Low frequency noise in a 12B4 preamp - diyAudio
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Old 29th May 2004, 07:33 PM   #1
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Default Low frequency noise in a 12B4 preamp

Brett, I think all of your advertising has worked, because hereís another 12B4A preamp.

I just finished it a couple of days ago but have a problem, maybe one of you here can help me. Iím getting a random, very low frequency noise out of the preamp, and I canít figure where itís coming from. On a scope, with the SEC/DIV setting on 0.1s, it looks like the output is tracing the contours of a mountain range, very random and very jagged. The amplitude of the ďmountainsĒ are generally around 40mV peak, but Iíve seen them jump to around 120mV. The signal seems to randomly swing negatively or positively. Looking at the speakers moving around, the mountain shapes appear around twice every second or so.

Iíve also looked for the signal on the power supply, but this is hard to do. Iím not sure how far Iím allowed to move to the vertical position around, so I used the scope AC coupled, and it seems that the signal also exists on the power supply. This problem exists equally on both channels, and does not seem to interfere with music signals played thru it. Not that I know this for sure, but the preamp sounds pretty good.

I originally had no film bypasses in the amp, but because of rectifier induced noise, I added 0.047uF film caps to each 50uF cerafine. This completely solved the rectifier noise problem, but the sound turned very sterile; slow and even colder than Harryís diamond buffer. This morning I replaced these with some 4.7uF Hitachi polypropylenes, and these seem to work quite well. Iím finally getting clean warm sound. None of the bypasses caused the least change on the low frequency noise however.

Iíd keep listening to see how the sound changes as time passes, but I can hardly do so with the way the preamp is now. Any suggestions?

*edit: those are 100uF cerafines in the PS*
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Old 29th May 2004, 07:35 PM   #2
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Here's a picture of the inside. This is with the 0.047uF film caps in place. I've just managed to fit the new 4.7uF caps in next to the 1.5K Mills resistors.
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Old 29th May 2004, 07:37 PM   #3
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Oh, and here's a gratuitous pic of the outside:
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Old 29th May 2004, 10:01 PM   #4
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Nice layout. Have you tried different tubes? I know that sounds kind of silly but I was having a 60hZ hum problem in my new amp until I biased the heaters above ground (as I see you did) and changed the driver tube. Viola! no more problem.
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Old 29th May 2004, 10:25 PM   #5
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I'd be immediately suspicious of connections in the PS or maybe even leakage from a cap or a diode. Swap some caps around (like the 50s for the 33s) and see if the problem moves.
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Old 29th May 2004, 11:33 PM   #6
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Just for my own personal learning experience, what did you see Sy? Or are you going by intuition? Learn me sumpin SY!!
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Old 29th May 2004, 11:40 PM   #7
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Well, it's correlated between the channels. That says, "Power supply!" You looked and, indeed, that's what it was. Now, what sorts of things can cause the power supply to jump around? There are lots of possibilities, but of the myriad I first look at the least reliable stuff- my own soldering work, then electrolytics, then diodes.

If you swap the 50s and 33s, BTW, either the problem will go away, will continue on both channels, or suddenly be confined to one channel. Each of these outcomes tells you something.
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Old 30th May 2004, 02:19 AM   #8
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To quote my favorite commercial, Brilliant!!!
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Old 30th May 2004, 03:40 AM   #9
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Default heater-cathode leakage?

I've heard that 12B4 are prone to heater-cathode leakage that can cause some strange noises, possibly low frequency. Nice preamp, by the way. I built a similar model myself and like it very much. I use a 20uF bypass on the heater voltage divider rather than the 0.22uF you show. I found that cut down on some hum noise.

As for the mountain range, both your speakers and your ears are better than mine if you can hear a 2Hz noise signal I have seen the same behavior on my scopes though, even on the output of actively regulated supplies. I actually think it might be a spurious signal coming out of the air. The pumping of your speakers is of course real, but I am not an expert on motorboating so I can't suggest a cure at the moment.

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Old 30th May 2004, 10:10 AM   #10
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One other thing to think about. On a 5842/417A there is more than one pin connected to the grid. The same goes for the 12B4A. Try soldering a 220 ohm resistor to pin 2 and another to pin 7 and solder the signal wire to both of them. It might help.
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