Preamp hum- Learn from this fool

[Conrad Hoffman]

My diy phono preamp, which I'm quite happy with, showed a 60hz spike in the spectral analysis, about 70dB down referred to 0dB tracks on an STR-100 test record. Plus a couple related harmonics. This bugged me, and I had used one of those little potted self contained +/- 15V power blocks for the power supply (not a switcher). Somehow I convinced myself that the problem was related to the magnetic field from the transformer, as I tried all manner of grounding and shielding, and probing the ps brick with a loop pickup showed strong fields. For the brief time the preamp would run with the line cord pulled, the 60hz spike went away.

Ok, so I found a nice toroid of the right voltage in my junk box (I have a very fine junk box), and designed up a supply board. Nothing fancy, just well implemented LM317 and LM337 regulators with a high speed well bypassed bridge. Laid it out, etched the board, built the board, tested the board, installed the board, and hooked everything back up. Can you guess? The 60hz spike moved from -70dB to -75dB, with all the same harmonics still present.

Then in an act of wild unpredictability, I unplugged the wall wart for my turntable from the power strip that feeds the preamp. The 60hz spike dropped to -90dB, near the noise floor of the preamp. The harmonics were gone. Now, there's nothing in the wall wart except a transformer (I think). It's just a 16VAC supply (Pro-ject) that powers my newer Thorens. I moved it a foot away to a different circuit, and the 60hz stayed gone. Now I have to figure out exactly what the interference is, and how it got in. I have trouble believing it's just proximity of the transformer, but what else could it be? That's today's project.

Anyway, the morel of the story is don't go off building some fancy solution to a problem unless you know exactly what problem you're trying to solve!

[Conrad Hoffman]

And yes, it turned out to be nothing more than proximity. My preamp is on the floor near the power strip. Putting the wall wart on a short extension and moving it around proved that the circuit it was on makes no difference, only the distance from the phono section. I left it on the short extension and placed it a couple feet away. Problem solved. Now if I could just get the residual pickup from the turntable itself down near the preamp noise floor, but that's probably unreasonable. Still, it's all way below even the quietest LP pressing.