Measuring A Capacitor

You are chasing the wrong problem with the aluminum foil. You have a grounding issue. They can be very frustrating and confusing as evidenced by the fact you touch it and it goes down. I was once trying to track down a hum problem by using a clip lead to connect various points on various pieces (preamp, power amp, phono stage, turntable) and after having no luck I clipped one end on the preamp with the other end hanging in space just as place to put it while I thought about it and the hum went way down. This was very repeatable and I called in others to make sure I wasn't insane.

Almarg gives good advice on things to try but his paranoia about static being amplified 10,000 times is just that, paranoia. In order for static to jump you need quite a few volts so you aren't going to get quite a few times 10,000 volts out of your system. At worst you will get a very, very, very brief pop equivalent to a very, very, very, brief clipping of the amp. However, it could be a problem because that static discharge into the input could damage the transistors it hits, so his advice is once again good even if the reasoning why is flawed.

Almarg gives good advice on things to try but his paranoia about static being amplified 10,000 times is just that, paranoia. In order for static to jump you need quite a few volts so you aren't going to get quite a few times 10,000 volts out of your system. At worst you will get a very, very, very brief pop equivalent to a very, very, very, brief clipping of the amp. However, it could be a problem because that static discharge into the input could damage the transistors it hits, so his advice is once again good even if the reasoning why is flawed.

Please re-read my post. I said nothing about static jumping, in the sense of a spark such as might occur if a static-charged person puts his finger near a grounded light switch. I also said nothing about 10,000 volts.

I referred to the static charge being directly conducted into the phono input, through a conductor (the foil). And obviously any significant discharge (greater than the few millivolts that the cartridge would normally put out) would clip the system before the full 80db gain (10,000x) were realized, but that is beside the point. The point is that there would be a very large transient put into the system, albeit a very brief one (as you stated). And its very brevity could very conceivably exacerbate the problem, because the consequent high frequency spectral components would cause the transient to be routed by the speakers' crossovers to the tweeters, which are much more vulnerable than the lower frequency elements.

I certainly can't prove that damage would occur, or even that it would be likely, but it would certainly seem prudent to turn the system off.

Regards,
-- Al

Please reread your own post. You did say it would be amplified 10,000 times. And it would not be "a very large transient." it would be a slight pop that clipped it for a minuscule amount of time. Look, it happens all of the time in the winter. I walk across the room to change the record and when I touch the tonearm I feel a discharge which creates a pop in the speakers. Over and over and over and over again. The tweeters still work just fine.

please reread my post. You evidently don't understand electrostatic discharge. You don't have to have a spark like the kind that jumps from your hand to the light switch for a discharge to occur, Most are never felt. For it to occur it would have to at some point jump the gap. When it did, however small it was, it would drive the system to clip for a instant. There is no way in the world that a very brief pop from the system would take out a healthy tweeter. It is however not unlikely that a static discharge through the input devices could damage one of them and that is the point that we both missed earlier. As a matter of fact, turning off the system would do nothing to protect those input devices. The static discharge through them would be the same no matter if the system was on or off. The only protection in that case would be to follow safe ESD practices.

You seem to be one of these people who loves to give advice but can't stand it when they are offered it. I stood corrected and admitted that my advice was bad because a static discharge into the input could damage the system by damaging the input devices. Suck it up and admit that this same pop would never take out a tweeter.

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You did say it would be amplified 10,000 times.

I neglected to say the obvious, which is that it would be less if the system were clipped, which will most likely happen.

And it would not be "a very large transient." it would be a slight pop that clipped it for a minuscule amount of time.

It would be very brief, that has nothing to do with its amplitude, which is what the phrase "very large" refers to.

I walk across the room to change the record and when I touch the tonearm I feel a discharge which creates a pop in the speakers.

The static is not going directly into the signal path, as it would be in the scenario I hypothesized. Big difference!

There is no way in the world that a very brief pop from the system would take out a healthy tweeter.

Let's say that it was so brief that it lasted only 50 microseconds (a single period at 20kHz). We are both agreed that its amplitude could be full power, to the clipping level of the amplifier. Say that is 100 or 200 watts. Will a single cycle of 20kHz at 100 or 200 watts damage a tweeter? I don't know, but it certainly sounds like a cause for concern. It certainly will not damage it via thermal effects, but the concern would be the over-excursion it would result in. If you are certain that it will not harm the tweeter, please provide a link to some documentation or technical rationale that is more persuasive than your simply saying it will not.

You seem to be one of these people who loves to give advice but can't stand it when they are offered it.

If you knew me better you wouldn't say that.

Basically, this thread has become unpleasant because you chose to describe my "err on the side of caution" advice by using the word "paranoid," twice. The fact that you said what could easily have been said in a constructive manner, in an inflammatory and disrespectful manner instead, would seem to say something about what kind of person your are.

You'll have the last word, because I don't intend to devote any further time to this utterly pointless and unnecessary argument.

-- Al

The hum is finally gone! The ultimate fix was a simple one. All I had to do was flip the preamp plug 180 degrees. I thought I had already done that. Live, and learn I guess. Now I have about 5,000 LPs that are waiting to get played... You all know what LP stands for, don't you? Most people think it mean Long Playing, but it really means "Licorice Pizza".

Thanks to all who contributed to my affliction with vinyl.

Lee