12AU7 based preamp hums when output cable is touched! - diyAudio
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Old 11th April 2009, 04:19 AM   #1
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Angry 12AU7 based preamp hums when output cable is touched!

Hi guys, I just finished building a simple 12AU7 based guitar preamp which I designed to give some good tube boost to a Peavey Transtube Studio Pro 112.

I'm very pleased with the results as I'm getting liquid like sustain and hairy metal distortion as I wanted from this solid state amp. It has very low noise even when I use the high gain input and at very high distortion settings, however I noticed that when I touch the output cable inside the amp (it is a shielded cable which is connected to a jack) the amp hums a lot (only when touched or if my hand gets close to the cable), also if I change the position of this cable (talking about the place where the cable is resting inside the aluminium chassis) it hums more or less depending on where I put the cable. The power transformer is far from the preamp and there are no motors, lamps or computer monitors near the amp nor the preamp.

The input and output jacks are grounded via the front panel (chassis), also near them I connected the star ground to chassis.

Any advice will be appreciated, thanks in advance!
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Old 11th April 2009, 04:31 AM   #2
pdul is offline pdul  Denmark
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sounds like a bad shielded cable or bad ground connection, very sensitive stuff with those very high impedances
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Old 11th April 2009, 05:24 AM   #3
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I've tried removing the volume pot, and the same thing happens, so you suggest to try a better quality cable? This problem doesn't happen with the input cable which is the same type of cable (shielded instrument cable). Also if I touch the cable that goes from the preamp to the amp nothing happens (it behaves normally).
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Old 11th April 2009, 05:32 AM   #4
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don't touch it?
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Old 11th April 2009, 05:36 AM   #5
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I made a mistake in the first description, the output cable is inside the preamp I built, not inside the Peavey amp.

Touching the output cable inside the preamp, moving the cable or just putting my hand near the output cable changes the hum level which is low (in the clean channel hum is not heard at all) but in certain areas inside the preamp it gets really loud, which made me think that I can get the preamp noise floor even lower while I'm using the distortion channel of the amp. As I mentioned the input and output jacks are grounded using the chassis' front panel (where the star ground is located) the other side of the cable is not grounded (as in some vintage Fender amps).
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Old 11th April 2009, 06:57 PM   #6
kevinkr is offline kevinkr  United States
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Do you have an oscilloscope? I suspect your little circuit might just be oscillating (probably in the VHF region) depending on the internal layout. The audible results and behavior I have observed in such instances closely match what you have described.

I would take a look with a scope first and see what is going on, if that's not possible I would suggest making sure that the 47K resistor R26 in the grid circuit is mounted right by the socket, and I would add a small resistor in series with the output to provide some isolation between the cable capacitance and the output of your circuit. This could best be placed in series with C25, and a 220 ohm - 1K resistor would be the place to start.

Make sure that your filament wiring is tightly twisted and not looping the long way around the socket.
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Old 11th April 2009, 08:32 PM   #7
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Thanks Kevin I'll try that, also I read on Randall Aiken's website that jacks must be isolated from chassis, I 'll try that as well and see what happens.
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Old 11th April 2009, 09:50 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by frickecello
Thanks Kevin I'll try that, also I read on Randall Aiken's website that jacks must be isolated from chassis, I 'll try that as well and see what happens.
Yes that is pretty important, and in addition there should be only one ground connection to the chassis. Might not be a bad idea to read up on good grounding practices as well.
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Old 12th April 2009, 12:54 PM   #9
Brit01 is offline Brit01  United Kingdom
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The same happens with my amps.

I have an Aikido linestage amp. It is dead quiet and has good grounding but when I touch or place my finger near the sensitive wires from the output of the potentiometer it hums a lot.

I also have a Carver SS power amp and it does exactly the same so I'm not worrying about it.

If everything works fine and it is quiet when you are not touching it I wouldn't worry about it.

As the other members mention, just check all your grounding first.

Good luck.
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Old 12th April 2009, 04:20 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Brit01
The same happens with my amps.

I have an Aikido linestage amp. It is dead quiet and has good grounding but when I touch or place my finger near the sensitive wires from the output of the potentiometer it hums a lot.

I also have a Carver SS power amp and it does exactly the same so I'm not worrying about it.

If everything works fine and it is quiet when you are not touching it I wouldn't worry about it.

As the other members mention, just check all your grounding first.

Good luck.
What you are describing is normal behavior, what the OP described is somewhat different and may be a symptom of oscillation or poor grounding practices. (Or possibly an issue of hum pickup from filament wiring as the signal levels are fairly low.)
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