popcorn noise in legacy amp

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My 20-year-old CT-400 amp occasionally generated popcorn noise in one of the two channels. I noted this problem on this forum prior to 2008. I traced the noise back to the second op-amp stage of the input since the noise continued when I cross wired the first op-amp stage from left to right and the noise disappeared when I closed the volume control on the back of the amp. Swapping the Motorola 2-stage op-amp chip did not change the situation.

Then my neighbour moved away 18 months ago and I have not had the problem since. My conclusion was that one of the neighbour's domestic wireless devices (wifi router or cordless phone) may have been broadcasting into my basement and then via the cabling into the Crown amp.

Another corrective measure which I undertook 5 years ago to reduce (but not entirely eliminate) the frequency of incidents of popcorn noise was to hot wire the internal fan on 24/7 in order to maintain a lower chassis temperature when idling.

Now that the noise appears to be permanently gone, I am thinking about removing the fan bypass jumper wire in order to reduce wear and tear on the fan and create a quieter background in the home theatre room.

If the popcorn noise stays completely away without the fan operating continuously, then I will conclude that the neighborhood wireless transmitters had something to do with the popcorn noise.

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Simple logic... If lowering the amps internal temperature helps then it cannot be only your neighbors fault, can it?

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What I remember doing a few years ago was swapping the quad op-amp chip between channels and the problem did not move.

Then I cross wired the output of the 2nd op-amp (right channel) of the quad to the input of the 3rd op-amp (left channel) of the other quad and the problem did not move.

That told me that the problem was in one of the first two op-amps stages of one particular channel and was not related to a particular quad op-amp chip.

Since the Crown CT-400 is basically a DC amplifier, there are no de-coupling electrolytic capacitors between stages.

Whatever few bypass electroylitics there are on the main board, I replaced these without improvement.

That left me with all the low wattages carbon resistors and ceramic capacitors and the volume control surrounding the first two input stages as being a potential source of the thermo-sensitive pop-corn noise.

My conclusion is that perhaps the passive resistors and capacitors in the first two stages are causing the op-amp chip to oscillate at RF frequency and the popcorn noise is a by-product of the oscillation.

I cannot prove that the cataylst for the noise is my former neighbour's wireless signal, but if my Crown amp was designed in the 1980's before the introduction of domestic 2.4 giga-hertz RF signals, then it is possible that the original circuit has no protection against this.

Again, we will see if my restoring the fan's original control thresholds causes the popcorn to re-appear. Thanks for you comment.

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Then I'd start looking at what was on the power circuit with me that would come on at apparently random intervals: refrigerator, freezer, air conditioner, fluorescent lights, dimmer switches, and anything else that could cause a quick power spike or loss in the circuit your amp is plugged into.

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I don't think the noise is activated by a spike or dropout on the power line. The internal power supply of the CT-400 is very robust and regulated and I doubt if a dropout would get through. There are varistors added inside the electrical panel and the amp is fed by current passing through a computer power director which contains additional varistors.

The amp has been in the basement since 1993. In 2002 it was placed in a rack inserted into a partition wall. The room temperature and humidity of the basement vary throughout the year, but the noise was more prevalent in summer.

The amp is left on 24/7 and feeds speakers in the basement and two other ground floor rooms. including the master bedroom.

I have been awoken from my sleep in the middle of the night by the bedroom's right side speaker starting to emit the popcorn noise.

If I shut down the amp and restart it imediately, the noise returns.

If the noise is precipitated by a neighbor's wireless device, I can't explain why such a device would be operated at 3 AM in the morning.

It may be a coincidence that the noise stopped when my neighbor moved away.

It may be that keeping the chassis as cool as possible by having the chassis fan stay on permenantly is the reason the noise has stopped occurring.

Today I removed the patch wire that was keeping the fan on all the time. The basement room is now very quiet compared to before.

It may be months from now before the same conditions repeat themselves which lead to the noise returning.

Thanks for your comments.

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