Hum induced in other rack gear

[heatwalk]

Hi all,

Built a 2ch tube preamp recently for a studio that sounded great on my bench, low noise floor, no hum etc. Sent it off to a studio and they are having issues with it inducing unusable amounts of hum in other equipment, most notably in the preamp directly above it in the rack, but even a few units down / to the right of it. I dont have a jam packed rack over and most of my testing was done working on this unit by itself so I suppose I wasnt set up for this to happen but now trying to help from a distance. The issue is directly linked to the offending unit being flipped on and off and somewhat proximity based so points to power transformer radiation, though the extent to it being an issue is somewhat surprising. Could this also be a pin 1 problem? Or does that only manifest in the unit itself.

The power transformer is mounted on the rear of the unit which I didn’t think would be a major issue as plenty of tube equipment mounts the transformer it off the rear, but apparently not the case here. My initial recommendation is for them to try remounting elsewhere in the rack, however if it is messing with units a few spots away I cant imagine itll be a smoking gun. The power transformer has no end bells on it - is this likely making things much worse / is this the root of the issue? I assume my best bet is providing them with a steel project box or some mu metal foil to mount over the power transformer? Would like to not have it returned.

 

[Tubetec]

Rack mounted gear is notoroius for all kinds of induced noises , the rackmount itself can cause ground loops . I've been a part of many sessions where the producers insisted their personal outboard didnt go anywhere near a rack for that reason .

 

[heatwalk]

Maybe isolating the chassis from rack with some nylon washers?

 

[pucho812]

Maybe isolating the chassis from rack with some nylon washers?

That’s a start. But you have to figure out how it’s happening so you can fix it. Plenty of gear has external power transformers and they do not cause issues. So what if anything is different here?
FWIW at a previous gig, we had an entire line( preamp, compressor, eq) where the power transformer lived in an external box, it made for the quietest of gear that way.

 

[JohnRoberts]

Transformer external magnetic fields in tightly spaced racks can be problematic. Sometimes changing orientation of the transformer can help but you might need to spend some bench time experimenting.

JR

 

[heatwalk]

Plenty of gear has external power transformers and they do not cause issues. So what if anything is different here?

Rack mounted gear is notoroius for all kinds of induced noises , the rackmount itself can cause ground loops . I've been a part of many sessions where the producers insisted their personal outboard didnt go anywhere near a rack for that reason .

Yea, I guess I didn't consider much of the possible issue regarding mounting in the build. Build and bench testing was not in a studio rack full of other preamps but it was quiet in my tests, played nice with interface and computer and monitors, but like I said not in a full studio full of gear. The chassis is unpainted steel (used an old Altec chassis), so thinking some kind of coupling could be happening with it making contact with rack?

Transformer orientation I did with respect to the input and outputs XFMRS also mounted on the back ( these are unaffected - the unit itself is quiet as mentioned and inputs don't pick up the hum)

 

[Ike Zimbel]

I ran a PA company back in the 90's and dealt with this, especially when mounting power amps and processors in the same rack. I do not think this is a ground loop issue, more likely radiated noise from the power transformer. What the chassis of the other rack gear are made of makes a difference, too. For example, gear that has an aluminum chassis will not shield against EMF from this device.
In all cases where I dealt with this, steel was the answer. If your client has some steel blank rack panels (or any other steel or iron material) lying around, ask them to experiment with inserting them between the transformer and the other devices (so holding the panel over/under the transformer horizontally, not mounting blank panels in the rack). If they at least hear a difference, that will tell you that you need to get some steel around that transformer. Make sure they know the difference between a steel panel and an aluminum one as both are pretty common. Also, if they take the unit out of the rack, and put it, say, across the room and run XLR cables to/from its patch points in the rack will prove/disprove the ground-loop theory pretty quickly. No hum? No ground loop.
In one use-case, we had some 6-U racks that held, originally, two Yamaha PD2500 power amps and two EV DM1122 processors. The Yamaha's were an early lightweight amp and had switching power supplies. When they died, we replaced them with QSC MX1500a's that were a conventional amp with a big toroid transformer in them. These induced hum in the processor nearest the top amp (the two amps were in the bottom 4-u with the processors above them). The fix for this was shoving the top cover from a 9" square electrical back-box in between the top amp and the bottom processor. My staff eventually cut the metal plates to fit and fastened them to the processor permanently. I'm not sure what gauge the steel plate was but it was probably around 2mm thick.

 

[Audio1Man]

1. Remove the preamp from the rack and place it on a chair.
2. Connect the preamp using longer cables and same power cord.
3. Check for HUM, if HUM is gone it is radiated field.
4. If HUM is still present use an AC POWER GROUND LIFT.
5. If HUM is still present, float pin#1 XLR INPUT.

 

[beatnik]

Is it just a specific preamp picking up the hum? If so that preamp might be lacking shielding in the input circuitry perhaps. Maybe no mumetal shield on the input transformer.

I would not fit a power transformer without bell covers hanging out the back of the chassis, it just seems a bit unsafe.

 

[JohnRoberts]

Back when I was working at Peavey we manufactured hum makers and hum receivers, so we had to work to make sure our products didn't cause hum in each other.

Some of our old amplifiers with huge EI core transformers could generate a noticeable magnetic field. Modern toroidal transformers are somewhat better wrt hum.

JR

 

[Cqwet Dbdfte]

Magnetic field induction can be expensive and problematic to cure. This is one reason why I got a (cheapo and small) SMPS for tube MC cartridge input amps. The switching noise is well above the audible range, and after some effort to make a filter the noise +B was reduced to a few mV pp. The heater supply was more of an issue, about 100mV pp, (the cheapo SMPS tied the negatives together) but does not seem to enter into the picture.
Output noise is about -60dBu unweighted from the amp, mostly sub 10Hz. Gain about 50dB 1kHz, input transformer adds another 27dB.

 

[AlgoRhythms]

I keep several 12x8 inch mu-metal sheets around to help minimize rack hum.
Strategically cutting and placing them has helped a lot.

 

[Graham Nystrom]

You can find steel enclosures for toroids that might help. But I agree with all that say its EMR from the power transformer. I am just getting into building and designing gear and this is one of the biggest head-scratchers for me. My first instict was to say to heck with it and make all my stuff with external power transformers. But now that I've tried that, I'd like to try getting them back in the enclosure. This one issue sure makes it make sense why companies use steel for chassis more than aluminum! If your PT is a toroid, you can spin it while listening to the noise to find its quietest orientation. But you never know what it going to be above and below it when the unit goes to its forever home.

Good luck and I am sure you can find a workaround for this unit and others. A good learning experience! (arent they all???) Take care.
Graham

 

[Graham Nystrom]

That’s a start. But you have to figure out how it’s happening so you can fix it. Plenty of gear has external power transformers and they do not cause issues. So what if anything is different here?
FWIW at a previous gig, we had an entire line( preamp, compressor, eq) where the power transformer lived in an external box, it made for the quietest of gear that way.

Hey Pucho- I'm curious where your gig was that used external PTs if I may ask?
Thanks!

 

[heatwalk]

Thank you everyone for chiming in thus far. We will see what they say when they get back to me after just pulling it out and moving it away from everything.

Is it just a specific preamp picking up the hum? If so that preamp might be lacking shielding in the input circuitry perhaps. Maybe no mumetal shield on the input transformer.

Unfortunately it is more than one item, but mostly preamps which makes sense to me given the gain they have. The biggest offender sound like a 1073 style unit directly above which reading a gearspace thread seems like is a unit which can be sensitive to pickup from items like LA2As - also external mount.

I would not fit a power transformer without bell covers hanging out the back of the chassis, it just seems a bit unsafe.

Ok. If it comes back I will see to mounting a project box over the transformer, regardless of hum.

 

[pucho812]

Hey Pucho- I'm curious where your gig was that used external PTs if I may ask?
Thanks!

You may ask. I sent you a pm with details.

 

[Graham Nystrom]

You may ask. I sent you a pm with details.

I respect a man who respects privacy!

 

[MidnightArrakis]

Maybe isolating the chassis from rack with some nylon washers?

Is your steel rack-chassis even grounded to the rack and/or the electrical mains GROUND? What or where does the incoming AC mains -- GREEN -- wire connect to, if anything?

Then.....there are these little doo-dads you can use to not only mount your rack-gear into a rack, but then also isolate the chassis from the rack itself:



https://www.ebay.com/itm/403671221191

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[MidnightArrakis]

providing them with a steel project box or some mu metal foil to mount over the power transformer?

Although I am guessing that your power transformer is -- NOT -- a toroid transformer, but if it is.....then, there are these steel toroid transformer covers that you could use:






And, finally.....should you want to use something else other than some kind of funky project box, if you are able to send me the basic -- height / width / depth -- mechanical dimensions of your power transformer (or, its datasheet), I could easily design something to mount over and enclose the transformer for you. EZ -- PZ!!!

Something like THIS!!!



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[beatnik]

Do the Neve input transformers have some sort of mu metal shielding or it's just a steel enclosure ?

Really the noise should be measured with a level meter at the output of the receiving unit and compared with the measurement with the emitting unit spaced afar.
Without an actual measurement you are not really sure what is going on.
It's also worth noting the receiving unit gain amount.
In the worse case scenario they have the receiving unit gain at max and the speaker monitoring level at max, that is obviously gonna make the noise floor very evident by ear.