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Old 11th March 2003, 11:48 PM   #1
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Default Need help with IC Based Amplifier "pop" problem (Integrated Component)

Panasonic SU-CH9 Integrated Amp

This is an IC based bi-amped integrated amp. Circa 1991. The left high frequency channel produces a "pop" during certain high frequencies when the unit volume is at least -35db or louder. At first, I thought possibly a short was occurring in a trace when the left high frequency channel hit a certain voltage level. However, I can find no evidence of this (flashover or charred remains). I am also doubting this is the case because it is very consistent ("pop" does not sound differently, always the same). I have one CD with a drum stick hit (on wood) that causes the pop to occur at that instant and not before or after. So it "pops" along with each hit of the stick. If I turn the volume up very loud, I can get a series of pops to occur in a row (I did not do this intentionally).

The sound reminds me of shorting a speaker to ground. Speakers have been eliminated as the problem by swapping speakers. I have also driven the right channel with the same problem causing source and the pops are not reproduced in the right channel.

I've come across another post that mentioned a popping sound occurring with bad ceramic caps. Is this the culprit or should I be looking for something else? clipping? Any help in isolating the cause is greatly appreciated.

TIA!
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Old 12th March 2003, 01:19 AM   #2
SY is offline SY  United States
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It might be a coupling cap, not necessarily ceramic, or it might be something else. Do you have a scope? That will tell you if the problem is an oscillation and from which stage the pop emanates.

When you say the problem occurs when the volume is at -35 dB or louder, are you referring to a specific setting of the volume control? Does that pop only happen when you've got signal running through the amp? On all inputs?
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Old 12th March 2003, 02:44 AM   #3
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It might be a coupling cap, not necessarily ceramic, or it might be something else. Do you have a scope? That will tell you if the problem is an oscillation and from which stage the pop emanates.

When you say the problem occurs when the volume is at -35 dB or louder, are you referring to a specific setting of the volume control? Does that pop only happen when you've got signal running through the amp? On all inputs?


Sy,

Thank you for the reply!

Unfortunately, I do not have a scope. The -35dB or louder is a specific volume setting. This unit has an LCD dB readout. The pop only occurs in the high frequency left channel with the unit on and music playing and when certain transients occur. Below this volume setting, the drum stick on block hits will not trigger the "pop". That is why I assumed it has something to do with the voltage level on that particular trace causing a short to ground (I could be way off). It does occur on all left channel inputs.

I again assumed that since the input signal is filtered high and low prior to the dual IC amp output stage, that it occurs somewhere after the filtering for the high stage. I "feel" that because the sound is similar to a shorting speaker pop and it does not seem to be distorting the music (more like "added to the music" or over the top of it), that it is occurring after the high frequency amp chip. However, the only components in the left output signal path after the high amp is a 100uF bipolar coupling capacitor, a 1k resistor to ground and a serial 4R7 resistor & 0.047uF cap (ceramic) to ground. I swapped out the ceramic cap with no luck and verified the resistance of the 4R7 resistor. Would there be any problem of shorting the bipolar coupling cap as a quick check (jumpering with a test lead) to determine if that is the problem? (getting to the bottom of the board takes some time).

Thanks again!
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Old 12th March 2003, 03:47 AM   #4
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The LCD comment is interesting. Does this use a digital potentiometer for volume/balance?
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Old 12th March 2003, 11:45 AM   #5
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Quote:
Originally posted by SY
The LCD comment is interesting. Does this use a digital potentiometer for volume/balance?

My mistake, it is actually a fluorescent display, not liquid crystal. The volume is controlled by a TC9212P IC chip.
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Old 12th March 2003, 01:50 PM   #6
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Well, that might be one more place to look. I think you're wasting your time trying to find an arc- the signal level is just too low.

See if you can trace the wiring well enough to separate out the preamp/control section from the power amp section. Then run the L preamp to the R power amp and the R preamp to the L power amp. Note whether the problem switches channels. That will cut the possibilities in half.
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Old 12th March 2003, 03:07 PM   #7
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Thanks again!

Good tip on switching channels. I can switch the L & R just before the high amp. If the noise stays with the left side, I will switch the high amp output L & R. This should narrow it down to the amp if defective or the output section of the left channel.

It may be this weekend before I will have time to tear it down again. The bottom main circuit board is not panel accessible, so the entire unit has to be broken down. I will solder in test leads, so I can switch channels from above board for the various stages.

I will post again after trying this. Thanks!
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Old 13th March 2003, 03:01 PM   #8
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SUCCESS!


SY,

Thanks again for your help. I could not wait for the weekend, so I crossed the preamp outs last night. This did not eliminate the sound from the left channel. Therefore, the problem was isolated to the amp and output stage. I swapped both 100uF output coupling caps, but this was not the problem either. This narrowed things down mainly to the amp itself. Upon close inspection, the solder joints at the amp pins looked suspect. I reflowed all of the pins on the high amp. So far no more "pop"! I've learned enough on older stuff to know that sometimes my direct actions are not what fixes things (sometimes you indirectly reseat, bend or move something inadvertently that fixes things at least in the short run.) But, for now, things seem to be working correctly!

I did notice that there are about 8 to 10 coupling capacitors in each channel. Can these be removed safely if there is no DC voltage drop across the coupling cap while the unit is powered on but no input signal? Is there a "minimal" amount of DC drop that is acceptable? I am assuming each stage has a coupling cap due to each stage being designed by a different engineering team, each wanting to ensure that their stage is robust despite what the other teams produce. Or is the DC on the signal line due to active devices occasionally creating DC voltage components on their outputs?
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