Measuring A Capacitor


I have a preamp (NAD 1155) that has a hum problem. It started when I got a new turntable, so I thought it was a TT grounding problem. But no... The preamp's phono section hums every time it is selected. Even if there is no phono present. All the other inputs are dead quiet.

Anyway, a fellow 'goner thought it is a capacitor that's going 'round the bend. I'm willing to take my volt/ohm meter, and find the offending component, and replace it. All I know about capacitors is that they are measured in picofarads, and they discharge their energy in bursts, when it's requested.

Any help in this regard is much appreciated. I have a mountain of vinyl waiting to be played.

TIA

Lee
licoricepizza
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
Outstanding! Phono hum is a very common problem, as others have noted above, so I'm sure the information brought out in this thread on the various possible approaches will prove useful to others in the future. Glad it worked out.

Regards,
-- Al
An update - This morning, the hum had returned, so I retraced my doings of the day. It turns out that the cause of the hum wasn't plug orientation, like I first thought. But rather, it was airborne RF interference caused by my amp. As long as there is a piece of foil between the amp and the cables going into my preamp, everything is fine.
Excellent! Make sure the foil is grounded and it will make an excellent EMI/RFI shield (or even a Farady cage if you're so inclined).