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RCA hum (Read 1689 times)
amit
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RCA hum
06/21/23 at 11:31:02
 
Hi all,

I recently completed ZKIT-1 on a wooden board (no chassis). I have a noticeable hum when connecting RCA cables. Once removed, the hum goes away almost completely. Things I've already tried:

1. connecting to ground
2. replacing all tubes
3. replacing diodes
4. doubling capacitance on both 3.3uF and 33uF
5. twisting all power wires
6. bypassing the potentiometer

Would very much appreciate your help in figuring this one out.
Many thanks!

Amit
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4krow
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Re: RCA hum
Reply #1 - 06/21/23 at 16:53:28
 
 As a precaution, replacing the interconnects that you are presently using with a different set will let you know if they may be playing a part here. In fact, even the interconnects between the source and preamp as well, if that is your case, just to rule out a simple fix for this problem.
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amit
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Re: RCA hum
Reply #2 - 06/22/23 at 14:17:47
 
Thanks -- yes, tried it, and changing interconnects helps but not significantly. Also tried different outlets.

Some more context: without interconnects I am getting a low hum, which is almost inaudible from the listening position, but certainly audible up close. I'd be more than happy to get rid of it, of course, but I think that is  typical, as it comes from the Edcor power transformer (low dull hum). With interconnects, I'm also getting the second harmonic noise (around 100Hz -- my outlet is 240V 50Hz), regardless if connected to a source.

Might also be relevant: I'm using Japanese output transformers 3.5kOhm.
Also tried undoing the Hazen mods, to no avail.

What I'm thinking to do next: adding a choke and/or installing a tube rectifier. Would appreciate any recommendations you might have on these or any other ideas...
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Mannytheseacow
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Re: RCA hum
Reply #3 - 06/22/23 at 14:20:16
 
I read in another post that someone solved this by wiring the RCA's directly to the volume pot and skipping the board.  Haven't tried this myself but easy enough if you want to trouble shoot.
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amit
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Re: RCA hum
Reply #4 - 06/24/23 at 14:41:11
 
Thanks for this. I also saw that thread a while ago, and decided to try it out today. Alas, so improvement... Bypassed the pot altogether and connected the inputs directly to the 1K resistors going to legs 2 and 7 of the 6N1P. It did result in a bit more bass, I think, but no reduction in hum/buzz. Maybe even slightly increased.

I'm thinking to try a choke next. Any suggestions? Do I need to change the filtering circuit, other than lowering the 1K resistor to combine with choke resistance?

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Mannytheseacow
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Re: RCA hum
Reply #5 - 06/26/23 at 23:16:14
 
I can’t speak to the 240v version but my thought is that the circuit should run quiet(ish) as designed. Check over all your parts, solder joints, and wiring carefully. The potentiometer is fairly delicate and can be overheated easily when soldering, though it sounds like you have already bypassed the pot and that’s not the problem.
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dank
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Re: RCA hum
Reply #6 - 06/28/23 at 00:31:43
 
Try running a ground wire to the ground on the RCA's that are not connected to your Zkit1.  These should be plugged into your source or preamp I presume.  Perhaps they are floating or AC grounded and this is not compatible with your Zkit1 setup.   If this fixes it you can leave as is or go into the source/preamp and add a ground wire to the RCA's.

Also, something to think about, I have discovered on a different power amp that the amount of hum at idle can very significantly with the rotation of the power transformer.  Might be worth a try to see if that effects your low level hum at all.

Dan
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amit
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Re: RCA hum
Reply #7 - 06/30/23 at 16:23:52
 
thank you -- tried it, no improvement...

also tried powering the amp without the input tube. hum was lower and dull. this suggests, I think, the upper harmonic noise comes from the input circuit somewhere. which components are to be suspected first?
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amit
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Re: RCA hum
Reply #8 - 07/05/23 at 15:50:05
 
just tried adding a 5H 100ma choke -- still no improvement Sad maybe too low inductance. really frustrating!
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amit
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Posts: 7
Re: RCA hum
Reply #9 - 07/10/23 at 08:56:27
 
another update in this odyssey: added a simple stepped attenuator, and when dialing volume to the max hum+buzz are most pronounced. two notches down, less so. about 2/3 volume, mostly hum but no upper frequency buzz. could it be faulty caps or resistors somewhere in the input circuit (already tried replacing tubes)
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