Noise & attenuation

[Alfetta87]

Hi there,

I bought some time ago a preamp kit from asia using the 6N3 tube, with a valve PSU.
I'm really happy with the sound, although I have some issues:
- I have a 50Hz hum noise, whatever I do, central grounding, separate PSU for heater and HV, and even with the simplest wiring (no input selector) I can hear it, especially when the volume pot (100k log. mounted before the preamp.) is in the middle position.
- The output level is way to high; at 9 o'clock, I have already full power, and the potentiometer is not so accurate at that low position.

My thoughts: the hum is not extremely high, so when I'd attenuate the output level by let's say 10dB, or even more, both problems would be resolved I guess.

At the output I'm driving a Lundahl LL1547 to connect an XLR output socket.

Questions:
Where should I place the attenuator: just after the output stage, and before the transformer?
.. or after the transformer?
What would be the right values for the serial/ parallel attenuator to preserve the output impedance (a friend of mine calculated about 600 ohms)? I'm running it with an UcD400 amplifier.

Thanks,
- dan

[sangram]

I would insert a resistor in series with the potentiometer.

A 220K resistor should get you approx 10dB attenuation - though both this value and the pot value seem high - a 47K resistor/47K pot should work better.

[Alfetta87]

OK thanks, but my potentimeter is BEFORE the stage - thus I can attenuate the signal but not the hum.

[sangram]

Ouch.

Is the filament supply AC or DC?

[Alfetta87]

DC, regulated (LM317).

[jrenkin]

I am no expert, but it seems that the hum must be either a grounding problem or coming in from your source. Where would 50Hz come from? Not your power supply (should be 100Hz, right?), not your regulated DC heaters (100Hz and minimal). I would recheck the grounding scheme, especially the mains grounding and power trafo grounding and then try a different source and finally look at the transformer orientations. Just some thoughts.

[sangram]

On either side of the transformer, the output impedance will be affected by the L-pad, and a receiver will affect it once again. Plus this way, you are essentially throwing away gain, which is kind of suboptimal.

Where is the cold terminal connected to and where is the ground? Do you also get the hum with no source connected and the pot set to zero? If the hum varies with pot position and the pot is before the stage as per your first two replies, I would think of eliminating the stage as the source of the hum.

It does sound like a ground loop to me if we can rule out the tube supplies. High input impedance + high pot values + XLR wiring can all lead to hum issues.

[Alfetta87]

Tried everything, transformer orientation, ground loop (any ground connected to a single point), even w/o wiring the XLR socket or even the output trannies.
Hum is minimal when the pot is in zero or max position, and increases when middle position.

But the hum is not of high level, thus my thoughts to attenuate the output... no way to do this without increasing the output impedance?

[danzup]

did you ever think that hum come from not enough anodic filtration ?
Just add condenser on +B voltage and see if thing are improving .

[Alfetta87]

Originally Posted by danzup

did you ever think that hum come from not enough anodic filtration ?
Just add condenser on +B voltage and see if thing are improving .

I tried this,but no improvement.