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Old 15th January 2009, 11:22 AM   #1
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Default help! annoying preamp noise

I've also asked this question on Ask MetaFilter, but I had a feeling that I'd be more likely to find a solution to my problem here, given the high concentration of circuit experience...

I have an Air Tight ATC-3 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 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 a socket tester.

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.

This noise is really loud and makes my system unlistenable... 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. Anybody have suggestions? I have a digital multimeter and oscilloscope at my disposal for the collection of further data.
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Old 15th January 2009, 12:39 PM   #2
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All I could hear was 120hz buzz. You need a line filter and probably interconnects with superior shielding. Small value film or ceramic caps across the inputs might also be required too if that doesn't get rid of the RF.

Bill
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Old 17th January 2009, 08:26 AM   #3
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Well, I figured out the cause of the 120Hz buzz. My power is filthy. This is what my scope sees when I superimpose my home AC over the outputs of the preamp (driving the line-in of my laptop):

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

It's kind of hard to tell in a still shot, but when the scope is sampling live, it's obvious that a large amount of the non-flatness of the preamp output is a result of those little hiccups in the AC sine wave. It's easier to see in the close-ups. I guess I need to look into some sort of line filtration to get rid of the buzzing. It still doesn't explain the FM radio interference, however

Any suggestions on power filtering solutions and methods of hunting down the RF interference?
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Old 17th January 2009, 12:26 PM   #4
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I'm no expert in this field but there are many different devices you can build or purchase. You could try replacing the power inlet jack with a unit that has a filter built into it, wire in a stand alone unit, both available at places like Mouser for a few bucks, or build one. You could try a ferrite core on your power cord, cant remember what exactly they call them. Try a power conditioner, store bought or DIY. Some of the better surge protecters have filtering built in. It does sound like your power grid wiring is acting like a big antenna for your preamp. The ultimate solution might be an isolation transformer unit, but that is also the most costly. Try using the search on diyaudio for other ideas about building filters and such, and dont forget GOOGLE is also your friend. Hopefully someone on this forum has had similar problems and can give you other solutions.

Best, Bill
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Old 9th September 2010, 06:55 PM   #5
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Did you ever fixed the noise problem with your ATC-3?
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