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Hum from amp

post #1 of 6
1/16/10 at 9:00am
Thread Starter 
I have a hum in a new amp I built. Very faint. Independant of volume. With/without input rca connected. I checked all my grounding and it looked good. Only present on low impedance phones (grados.) The odd thing is if I move my head/headphones within a foot of the amp the hum goes away. The hum is either 60 or 120 hard to tell from the frequency spectrum I tested with audiotester. Any ideas?
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post #2 of 6
1/16/10 at 1:04pm
Is the hum in both channels equally and independent of tubes?

It may be coming from anywhere, ground loops; rectifier, proximity of transformer to tubes. The finding that it reduces when you get physically closer is a little strange.

You don't have access to a scope to show that this is a AC hum?

..dB
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post #3 of 6
1/16/10 at 1:19pm
Thread Starter 
Quote:
Originally Posted by dBel84 View Post
Is the hum in both channels equally and independent of tubes?

It may be coming from anywhere, ground loops; rectifier, proximity of transformer to tubes. The finding that it reduces when you get physically closer is a little strange.

You don't have access to a scope to show that this is a AC hum?

..dB

Independent of tubes, same in both channels. No scope but the heater is rectified filtered DC (no reg) and it has .12V AC. Thinking of adding a choke to the DC heater filtering. It is very odd that it completely goes away if I am within a foot or two of the amp, I also ordered a ferrite clamp to put on the headphone cable.
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post #4 of 6
1/16/10 at 1:30pm
Quote:
Originally Posted by regal View Post
Independent of tubes, same in both channels. No scope but the heater is rectified filtered DC (no reg) and it has .12V AC. Thinking of adding a choke to the DC heater filtering. It is very odd that it completely goes away if I am within a foot or two of the amp, I also ordered a ferrite clamp to put on the headphone cable.
how are you grounding your DC heaters? re: your other post in the Bijou thread, i thought that you don't need to lift the heaters if DC - just ground it star ground. do you have a schematic of the heater circuit?
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post #5 of 6
1/16/10 at 1:40pm
Thread Starter 
Basically it’s a 10,000 uF cap + .2ohm resistor, lifted with the bias tap from the transformer connected to the CT of the heater winding via a single rectifier diode which is in turn grounded with a cap and resistor.
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post #6 of 6
1/16/10 at 2:21pm
Quote:
Originally Posted by regal View Post
Basically it’s a 10,000 uF cap + .2ohm resistor, lifted with the bias tap from the transformer connected to the CT of the heater winding via a single rectifier diode which is in turn grounded with a cap and resistor.
i have not seen a DC heater done this way before. i can only guess that you still have quite a bit of ripple.
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