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Post by plinker on Dec 22, 2021 7:38:15 GMT -6
I have a compressor that is creating noise in a preamp that is directly below it (when free-stacked). The compressor has a linear PSU that sits directly above the preamps SMPS. The attached picture shows the style of toroid power transformer inside the compressor. It feels like the compressor case is made of aluminum, so maybe not as shieldy as steel. I'm about to rack these two pieces and need to hack something to stop the noise. Any thoughts? Thanks!  |
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Post by mattbroiler on Dec 22, 2021 10:36:38 GMT -6
I use these humfree tabs that separate rack gear they are supposed to help eliminate ground loops between rack pieces I have not had to fight that issue so don't know how well it does what it says but they are inexpensive to buy and try www.sweetwater.com/store/detail/IsoTabs--humfrees-rack-isolation-tabsI think the idea is that the rack units are isolated by being washered front and backside where it mounts into the rack rail. You can probably achieve the same thing by putting fat plastic washers on both sides of your rack pieces. |
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Post by yewtreemagic on Dec 22, 2021 10:45:30 GMT -6
Cut out a piece of Mu-metal and place it between the two (the foil is easy enough to cut with scissors).
Humfrees are great for solving ground loops, but won't help with radiated noise.
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Post by tkaitkai on Dec 22, 2021 11:04:32 GMT -6
I've experienced a very similar issue. My solution was selling the compressor.  Right now, I have 1ru between each piece in the rack and everything seems to be playing well. But yeah, when I had this problem, it was a GAP LA3A that was crammed right underneath the mic pre — it would introduce this weird, fizzy-sounding junk even when it wasn't in the signal path. Did multiple A/Bs with it on/off to confirm I wasn't going crazy, and ultimately concluded everything sounded better with it completely out of the rack. I like the mu-metal suggestion, though. I'll keep that in mind if I ever run into this again. |
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Post by svart on Dec 22, 2021 11:58:35 GMT -6
I have a compressor that is creating noise in a preamp that is directly below it (when free-stacked). The compressor has a linear PSU that sits directly above the preamps SMPS. The attached picture shows the style of toroid power transformer inside the compressor. It feels like the compressor case is made of aluminum, so maybe not as shieldy as steel. I'm about to rack these two pieces and need to hack something to stop the noise. Any thoughts? Thanks! <button disabled="" class="c-attachment-insert--linked o-btn--sm">Attachment Deleted</button> Swap them in the rack. put the preamp above. Also, it could be that the preamp shielding is not good. Also, aluminum is what I use for RF shielding. Steel is hard to work with for small shields. Aluminum is FINE for RFI egress and ingress issues, however the one thing it doesn't do is block electromagnetic coupling, so if your issue is that the magnetic field from the transformer is coupling over, then the aluminum chassis isn't helping much. As for those suggesting MU metal, it does work for emag issues, but you'd still need to encapsulate the area of radiation and ground it properly in order to stop the field entirely. Simply laying a sheet of mu metal over it won't do much. |
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Post by plinker on Dec 22, 2021 13:03:38 GMT -6
Thanks for all the suggestions. I'm going to try swapping them top/bottom for now.
They aren't racked yet, so I have plenty of options. I might even try putting a steel sheet between them to see if that makes a difference. My existing rack shelf (for those compressors -- I have two of them) has gaps in it. It could be that a solid steel shelf might help too.
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Post by Ward on Dec 22, 2021 14:55:39 GMT -6
I have enough BX isolated power circuits in my building not to have to worry about it, and 7 different grounds run. And do NOT use new power bars. If you can find some 90s ones with the braided power cabling, those will work.
Always best to consult an electrician and if gear won't stop humming, send it packing!
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Post by WKG on Dec 22, 2021 17:35:11 GMT -6
I had issues with a headphone amp causing hum. Moving it a few spaces lower in the rack solved it.
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Post by plinker on Dec 22, 2021 18:26:56 GMT -6
I'm sorry -- I should have been more clear. This is not a grounding/circuit issue. It's being caused by the proximity of the compressor's PSU transformer to the preamp. When I move it away the warped sounding hum/noise goes away. It's definitely being caused by the compressor.
Thanks!
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Post by michaelcleary on Dec 22, 2021 20:56:36 GMT -6
I had that issue with a Cranesong Trakker. It has some sort of massive heat sink in the back (not sure if that was the cause itself or not) but it was causing hum in a preamp above. Moving it away helped.
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Post by plinker on Dec 23, 2021 0:34:39 GMT -6
Unfortunately, swapping position top/bottom didn't fix it. It's 16 gauge steel, with a bunch of mounting slots.
Putting it in between the compressor and the preamp significantly reduced the hum. I think I'll try going with the solid, no-slots/holes version of the shelf. That should do it, I think. Kinda sucks because I really don't feel like spending $50 just for a hum solution, but... Thanks again. |
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ericn
Temp
Balance Engineer
Posts: 15,011 
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Post by ericn on Dec 23, 2021 16:15:30 GMT -6
Make sure it is a proximity problem vs a grounding issue by at least seeing if lifting grounds solves the problem.
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