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Passive preamp introduces hum
Passive preamp introduces hum
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Old 18th March 2017, 03:01 PM   #21
Lohengrimas is offline Lohengrimas  Netherlands
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Quote:
Originally Posted by scottjoplin View Post
How long are your interconnects? Are they thin? Sounds like you need to lower 0V impedances. Try using screened cable in the box. Also pot is plastic case so is offering no screening' Have you a metal case you could use instead?
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.
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Old 18th March 2017, 03:05 PM   #22
Nico Ras is offline Nico Ras  South Africa
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Quote:
Originally Posted by Lohengrimas View Post
What still puzzles me, is that there is only hum when the pot is between either ends; at maximum "volume" (i.e. turned fully open) the hum disappears.
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.
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Old 18th March 2017, 03:26 PM   #23
Mooly is offline Mooly  United Kingdom
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Passive preamp introduces hum
Quote:
Originally Posted by Lohengrimas View Post
What still puzzles me, is that there is only hum when the pot is between either ends; at maximum "volume" (i.e. turned fully open) the hum disappears.
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.
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Old 18th March 2017, 03:32 PM   #24
scottjoplin is offline scottjoplin  Wales
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Also at full volume the pot is effectively out of the circuit, I think the pot itself is picking up most of the noise because it itself has a plastic case and it's in a plastic box
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Old 18th March 2017, 03:34 PM   #25
DF96 is offline DF96  England
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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.
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Old 18th March 2017, 03:41 PM   #26
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by Lohengrimas View Post
What still puzzles me, is that there is only hum when the pot is between either ends; at maximum "volume" (i.e. turned fully open) the hum disappears.
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
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Old 18th March 2017, 03:44 PM   #27
Lohengrimas is offline Lohengrimas  Netherlands
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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.
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Old 18th March 2017, 03:50 PM   #28
Nico Ras is offline Nico Ras  South Africa
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Oops, check your external wiring carefully. Check that the connections are made through from your power amp to the source with a multi-meter, it seems like a wiring problem.
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Old 18th March 2017, 03:52 PM   #29
scottjoplin is offline scottjoplin  Wales
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Ground loop. You could try reconnecting it with a small resistor in line, say 4.7 ohm. Sorry I miss understood, one of the ground connections?
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Last edited by scottjoplin; 18th March 2017 at 03:54 PM.
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Old 18th March 2017, 05:11 PM   #30
basi is offline basi  Croatia
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I can't believe that in 2017y. u guys make 3 page on a thread of most basic and simple connection in audio.

If you can't explain it simply, you don't understand it well enough. Albert Einstein
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