I have three noisy bits of equipment that are giving me enough trouble to want to deal with.

1) M-Audio MIDISPORT 8x8 (MIDI interface, racked, 9V DC wart)
2) Boss CE-300 rack chorus (US version, running from a 45W Maplins step-down wart - which has no earth pin, just a plastic one - not hooked to patchbay as I use the front mounted jacks ad hoc)
3) Alesis Microverb IV (racked with the above, not currently patched in - powered by a 9V AC wart)

I'm getting what sounds like mains hum when I plug any of the above into the power, regardless of audio connections.

My mixer is hooked up to my balanced patchbay using unbalanced leads at the moment for the 16 channels, and today I've finally got round to balancing the Aux sends. I have found though that these audio connections don't seem to have any effect on the hum - it's only by unplugging the above three pieces of kit that the hum goes away. I noticed it mainly on the reverb sends/returns, which is why I balanced those.

Because the two audio devices are not patched in, yet still generating hum when plugged in - switched on or not - I'm guessing that the trouble with all three is the power supplies they use. Would that be a sensible guess?

I can do without the Microverb, and indeed it produces the least hum. The MIDI interface though is necessary. I'm using a standard chunky-transformer 500mA 9V wart which has a proper ground pin. The Boss chorus is also something I want to use - the cheapo 45W step-down that I use for it has a plastic pin instead of the metal ground pin that I would expect on something plugged into the mains... all these units are racked with my other synths & outboard, so my other guess is that the hum might have something to do with the metal rack unit.

Two things spring to mind - am I looking at replacing power supplies and/or the step-down? Also, would those Humfree things that clip onto the rack ears be any use here?

The other thing is that I'm running the low-voltage power leads alongside the mains ones up the rack to each unit. Would that affect things? All my rack kit is powered from the same wall socket via a multi-way tower.

Hopefully this is going to be the end of all this bother - I've been plagued by unwanted noise of various kinds for too long... lost count of how much I've read about it and how many threads I've ended up in...

EDIT: I should add that I do intend to balance or pseudo-balance my other connections as time and money allows.

Quote nathanscribe:

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I'm getting what sounds like mains hum when I plug any of the above into the power, regardless of audio connections.

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Do you mean you can hear an acoustic hum from them (or their power supplies) when you plug them into the power?

If so, then you have the familiar and annoying transformer laminations rattle, and there is virtually nothing you can do about it. It's caused by inferior mains transformers.

On the other hand, if you mean you can hear hum through the rest of the audio system, even though here are no audio connections made at the equipment itself from the affected units, then it could be down to radiated magnetic fields from the transformers in those units, breaking into nearby equipment and inducing hum magnetically.

In these cases, moving the offending power supplies well away from the more sensitive units (or their wiring) is the cure.

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Because the two audio devices are not patched in, yet still generating hum when plugged in - switched on or not - I'm guessing that the trouble with all three is the power supplies they use. Would that be a sensible guess?

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Ah... the story changes! Do you mean that these offending units have their audio outputs already connected to the patchbay? If so, then you already have an earth loop via the patch bay and the offending units' power supplies.

The cure is to break the screen connections on the audio cables from the offending units to the rack, at the rack end.

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EDIT: I should add that I do intend to balance or pseudo-balance my other connections as time and money allows.

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Good plan.

Hugh

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Technical Editor, Sound On Sound

The two effects processors are not hooked up to the audio patchbay at all - the Boss has jacks on the front panel that I use as & when, and the Alesis is rarely used any more and so is not hooked up at all by default. The MIDI interface is only connected to the other rack gear via MIDI cables, and there's a USB cable to the computer; all the above have wart-style power supplies. All are mounted into a Quicklok metal rack with about 20U of gear in it.

The noise is in the audio chain - any of the above units being plugged into power only (but not patched into audio) generated hum which is audible in my mixer channels that come from the rack - and also, for some reason, on Aux send 4 but not the others!

There is also an element of non-audio-chain tranformer hum but I have learned to live with that...

I'll try another power supply for the MIDI interface, and will try a longer extension to move the warts away from the rack a bit - it's definitely these three causing it, as without them plugged into the power, there is very little noise at all in my audio chain.

One thing that I wondered about was the Boss unit - being a US import, it has only a 2-prong mains lead going into a non-earthed step-down. Would that mean it was earthed only through the rack- and it that actually safe?

Are you using decent screened cables everywhere? What happens when you disconnect everything from your mixer apart from the headphones and then plug in one lead at a time? Is there one particular lead that it causing the problem? Or is there something in the mixer susceptible to the fields from the transformers?

James.

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JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

I did the whole "unplug and start again" thing with both audio and power connections, and found that no hum was present on the audio signal unless any of those three particular units was plugged into the power. No other units in the rack hum. Bizarrely, when the offending units are plugged in (again, power only, no audio connections) there's hum on the mixer's Aux send 4 too - which means I get a permanent reverbed mains drone...

Removing the three offending articles form the power supply cures the problem.

All my cables are home-made to fit using Van Damme cable and Switchcraft/Neutrik jacks. I've run the mains leads for the rack kit up one side of the rack tower, and kept audio away and not parallel, as much as possible.

Quote nathanscribe:

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2) Boss CE-300 rack chorus (US version, running from a 45W Maplins step-down wart - which has no earth pin, just a plastic one - not hooked to patchbay as I use the front mounted jacks ad hoc)
sends/returns, which is why I balanced those.

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The hum may be introduced by your step-down transformer if it is an isolating type. Since the Boss CE-300 does not have a mains earth connection, it may be developing an earth potential causing the ground loop.

What happens when you eliminate the CE-300 from your setup?

Taking out the Boss removes the hum cause by the Boss. If the other two units are still plugged into the power, they still hum like the Flying Pickets.

Do you have a tester to check whether the main socket you are using has an earth connection?

Have you tried swapping the mains distribution extension block(s) that you use?

How many extension blocks do you have? If more than one, are they daisy-chained?

The fact that you are getting induced hum even when the units aren't connected to anything via the audio connections implies the hum is being coupled magnetically.

So try moving the three offending units as far away, physically, from the rest of the gear as you can and see what difference that makes.

Hugh

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Technical Editor, Sound On Sound

Righto:

Take the offending adapters out of the power tower, remove one of the offending units from the rack, plug in its power adapter (unit not connected) = hum.

Remove adapter again. Plug in an extension lead to the power tower, which places the mains plug for testing about a metre away. Plug in the removed unit = no hum!

Move the newly-distant adapter back towards where it was originally = increasing hum.

Moving the adapter around = varying hum.

Repeat for each unit. Results? The units being racked is of no issue - it's the location of the adapters that's generating the hum. I suspect a combination of the physical construction of the rack and the way I've laid the wiring around the place.

I take it I should simply distance these adapters from the rack? Or should I reconsider my neatly bundled cables?

Quote nathanscribe:

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Take the offending adapters out of the power tower

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When you say power tower, you mean one of those PC-type mains distribution blocks that are vertical as opposed to horizontal?

Have you tried swapping that out completely?

How about powering everything from a different socket from another power circuit in different room?

Part of your problem seems to be proximity of audio cables to mains cables but it shouldn't be causing a loud hum if you are using good screeened cables.

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When you say power tower, you mean one of those PC-type mains distribution blocks that are vertical as opposed to horizontal?

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Yep.

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Have you tried swapping that out completely?

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Nope. Nothing else with enough sockets.

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How about powering everything from a different socket from another power circuit in different room?

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Not possible. The power in this room is the power in this room. I have to use it, and that's that!

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Part of your problem seems to be proximity of audio cables to mains cables but it shouldn't be causing a loud hum if you are using good screeened cables.

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I do want to balance as much audio as possible, but that's ongoing as I can get round to it. As for separation between power/audio cabling, I can only assume that if the cabling is at fault I have a rogue bad one, as almost all my cabling has proved fine before. Not only that, but the audio cables are not near the power supplies. They pass near mains leads, but as I have previously stated, not parallel and to as minimal an extent as I can manage.

I understand that you have to use the sockets in that room however, you may have a problem with the supply. It would be good to try an alternative socket elsewhere.

Also, those power towers do occasionally have earth faults/loose sockets. Try using another mains distribution block with fewer items connected. Use the same ones that have the humming problem and then swap out the power tower.

I know you are stuck with your setup but, you need to try and eliminate the source of the problem.

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Also, those power towers do occasionally have earth faults/loose sockets. Try using another mains distribution block with fewer items connected. Use the same ones that have the humming problem and then swap out the power tower.

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I already tried different sockets on that tower with the offending adapters, and that's not the issue. I also tried quiet gear in the sockets I'm using for the offending units, and they were still quiet, suggesting it's not a case of bad sockets.

I have realised that the only units causing hum here are the three with external transformers. All the other gear on that tower has regular plugs with mains leads going to internal power supplies of whatever type. Wall-warts are what these three noisy units have in common, and moving them physically away from the other gear removes the hum.

What I meant was swap out the power tower temporarily not swap around sockets - they aren't very good for professional audio anyway.

Do those three items have earth terminal screws on the back?

If they do, you can connect them all together.

Hi Nathanscribe,

It’s so good to see someone being systematic with a hum/ground loop problem.

In your case I suspect the best approach is to plug all three wall warts into their respective units, group the three rack-to-wall-wart cables together and away from the rack as one umbilical, tidy up the excess and then place all three wall warts some distance away.

You should then be able to plug them all into one 4-way distro board and have extremely low hum levels.


Martin

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YewTreeMagic

Quote nathanscribe:

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Remove adapter again. Plug in an extension lead to the power tower, which places the mains plug for testing about a metre away. Plug in the removed unit = no hum!

Move the newly-distant adapter back towards where it was originally = increasing hum.

Moving the adapter around = varying hum.

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Well there you go. The source of the problem is clearly the radiated magnetic field from the wallwart/power adapter.

The next question to answer is where is that magnetic field being picked up?

Balanced cabling should be immune (provided it is more than a few inches away from the transformer). Starquad balanced wiring is even more immune. Unbalanced cabling will be prone to picking up magnetically radiated hum.

So re-working your audio cabling to keep it as far away as possible from the troublesome mains adapters is an obvious first step... and rewiring everything with balanced (or pseudo-balanced) cabling is the second.

But it's also possible that the magnetic field is being picked up by other hardware devices -- particularly anything with input or output transformers -- so moving the wallwarts /power adapters well away from those units will help too.

Might be worth turning off, fading out or disconnecting your other audio equipment to see where the hum pickup is getting into your system. Start with the speakers/power amp and the cabling to them, and work back up the signal path from there.

hugh

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Technical Editor, Sound On Sound

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How about powering everything from a different socket from another power circuit in different room?

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While this will remove the wallwart/power adapters radiated magnetic field from the scene, it is guaranteed to create the mother of all ground loops when the audio is hooked up again! So fine as a fault-finding tactic but not a viable solution in the long term.

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Part of your problem seems to be proximity of audio cables to mains cables but it shouldn't be causing a loud hum if you are using good screeened cables.

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Cable screening only protects against radio frequency interference and nothing else. What we are dealing with here is a radiated magnetic field and the only way of rejecting that is with a balanced cable in which the magnetically induced currents in the two signal wires are cancelled out by the differential receiver at the balanced input.

hugh

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Technical Editor, Sound On Sound

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I understand that you have to use the sockets in that room however, you may have a problem with the supply.

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The description of the way the hum comes and goes with the proximity of the wallwarts to the audio equipment and its cabling strongly suggests a radiated magnetic field from that wallwart to be the problem -- which is not that uncommon -- and not a mains supply or mains socket problem.

hugh

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Technical Editor, Sound On Sound

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Do those three items have earth terminal screws on the back?
If they do, you can connect them all together.

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Again, there is no logic in this. It is very clear that the issue isn't a ground loop but a radiated magnetic field. Adding additional earth bonding to the units cases won't help.

hugh

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Technical Editor, Sound On Sound

Quote Martin Walker:

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It’s so good to see someone being systematic with a hum/ground loop problem.

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Absolutely! The only way to get to the bottom of these kinds of frustrating issues.

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In your case I suspect the best approach is to plug all three wall warts into their respective units, group the three rack-to-wall-wart cables together and away from the rack as one umbilical, tidy up the excess and then place all three wall warts some distance away. You should then be able to plug them all into one 4-way distro board and have extremely low hum levels.

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Absolutely -- I concur. You may also find that rotating the wallwarts plug-board relative to the rest of your system affects the level of hum. Most transformers radiate more strongly in some planes than others, so finding the 'null plane' and aiming that at the equipment might help.

And keep all unbalanced audio wiring and other mains cables as far away from these offending wallwarts as possible! Also, try to arrange your cables so that the audio cables cross over mains cables at 90 degrees to minimise the risk of magnetically induced hum generally.

hugh

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Technical Editor, Sound On Sound

Thanks for the replies chaps!

I've plugged the offending warts into a cheap 4-way trailing socket for now, which I'll replace with a sturdier one later - and I've placed that outside the rack frame to one side. The hum has gone.

However, in keeping with the general impedance to workflow, I've discovered a dodgy pair of unbalanced leads which seem to have some extraordinary noise on them for no apparent reason, and now my Lexicon MPX1's inputs are playing up. One thing after another...

So, today's job is indeed to pull the rack into the middle of the room, pull out all the audio connections, double-check all the power cables, and make up as many balanced/pseudo-balanced cables as I have the bits for. I'm also going to bypass the patchbay-mixer cabling till I can make a 16-way balanced loom.

Things used to be easier when I was 10 and only had a cheap Casio.

Glad you've resolved the hum... even if that has now revealed other foibles!

BTW, the usual psuedo-balanced cable is a great tool for curing ground loops, but it doesn't offer the same rejection of magnetically-induced hums as a proper balanced interface because the impedance to ground is different on the hot and cold wires, and thus the cable isn't truly balanced (hence its name I guess! )

Hugh

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Technical Editor, Sound On Sound

Quote Hugh Robjohns:

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Glad you've resolved the hum... even if that has now revealed other foibles!

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Yes, I think by the time I've finished I might be a little pseudo-balanced myself...

We all end up a little like that nathan

Glad you're largely sorted


Martin

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YewTreeMagic

Well it wasn't quite as horrid as I feared - a couple of cups of tea's worth of cable-checking, re-routing and whatnot - turns out the main problem aside from the transformers was a duff output on an old cassette deck, which has now been removed. It was shoving disgruntling frequencies around in places it had no business to, and since removal things are much better!

I still need to balance more as there are minor bits of hum when patching between some things, but that's part of my ongoing plan and Farnell can expect some of my money next week I think - I've got a DI and ART Cleanbox II that are sorting that out in the meantime.

Thanks again chaps!

Quote nathanscribe:

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I did the whole "unplug and start again" thing with both audio and power connections, and found that no hum was present on the audio signal unless any of those three particular units was plugged into the power.

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I know you've found a solution already, but I would also have kept the offending transformers in place and disconnected all the audio connections from the mixer. Then replace the audio connections one by one until you find the one susceptible to the hum. Or is it the mixer itself that is susceptible?

James.

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JRP Music - Audio Mastering and Restoration.
http://www.jrpmusic.net

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I know you've found a solution already, but I would also have kept the offending transformers in place and disconnected all the audio connections from the mixer. Then replace the audio connections one by one until you find the one susceptible to the hum. Or is it the mixer itself that is susceptible?

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It's been a couple of days now so the information has already left my brain... but basically, I don't think it mattered what was plugged into the mixer, if those transformers were plugged in under the rack gear, there was hum. Also, there was another load of noise getting in from the dodgy outputs of a tape deck that I'd forgotten was patched in to the back of the patchbay and thereby to the mixer - with the tape deck gone and the transformers moved, it's fine.

I did indeed remove all the audio connections one by one and re-try them individually and accumulatively before moving the transformers, as I'd originally though it to be a ground loop rather than induced hum - I can't recall now whether particular channels were more hummy than others, but I had not realised at that time that the tape deck was chucking crud out as well - that was when I decided to remove everything and start again, one piece at a time.

As for the mixer being susceptible I don't know. It's an A&H Zed R16 so I'd expect it to be decent. It sits just to the right of a 4-foot-ish rack (a couple of Quicklok units atop each other, so all metal with perforated sides and crossbars at the back). I've about 20U of gear in there, most has a regular 3-pin UK plug, three units (as above) have external supplies, and only the Boss uses a step-down. I only switch on what I use at the time. I've strapped the mains cables together down one side of the rack, and they branch off to each unit at height. The audio to/from the mixer is a 16-way unbalanced loom that comes into the back of the rack at the top and then descends to the patchbay on the opposite side of the unit to the power cables. Remaining audio and MIDI connections are somewhat free behind the modules, with a couple being tied together for convenience.

I don't honestly know how the hum was getting into the audio chain, as the transformer units were on the floor at the base of the rack and not near any audio cables. They were in the middle of a fair bunch of mains cabling though.

None of the rack gear is isolated from the rack itself - it's all just bolted in.

The whole stack goes back to one wall socket.

In short, I don't know enough to say exactly why these various noises were appearing, but they were, and now they're not. Hopefully I can keep it that way!

Quote nathanscribe:

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...the transformer units were on the floor at the base of the rack

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What equipment is/was installed in the bottom half of that rack? Could it have been mic preamps or other products with input or output transformers?

hugh

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Technical Editor, Sound On Sound

The bottom unit is about a foot off the floor, and is a MOTU MIDI Express 128. Above that, in ascending order: ART Pro VLA ii compressor (no audio connected), Alesis Microverb IV (no audio connected), Lexicon MPX1 (patched from 2 aux sends and back into two channels via patchbay - now with fully balanced inputs and pseudo-balanced outputs), Korg DRV1000 reverb (hooked to a separate unbalanced patchbay in the same rack), Boss CE-300 (no audio connected), 2x Digitech RDS1900 delays (1 has no audio connected, the other is patched into the 2nd, unbalanced, patchbay), unbalanced patchbay, balanced patchbay, M-Audio Midisport 8x8 MIDI interface, Yamaha TX81Z (patched in to unbalanced patchbay), Ensoniq Mirage (into unbalanced patchbay), Roland U-110 (unbalanced patchbay) and finally, at the top, a Denon tape deck which was originally patched to the mixer on two channels, but is now out. Perched on top of the lot on a wooden shelf is a UA710 pre amp, but I took that out for the purposes of fault-finding.

I don't think any of these have transformer outputs.

I know from experience that small, sub 20va say, wall rat transformers radiate quite an extensive field. This is probably due to the cheap construction of the transformer and the fact that a unit is often chosen that is only just good enough for the task, transformers radiate best when worked close to saturation!

One solution if you are of a DIY bent is to replace the rat with a decent toroid in a steel box, a 13A back box and steel blank top affords a few extra dBs of shielding. The toroid should be of generous rating, at least 50% up on the supply required. The regulation of toroids is so good that if you needed 12V at 1amp say and had a gash 120va job kicking around, that would not be THAT silly!

50Hz to DC output supplies can of course be replaced with SMPS units and even quite inexpensive ones have very little RF emmission these days, plus of course all modern kit should be well proofed against such interference whereas it is hard to keep 50Hz out.

Dave.