Measuring A Capacitor

OK, I did the shorted IC exercise, and there was no hum. What does that tell me?

It says that the problem is not the preamp! It is outputting hum because hum is going into it. There are many threads at Audiogon on phono hum problems that you can search through. A few possible causes that come to mind:

-- Your power amplifier is too close to either the preamp or the turntable or the turntable cable.

-- Lack of good grounding between the turntable and the preamp (as you mentioned in your first post).

-- The phono cable being in close proximity to ac power cords, or to the power amp.

-- Ground loop issues between the preamp and other components, which become more apparent in phono mode than when other sources are selected because the signal levels are much lower for phono. Try disconnecting everything from the system except for the turntable, preamp, power amp, and speakers.

-- Try reversing the polarity of the ac plugs of each of those components, and/or floating them (using a cheater plug).

-- When the turntable is connected, is the hum present both when it is turning and when it is not? What kind of cartridge are you using? Grado's in particular often have a problem picking up hum from the turntable motor (when it is on), because they are unshielded. (It could be that the hum you hear when the turntable is connected is not the same hum that you heard when nothing was connected to the preamp inputs).

HTH,
-- Al

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

Yet another reason why I gave up on LPs: phono pre-amps :)

Lee, the tin foil must be grounded to be an effective shield. Simplest way to do that is to use an alligator clip from the a known ground on the (pre/integrated)amp (chassis ground lug, chassis, outside of a RCA jack,etc) on one end and then clip the tin foil on the other. That will create an effective EMI/RFI shield... which still may not be your problem.