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Old 21st December 2012, 08:31 PM   #1
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Default Hum in tube amp

I have build small SE tube amp using 6L6 and I can hear undesired audible hum from it. Few details:
- it is PCB based (commercial so I belive PCB is not a source of hum): 6L6 SE Tube Amplifier Kit 10+10W (Stereo)_Power Amplifier Kit_Valve Amplifier Kit_Analog Metric - DIY Audio Kit
- toroidal power teansformer is inside the chassis (as well as PCB)
- output transformers are outside the chassis
- power supplt has no choke - just capacitors and 10R resistor
- AC heating is center grounded - ok
- I use start grounding and both input and output jacks are isolated from chassis
- I use 3 wire power cord (PE connected to chassis)

What I have observed:
- after power on hum rises and after few seconds it is audible (heating / warming up?)
- hum disapears immediately after power off
- hum is still audible even if I shorten input pin to gnd

Based on these observations what is the source of the hum?

I don't think there is an issue with ground loop or circuit (commercial PCB kit).
Two ideas coming to my mind:

1. better power supply filtering (add a choke?)
2. "better" wiring - twist pairs of wires tightly, placement of wires - signal / output transformer / heating / power

Any ideas or recommendations where the hum comes from?
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Old 21st December 2012, 08:47 PM   #2
chip647 is offline chip647  United States
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Hum like you describe is almost always the result of a ground loop. It is one of the most difficult things to track down, but it is fixable. You don't need any new parts. Here is a list of possible causes:

1. A "cold" solder joint. Very easy problem to create with lead free solder. Check all of your joints for any looseness. Re-flow the ones that look suspect. I have had joints that looked perfect make lousy connections.

2. A ground that runs to two different places. Each item should have only one path to ground. Think Star ground.

3. Unplug your interconnects and short the amp's inputs to make sure the problem is contained within the amp.

4. Make sure everything that should be grounded is grounded. Use your meter to check every grounded item.

You will find it. When I started out doing this it would sometimes take weeks to find the problem. (Not so much anymore).

Good Luck!
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Old 21st December 2012, 09:09 PM   #3
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I'm guessing that you should add a choke to the power supply. According to Analog Metric, it's an option.
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Old 21st December 2012, 09:12 PM   #4
DF96 is offline DF96  England
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Is the hum 50Hz or 100Hz? Pure hum or buzz?

The circuit shows a 100R resistor in the PSU. You say 10R?

It only claims 85dB S/N, which is not particularly high.

The power rail decoupling looks OK, provided they have got the grounding right.

Some hum may be expected from the heater supply, as the PCB can't do twisted wiring.
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Old 21st December 2012, 09:19 PM   #5
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"It only claims 85dB S/N, which is not particularly high."

That S/N is at 1kHz. They don't give a figure <1kHz.
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Old 21st December 2012, 09:43 PM   #6
DF96 is offline DF96  England
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S/N sometimes includes hum in the noise figure. It shouldn't, but sometimes it does.
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Old 21st December 2012, 10:30 PM   #7
roline is offline roline  United States
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I tried to not use an inductor in the power supply only to have to add it to get the hum out... C-L-C(B+)-R-C (2nd stage)-R-C(first stage) Also be sure your heater wires are tight twisted and 1/2" off the PCB or away from the active components. Use a hum pot or verify 6.3v CT is connected to ground, checked. Watch your input to the first stage and verify no ground loops, shielded cable or tight wound.... Verify not a ground loop on the power ground and incoming signal ground, another source....
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Old 21st December 2012, 10:42 PM   #8
jrenkin is offline jrenkin  United States
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I think it is important to know if it is 50hz or 100hz (60/120 in USA). 50hz= ground loop, 100hz = power supply. If it is the ps, us a choke, increase caps or add an RC stage if you have the headroom. Otherwise, find that ground loop, and twist!
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Old 22nd December 2012, 11:44 AM   #9
fotios is online now fotios  Greece
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I bet that is a power supply ripple noise.
What is the difference between the terms: "HUM" and "BUZZ"?
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Old 22nd December 2012, 11:46 AM   #10
DF96 is offline DF96  England
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Hum is a sine wave. Buzz is a sawtooth, with lots of harmonics.
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