass is lacking compared to mid and hf then a squarewave test will show a problem and it will look like in the picture. As the frequency is lowered the effect is more pronounced.

Which channel is the faulty one The one that sounds obviously wrong. If the other had to much bass then the one you are looking at would be "neutral".

You mention the pots... prime suspects tbh. They can get damaged and have a crack in the resistive track.

If you use something like a 100hz squarewave and set the bass and treble to mid point, then the amp should pass the signal untouched.

Once you've confirmed that the signal is reasonable from whatever source you are using, then you just use one trace and follow the signal through the amp.

What about using something like "Audacity" to generate a squarewave? Save it as an MP3 file and burn to CDR.
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Old 12th February 2013, 06:16 PM   #8
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If the bass is lacking compared to mid and hf then a squarewave test will show a problem and it will look like in the picture.
Which picture?
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What about using something like "Audacity" to generate a squarewave? Save it as an MP3 file and burn to CDR.
Yeah could do that, will see what the output of my phone app looks like and compare it with audacity. Thanks Mooly.
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Old 12th February 2013, 07:14 PM   #9
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Which picture?

Yeah could do that, will see what the output of my phone app looks like and compare it with audacity. Thanks Mooly.
Well spotted... a prawn sandwich took priority

As you boost and cut the bass you should see something like this happen to the squarewave. Try it on the good channel too. Depending on the type of circuitry it may look more of a "hump" or "saucer shape" as altered but you get the idea.
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Old 12th February 2013, 07:56 PM   #10
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Cool! That's kind of like a circular-saw-tooth waveform So there will be less of this effect on the good channel yes?
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