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Old 21st April 2012, 01:41 AM   #1
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Default Extreme hum in EQ output

Hi there,

If anyone remembers I posted about some tone controls a little while ago and decided to modify an old preamp into something more of an EQ, only problem is, there's a lot of hum. I've found that it has pretty much nothing to do with the signal path, as the hum is still there when theres no tubes in the thing. I've changed the heaters from elevated AC to DC (12v), it didnt really change a lot. The power supply is 40VAC with a quadrupler, then stabilized with a zener, ive checked it and its fine. As for grounding, its layed out in a little bit of a mess, but a way that i can see everything clearly and i see no problems. Its really started to bug me.

I'll attach a schematic (sorry its only hand drawn), and I'll say now, i like how it sounds, apart from the hum or course... the filter capacitors are shown on the power supply part of the schematic (there are labels on them to show where they are grounded in relation to the rest of the pre-amp), and the signal chain part, i'm sure you can match the components up. The "G" shows where everything is connected at chasis ground.

Edit: Forgot to add, the hum is there immediately after switch on before the tubes have warmed up. Also heater voltage sometimes falls to 11.9V depnding on mains voltage.

Any help would be greatly appreciated!
Ricky
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Last edited by razorrick1293; 21st April 2012 at 01:43 AM.
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Old 21st April 2012, 01:59 AM   #2
Gilgy is offline Gilgy  United Kingdom
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Just a thought, is the .5 H choke picking up hum?
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Old 21st April 2012, 10:03 AM   #3
DF96 is offline DF96  England
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Almost certainly due to grounding. With no valves (or cold valves) there is little else that could do it.

Make sure your PSU is grounded at the output end, not the rectifier end. Otherwise you are injecting charging pulses into your ground. Your diagram shows a ground at the rectifier end.
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Old 21st April 2012, 10:33 AM   #4
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I'm not sure I understand quite what you're saying, and am i looking at the heater supply, or HT supply? I have grounded one side of the secondary in order to use the multiplier, the next grounded point is after the rectifier for the smoothing cap.
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Old 21st April 2012, 10:34 AM   #5
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As for the choke, no its fine, there is nothing around it than can give it any hum and when the mid control is at full boost the only increase is in noise but nothing more than expected.
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Old 21st April 2012, 10:56 AM   #6
DF96 is offline DF96  England
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Could be either PSU. It sounds like you have grounded the HT supply twice? If so, you have injected charging pulses into your ground. Both supplies should only be grounded at the clean end. Your diagram shows them grounded at the dirty end. If this is what you have done then the voltage drop across the PSU ground conductor caused by the charging pulses is being fed directy to your output.
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Old 21st April 2012, 11:16 AM   #7
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So are you saying i should remove the ground from the PT secondary?
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Old 21st April 2012, 12:55 PM   #8
DF96 is offline DF96  England
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Yes and No. The secondary obviously needs to have a ground connection, but it must not have its own separate ground connection. It must be connected to the negative of the smoothing caps. Then the negative of the smoothing caps must be connected to the audio circuit ground bus at the output end. Finally, somewhere in the circuit there may be a single connection to whatever your ground reference point is.

Grounding within a piece of equipment is fairly simple, but people often get it wrong by blindly following some 'ground scheme' instead of thinking about where the currents go.
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Old 21st April 2012, 01:09 PM   #9
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Ok, so connect the output stage ground to the other end of the power supply, thanks for the help, il try this now.
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