Pretty long, yeah, I think a meter or so? Just regular big box store cables. I might have to find screened cables and a metal case then... It would be a bummer to find out it's just a dinky pot...
Anyboy that can explain why I don't hear the hum when the pot is at max volume? That just doesn't make sense to me if the shielding of the cables or case would be the problem.
Trying what I said will take less time than writing your next response and will save you the trouble of writing it. After it worked I will try to un-puzzle you.
That may be down to the power amplifier and nothing at all to do with the 'preamp'.
Do you still get the hum as you rotate the control if you apply shorting plugs to the preamp input sockets ?
If you do, then it sounds like some issue within the power amp (such as it not liking a high impedance feed).
If not then you need to be looking at the overall set up of source components and power amp and seeing if any loops exist.
Much easier if you are sat in front of the thing, its very hard to describe exactly how to test it all.
Hum which peaks in the middle of the pot range means that it is being picked up on the output side of the circuit - which includes the input of the power amp. I think you said that the power amp uses the same poor wiring layout as you originally used? So first thing is to correct the input wiring in the power amp; ideally shielded, but at the very least twisted.
Second thing is: what resistance value is the pot? Too high a pot resistance could do it too - especially with poor power amp wiring.
Making a simple 'passive preamp' is easy, but not so easy that it is impossible to get wrong.
Second thing is: what resistance value is the pot? Too high a pot resistance could do it too - especially with poor power amp wiring.
Making a simple 'passive preamp' is easy, but not so easy that it is impossible to get wrong.
Important observation. When the pot is either fully open or fully closed, the wiper is either at ground or at the input RCA. Both conditions are fine.
When the wiper is say in the middle, it has half the pot resistance to either ground or the input RCA. Now it hums. Why?
The relatively large resistance from wiper to either of the 'clean' points (gnd or input RCA) makes it sensitive to pick up hum 'in the air' from all the stuff you have running. Try to put your hand on the pot housing, chances are that the hum changes significantly.
Solution is to screen the pot with something metal to gnd, or use a metal box and connect it to the input- or output gnd of an RCA.
Jan
The power amp i'm testing it with is a simple TA2024 with a wall wart; not the amp I'm going to use it for (building it in a different location...).
I tried Nico Ras's solution, but that didn't help.
the pot is 50k, log, Alps blue velvet.
Another weird thing: apparently one of the solder joint on the RCA's came loose: when I connect that loose end (ground) to the signal post of the RCA, all the hum disappears, yet the sound comes through fine.
I tried Nico Ras's solution, but that didn't help.
the pot is 50k, log, Alps blue velvet.
Another weird thing: apparently one of the solder joint on the RCA's came loose: when I connect that loose end (ground) to the signal post of the RCA, all the hum disappears, yet the sound comes through fine.
Let us be clear: a hum problem which peaks when the volume pot is at the electrical halfway position (which will be different from mechanical halfway for a log pot) and disappears at max and min volume is almost certainly caused by something injecting a hum current into whatever is attached to the wiper. In most cases it will be stray capacitance, and the answer is shielding - but note that you may sometimes get sufficient relief by 'back-shielding' (i.e. twisting wires so the signal wire is always near a grounded conductor even if not fully shielded by it). One way to check is to unplug the source and see if the hum still disappears at max volume setting.
It is unlikely to be a hum loop or poor grounding. It could be due to RF parasitic oscillation in the amplifier, but unlikely.
It is unlikely to be a hum loop or poor grounding. It could be due to RF parasitic oscillation in the amplifier, but unlikely.
It is precisely because it is 2017 that 3 pages are needed to correct missing or incorrect thinking about simple issues; in olden times this would not be necessary as people would know how a volume pot works and understand the need for shielding, because they would have been awake during their school science lessons when they learnt about electricity rather than the politics of fuel supply.basi said:
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