Ok...I need help. Yes I've read and read and just got more confused.
So a little while ago I tried a basic passive with a single input, and a single output and a khozmo shunt attenuator. It worked beautifully and I was in love with the sound, so I bought a case (mine was in a cookie tin), some fancy RCA (for fun, to try) and an Elma 2 pole 6 way rotory selector.
I put this all together and...ground loops + the sound comes through independently of the volume control. As in, it comes the same if volume is at zero or max.
My explorations with multimeter seemed to make sense to what I would expect.
So I guess issue 1 is, my grounding obviously isn't working. Worked fine in my cookie tin, didn't work with exactly same configuration in my aluminum case. Confusion.
Issue 2 is, the fact that the volume control isn't working except as a pass through. I can't see any other way to wire the selector switch except what I have done. Everything I find says I did it correctly.
Image 1 is the original cookie tin that worked perfectly.
The others are how I think it should go, with some guesses as to grounding...I'm hoping for advice on this grounding. Should it be grounded to the case? I tried doing so with zero response - hum didn't change.
I feel like I've tried every configuration today and am flailing. Ie, I went back to exact original configuration and it worked again. Tried same thing but with new RCA, and it hummed. And so on and so forth...
Image 1 was original, that worked. Image 2 is another option and maybe the only I haven't tried yet. Image 3 I've tried, and failed.
So a little while ago I tried a basic passive with a single input, and a single output and a khozmo shunt attenuator. It worked beautifully and I was in love with the sound, so I bought a case (mine was in a cookie tin), some fancy RCA (for fun, to try) and an Elma 2 pole 6 way rotory selector.
I put this all together and...ground loops + the sound comes through independently of the volume control. As in, it comes the same if volume is at zero or max.
My explorations with multimeter seemed to make sense to what I would expect.
So I guess issue 1 is, my grounding obviously isn't working. Worked fine in my cookie tin, didn't work with exactly same configuration in my aluminum case. Confusion.
Issue 2 is, the fact that the volume control isn't working except as a pass through. I can't see any other way to wire the selector switch except what I have done. Everything I find says I did it correctly.
Image 1 is the original cookie tin that worked perfectly.
The others are how I think it should go, with some guesses as to grounding...I'm hoping for advice on this grounding. Should it be grounded to the case? I tried doing so with zero response - hum didn't change.
I feel like I've tried every configuration today and am flailing. Ie, I went back to exact original configuration and it worked again. Tried same thing but with new RCA, and it hummed. And so on and so forth...
Image 1 was original, that worked. Image 2 is another option and maybe the only I haven't tried yet. Image 3 I've tried, and failed.
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Making a reliable contact with aluminium is not straightforward because it gets an isolating oxide layer at its surface as soon as you expose it to air. Can that have something to do with it?
I basically just held one end of a wire against an empty cut out and touched the other end against everything possible. I think a connection must have been made, but it made no difference.
I'm sure it can be both.
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Another note...hmm...I just noticed that if I held an RCA - not connected - and touched the case it would hum. I'm wondering if this suggests my ground loops is coming from a different source, and not the passive preamp at all. I did reorganize my equipment at the same time as doing up this version 2 of the preamp. Perhaps there's some coincidence at play.
Also, it hums, and then when I turn the source on, it really hums.
Of course, I did have hum free operation in this configuration with the version 1 reattempt today so maybe that's not it at all. Sigh.
Also, it hums, and then when I turn the source on, it really hums.
Of course, I did have hum free operation in this configuration with the version 1 reattempt today so maybe that's not it at all. Sigh.
I'm not totally sure if I should be grounding anything to chassis for a passive. I didn't in my first attempt, just RCA to attenuator to RCA grounding...
I checked the Elma part # and is standard non-shorting and only shorting if requested, not that would necessarily be the issue but at least it's answered.
I think I'd focus on Issue 2, the attenuator not working and getting full volume constantly, since it would seem to be a mis-wiring. I've looked at the second set of pictures you posted and I don't see anything obvious but I'm just zooming in and visually tracing things. Have you thought of wiring the attenuator directly to one set of input/output RCAs and by-passing the selector? It might also be good to disconnect the controller board since it has a separate power source. I think you are going to have to test things step by step by first getting the attenuator working.
I think I'd focus on Issue 2, the attenuator not working and getting full volume constantly, since it would seem to be a mis-wiring. I've looked at the second set of pictures you posted and I don't see anything obvious but I'm just zooming in and visually tracing things. Have you thought of wiring the attenuator directly to one set of input/output RCAs and by-passing the selector? It might also be good to disconnect the controller board since it has a separate power source. I think you are going to have to test things step by step by first getting the attenuator working.
Yeah I have been mostly trying to solve the hum first, and worrying about the selector second. The selector issue seems...less likely to be as hard to track down.
I've tried just about everything I can think of.
I put everything back in the cookie tin as original, and it worked. I tried using the RCA I had in the new case, and it hummed. So I tried the RCA from the cookie tin in the new case (leaving all wiring at attenuator side the same) and it hummed. Tried different ways of connecting the grounds at the attenuator. Tried connecting attenuator ground to case.
I just put everything back into the cookie tin, using a third set of RCA, and it's back to working just peachy. 1 input, 1 output and attenuator. Same config exactly doesn't work in the other case.
Could it be something like the nut holding the attenuator shaft is grounding to the case in the cookie tin, but not the coated aluminum case? And that's helping?
So baffled.
I've tried just about everything I can think of.
I put everything back in the cookie tin as original, and it worked. I tried using the RCA I had in the new case, and it hummed. So I tried the RCA from the cookie tin in the new case (leaving all wiring at attenuator side the same) and it hummed. Tried different ways of connecting the grounds at the attenuator. Tried connecting attenuator ground to case.
I just put everything back into the cookie tin, using a third set of RCA, and it's back to working just peachy. 1 input, 1 output and attenuator. Same config exactly doesn't work in the other case.
Could it be something like the nut holding the attenuator shaft is grounding to the case in the cookie tin, but not the coated aluminum case? And that's helping?
So baffled.
When you put the attenuator into the aluminum case with just the in/out RCAs and got hum, did the attenuator work? Without a signal GND the attenuator won't work and would probably cause a lot of hum:
Hmm, I think I'm not sure, if I turn the source on the hum gets really intense and I just shut it off again immediately to avoid damage.
I did have my input RCA tied to the ground on the attenuator, so that should do it, yes?
I did have my input RCA tied to the ground on the attenuator, so that should do it, yes?
I think the cookie tin has been your signal GND and moving it to the aluminum case caused things to be isolated, like Marcel mentioned. You should tie the input and output connector GND to each other and also connect them to the attenuator GND for each channel. In the picture of the connections in the tin look wrong now that I understand what's going on. In it you have the IN GNDs tied to one side of the attenuator and the OUT GNDs to the other, the tin was conducting between the IN and OUT jacks to complete the signal GND.
Yes....this feels right. I think you're onto it.
That kind of what I kept coming back to, the janky tin was helping by being a ground.
So this drawing is likely the correct one...it's the only one I didn't try yet...
So in this situation everything will be "floating" (if I understand that term), ie no ground to case. RCA isolated, and no specific grounding to case.
Btw, thanks so much for the help, I know basic troubleshooting for newbs isn't the most fun and it's really appreciated when folk indulge us as we try to learn.
I'll report back when I try that approach tomorrow.
That kind of what I kept coming back to, the janky tin was helping by being a ground.
So this drawing is likely the correct one...it's the only one I didn't try yet...
So in this situation everything will be "floating" (if I understand that term), ie no ground to case. RCA isolated, and no specific grounding to case.
Btw, thanks so much for the help, I know basic troubleshooting for newbs isn't the most fun and it's really appreciated when folk indulge us as we try to learn.
I'll report back when I try that approach tomorrow.
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Hopefully the case of the cookie tin has been solved. All the clues and evidence are in your first post but it took some time to sort it out. If you look closely at the picture of the tin installation you can see the RCA jacks are not isolated even though they have plastic washers, the GND washer is underneath the plastic washer against the tin. And the yellow GND wires go to each channel of the attenuator so the IN and OUT are isolated, the GND through the tin was the only reason it worked.
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So I hooked everything up carefully today, and it was quiet, until I touched the volume knob and then it blasted out full volume. Definitely the volume control, and selector were causing issues by touching the case.
I went out to the garage and whipped up a wood and plexiglass case and everything is working beautiful now. I think I'll just stick with that, and buy some shielded wire, which I hadn't been using. I don't actually hear any noise at all but I suspect shielded wire would just be smart.
I went out to the garage and whipped up a wood and plexiglass case and everything is working beautiful now. I think I'll just stick with that, and buy some shielded wire, which I hadn't been using. I don't actually hear any noise at all but I suspect shielded wire would just be smart.
Interesting, in your cookie tin implementation the case was connected to the signal GND. Trying to isolate things from the case may be the issue, there are different camps of thought on this but I've generally found more issues with isolation than bonding things together. Hopefully you didn't damage anything with the full blast of volume. With your multimeter can you see if the metal shaft of the attenuator or selector is connected to anything else besides the case? Something seems still a miss with the attenuator GND wiring.
You're correct that I initially had the ground tabs on the wrong spot, but actually when I rebuilt the original I put them in the correct place, and it worked. Either way I think the volume control touching the case in the tin helped, and touching the case in the aluminum case hurt. Not sure why.