Measuring A Capacitor

Here's what I did: I used shorted RCA cables on the phono input - no discernible hum. Then I used tin foil (as a shield) between the phono cable and the power amp power cable - hum still present. Then I went wild with the tin foil, and put a few 2" squares between the phono input, and the adjacent input (tuner - unused) on the preamp - buzz still present. Then I put a sheet of it between the amp, and the preamp - buzz still present. So, either tin foil is a bad shield material, or the problem is in the preamp. Here's why I say that: the hum can be reduced, or eliminated by my touching the faceplate of the preamp. It's possible that when I did the shorted RCA plug test, the hum was gone anyway.

FYI, the TT I am now using is an MMF 2.2 LE, with an MMF Tracker cartridge (Goldring Elektra) moving magnet type. It has a 2 pronged plug. The same can e said of the NAD preamp. The amp, which is a very recent addition, is a B&K Sonata ST-140, with a detachable IEC, 3-pronged plug/cable. My old Carver M-500T was put back in place as a test - still buzzed. I used to have an old Denon DP37F in the place of the MMF. It had a Sumiko Blue Point MC on it's tonearm (and it never buzzed).

What do you think my next move should be, given all that?

Yours, in vinyl,

Lee

Most problems of this type are not caused by pickup of electromagnetic fields through the air, which would be potentially helped by shielding (if it is properly grounded).

They are more often caused by differing ground potentials between the chassis of the components that are involved, which causes extraneous currents to flow in the shields of the interconnects between them (which are connected to chassis, and which serve as the return path for the signals the interconnects are carrying, resulting in the extraneous currents being seen as signal by the load device). That will only be helped by foil if it provides a path between the components that is both complete and of lower impedance than the shields of the interconnects.

With both the TT and the preamp having 2-prong plugs, and ac safety ground therefore not being connected to the chassis, they both have chassis that are pretty much floating with respect to ground. If they happen to float to different levels, it would result in the extraneous current flow that I spoke of. When you touch the faceplate, you affect the level to which the preamp chassis floats.

So I would again suggest trying all possible combinations of ac plug polarities (which affects the levels to which the components float). Also, try connecting a wire between the preamp chassis and ac safety ground (the screw at the center of the ac outlet cover). Also, the other things I mentioned in my last post that have not yet been addressed. And seeing if anything can be done to improve or shorten the connection between preamp and turntable ground.

HTH,
-- Al

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