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January 15, 2009 12:59 AM

(Electronics|Stereo)filter: What is causing this infuriating noise in my preamp and how do I fix it? Links to sound files and long list of symptoms in extended.

I have this preamp and ever since moving to my current apartment in San Francisco, it generates a really loud noise that sounds like a combination of 60 Hz hum and FM radio broadcasts. The noise is loudest when the volume knob is turned all the way down, but it doesn't go away when you turn the volume up. And this is what it sounds like when you go from zero gain to full gain and back. The noises don't sound that loud when played back on a computer, but that level of noise is pretty dang loud when sent to the input of a power amp.

The maybe pertinent facts (numbered because there are a lot of them):

1. I recorded the linked sound files by plugging in the output of the preamp into the line in of a MacBook Pro. No mixing or diddling with the file other than that.

2. You can clearly hear the station identifier for KOIT in the first sound file I linked. This is an FM station. What the heck? I was under the impression that FM was way too complicated for equipment to randomly pick up unless it was designed specifically to, you know, receive FM radio.

3. Changing the vacuum tubes inside the preamp doesn't change the nature of the noise.

4. I have other audio equipment (both solid-state and tubed) that doesn't make this sort of noise.

5. The noise is generated independent of what I have connected as an input or output (or if there's anything connected at all).

6. I can't seem to reproduce this noise outside of my apartment (friends' places, hifi shops, etc). Whenever I try, the preamp is noiseless and dead silent.

7. Physically moving the preamp around my apartment doesn't seem to change the noise either.

8. I live in an old apartment without grounding in most of the electrical sockets, but the noise doesn't change if I plug the preamp into a grounded outlet. I've checked that the grounded outlets in my apartment are indeed grounded with one of these devices.

9. I live sort of near Sutro Tower (but who in SF doesn't?). Wikipedia tells me that KOIT broadcasts from Sutro Tower. I also live right next to the N Judah MUNI line. Sometimes my lights dim when it goes by. I suspect the power is probably kind of dirty.

10. Using a different power cable doesn't do anything.

11. For what it's worth, here's a look at the circuitry inside the preamp.

I've been investigating this problem on and off for the last year or so and I still haven't figured out what's causing the problem. Do any electrical engineer types have an suggestions for further testing or solutions to this problem? I have a digital multimeter and oscilloscope at my disposal for the collection of further data.

I'm quite attached to this preamp (more so when it isn't making this noise), so please don't tell me to replace it.

posted by strangecargo to technology (6 comments total)

The fact that it only happens in your building sounds possibly like a line-noise problem. This seems like a long shot, but perhaps someone in your building is piping one of their speaker outputs to ground? Perhaps they are using an audio-over-electrical-wiring device to send music from one room to another?
posted by zippy at 1:09 AM on January 15, 2009

since it's 60hz, I suspect the power as well- can you get your hands on an isolation transformer? Tripp Lite makes several, for around $100. They may not solve the problem, so try and get it somewhere that you can return it if it doesn't do it for ya.
posted by jenkinsEar at 4:23 AM on January 15, 2009

Can't listen to the sounds here, but my guesses

1) As mentioned, power line noise. If you have a UPS, you can use that to isolate from the mains -- plug the preamp in, unplug the UPS from the wall, see if the sound changes.

2) Bad ground in the amp. The easiest way to test is a multimeter, make sure the chassis is grounded by putting a meter across the chassis and the ground prong on the power cable (unplugged, obviously.)

3) It looks like there's a poly cap between the chassis and the ground pin -- hard to see the orientation. It's in the upper right, behind the power transformer and above what's either a power line choke or a resistor. That cap is supposed to short high frequency noise to ground, and I'm wondering if it's bad. Only way to truly test is to pull it out and meter it with a capacitance meeter.
posted by eriko at 5:27 AM on January 15, 2009

I put the sweep on into audition, and did a spectral analysis on it.

Shows a peak right around 15800 or so. That's definitely a television of some kind. Try unplugging your TV.

The hum doesn't show very well, but it's a 60hz square wave.

The radio thing is weirder- my guess is that something in your setup has a resonance that matches the frequency they broadcast on. Or a harmonic of that frequency. Or maybe a harmonic of the frequency they (might be) using to send their studio signal to the transmitter.

What condition are your signal cables in? Have you eliminated them as a possible source? Maybe one of them has a busted shield, turning it into an antenna. Check all the grounds. Even inside the box- maybe you've got a cold solder joint that's making it unshielded.

Strip the setup down to nothing. Plug the box into the wall, and headphones into the box. Nothing else. Can you still hear it? That eliminates a lot. Next would be to try a different circuit in the house, or plug it into a UPS, if you have access to one. See if there is a difference between when the UPS is plugged into the wall, and if you unplug it, simulating a power failure. Or, does the preamp have an option to be battery powered?
posted by gjc at 5:49 AM on January 15, 2009

Tube gear sometimes picks up radio. I've had guitar amps that do that. It's pretty location dependent, not so much within an apartment but the apartment's proximity to the station. You could put a grid stopper resistor on the first preamp stage (read about them here). That link does mention AM but I think I've experienced it with FM, too. Not entirely sure on that one. It will roll off your high end so you want to use the lowest value you can get away with to stop the noise. Also, it should be mounted as close to the tube as possible, preferably soldered directly to the appropriate pin on the socket.

That's what I'd do if it was out of warranty. You may want to contact the manufacturer.

The 60 Hz hum/buzz is almost always caused by a ground issue: bad ground, ground loop, etc. If in doubt, check your grounds. It can be pretty frustrating to troubleshoot, fwiw.

If possible, I would test it at a friend's place that lives near you but not in the same building.
posted by 6550 at 9:22 AM on January 15, 2009

I've done some investigation with a scope and have discovered that at least some aspect of the problem is due to line noise. The I suspect the mains hum/buzzing in the samples is actually 120Hz noise:

Here's a shot of my AC power superimposed over the output of the preamp (driving the line-in of my laptop).

Close up of the top of the sine wave.

And the bottom of the sine wave.

@eriko: Haha. Hilarious results with my UPS. The noise got worse (for obvious reasons).

Since I know what's causing the buzz/hum now, I'm going to look into eliminating that aspect of the noise, then move on to the RF interference. If anybody's still paying attention to this question, any suggestions on where to go from here?
posted by strangecargo at 10:24 PM on January 16, 2009