M20d Live Mix -- Vocal Feedback - Gain Issue?

[ncockerill]

Could use some advice from some folks more experienced in live sound than myself.  I am a guitar player, and my 4-piece band utilizes the Stagescape M20d with Stagesource L3t and L3s speakers via L6 link.  We have a friend running FoH sound for us via an iPad.  He is not a sound man by trade, but generally does a good job based on what we hear from audiences and club owners.

 

Ours is an interesting setup --- I’ll need to explain as it’s important to understand this for any diagnosis.

 

My guitar, the bass guitar, and the rhythm guitar all run direct from our various guitar processors into the M20d.  Similarly, our drummer’s electronic Roland kit runs direct to the M20d.  So, there are NO amps of any kind on stage.  So that we can hear ourselves onstage, each of us has a small Roland CM-30 Cube Monitor fed from the M20d monitor outs, and we use wired in ear monitors into the headphone jacks of those Roland CM-30s. Really sounds great in our heads.  The external speakers of the CM-30’s are muted.  Each of us have a vocal mic direct to the M20d.

 

At a few shows, we’ve been experiencing (low hum, not squealing) feedback from the vocal mics, and I just can’t understand it.  The Stagesource speakers are well in front of the mic line, so that’s not it.  And there are no amps/monitors on stage producing sound, so that’s not the culprit either.  But our sound man has a hard time getting our vocal channels to a good level when the overall main volume increases.  At a lower volume sound check, things are great.  When we start the show, and he starts inching up the mains to gig volume, he just can’t seem to get the vocal mics loud enough without the hum beginning.

 

Any thoughts?

[SiWatts69]

Low level hum doesn't sound like mic feedback to me.

 

First things to check:

Are your guitars and the kit going in to the M20d via outboard DI boxes or direct?

Have you "trimmed" the input levels for all inputs?

Are any of the guitars "active"?

 

Low frequency hum is often an indication of ground loop hum. This could be being generated by your electronic drum kit and even sometimes guitar pedal boards. Does the "feedback" present itself in your in-ears for everyone?

 

I don't typically plug instruments directly in to the M20, almost always using a DI box with the ground lift engaged to separate to two devices. Keyboards and other "powered" instruments are the biggest issue for ground loops but you won't necessarily experience a ground loop hum at every venue... it depends upon the mains circuits over which you have no control.

[ncockerill]

Thx SiWatts69 for the reply.  The three guitar processors are all direct to the M20d (via a snake) and the Roland drum kit is direct to the M20d (not to the snake – the mixer is typically next to the kit).  No DI boxes in use.  No “active†guitars, if you are referring to active pickups.

 

Three mics go to TC Helicon VoiceLive boxes, then to the snake to M20d.  The drummer’s mic is straight to the M20d (again because the mixer is next to the kit).

 

Yeah, we hear the low hum in our in-ears as well. 

 

We will typically use “Auto-trim†at line/sound check, and I believe trim-tracking is on for all channels.  Our sound man describes the mics as “very hotâ€.  He’s basically tinkering with mic trim and level settings throughout the show on those nights when things are misbehaving, and often resorts to just muting certain mics on songs where any particular person isn’t singing.  He insists that the source of the hum/feedback is definitely in the vocal mics, because when he mutes mics, all sounds great.

 

I sometimes wonder if the bass from those giant L3S subs are somehow getting back into our mics?

 

Again, thanks for any advice.  I know diagnosing sound issues on a web forum is mostly speculation.

[SiWatts69]

ok then, another couple of questions...

 

The gain knobs on the back of your speakers... Are they set at the 12 o'clock position?

What amount of reverb (or other FX) are you running?

What mics are you using?

 

 

 

Auto-trim really ought to be set and forget. If he's messing with trim levels during a performance it suggests there's something seriously wrong in his methodology because unless you all sing/perform at one level for sound check and then crank it up for the performance there shouldn't be any significant variance in the input level. BUT, having the input trim level too high is a surefire way to create problems.

 

The occasions where I struggle with feedback are when any one or more of the following is true;

Onstage backline volumes too high so monitors pushed high to get vocals over the top

Running without subs (as in these instances we're using backline for FOH for guitar and bass)

Weak vocal strength, meaning greater input trim required

Too much FX on a given channel.

 

BUT, in all cases, the feedback is typically always in the upper frequency ranges atypical of microphone feedback.

 

For want of a better suggestion, your guy has quite possibly tied himself up in knots, desperately trying anything and gradually making the overall setup deteriorate and it may be better to scratch the setup and begin again from scratch with a clean set of presets.

[ncockerill]

Gain knobs are at the 12 o’clock position.  And for the L3s, I typically keep it just below 12 o’clock in the hopes of somehow eliminating some of the rumble that could be a cause.

 

Only the drummer’s mic is getting reverb from the board.  We set reverb/FX levels on the TC Helicon VoiceLive devices for the other 3 vocals.  No need to apply additional reverb/FX to those channel strips.  Same for the guitar processors – we use the “bass modeler†or “guitar modeler†presets in the M20d which are pretty flat with no FX added.

 

Mics are SM58 and TC Helicon MP75.

 

I suspect your suggestion that our “sound†guy is tying himself in knots is true. He’s over his head, but in fairness, so am I.  And there’s little I can do from the stage.

We start each evening with our basic Setup, created from the previous night’s rehearsal.  Important to note that our rehearsals are “silent†--- we are not running mains at all and rehearse through in-ears exclusively.  Yes, I know, not ideal – but 3 young kids at home means I can’t be rattling the house all night.

 

 I think we’ll need to do a better job at sound check.  We typically don’t like to do long, loud sound checks while many patrons are still eating, talking, etc.  Generally we keep it brief and relatively quiet as to not disturb; just enough to know our mix is pretty good and everything is “liveâ€.   The problems start when the mains are turned up to actual gig level (and we are not a band that plays overly loud – we’ve actually been asked to turn it up by venue owners!).

[SiWatts69]

I think we’ll need to do a better job at sound check.  We typically don’t like to do long, loud sound checks while many patrons are still eating, talking, etc.  Generally we keep it brief and relatively quiet as to not disturb; just enough to know our mix is pretty good and everything is “liveâ€.   The problems start when the mains are turned up to actual gig level (and we are not a band that plays overly loud – we’ve actually been asked to turn it up by venue owners!).

 

I reckon you've perhaps hit the nail on the head in your last statement.

 

In your efforts to keep the noise to a minimum, when you create your mix, you're not firing on all cylinders and as such, any auto-trim level set at the outset is not going to be representative of your actual sound at performance time.

 

Blow the audience that might be in the venue at the time! They know its a live music pub when they arrive and likely know that at some point or other, there's the likelihood of the band needing to strike up for a short period. If necessary, announce that you'll be doing a short soundcheck and apologise, but you MUST soundcheck at full pelt.

 

Here's what we do! (there's two scenarios here with us since we don;t always roll out the full rig and often play backline only for drums and bass.)

 

Scenario 1 (Full rig) 2x L3m, 1 or 2x L3s, 4x L2m/t as monitors, everything mic'd and run through PA.

In this scenario, it is typically larger venues where we are less likely to have "company" during setup/soundcheck.

Drums auto-trimmed (kick, snare and a stereo overhead) one by one.

Bass (DI from back of amp head) trimmed once and rarely needs changing.

Guitar mic'd from small fender combo.

4 vocal mics

2x sax mics (radio)

 

We then perform a full song with monitors only (ie no level out front) and record all inputs. If necessary, monitor levels are tweaked based on individual needs.

We then perform a second song (with my other sax) again recording it as we go.

A FOH mix is then done in headphones from the two recordings.

Finally, we playback the recording and whilst all stood out front, the masters are raised and small tweaks made to adjust for the room (lifting vox above backline for example)

Once everyone is happy, we leave it well alone and just mute mics

Bear in mind, we pull the main outs up to full performance levels in this process, and with mics open on stage and monitors blaring, any feedback that might rear its head is often revealed.

I'll do a final walk across the mics, cupping each toward the nearest monitor, again trying to induce feedback. If we're on the edge, I will sometimes force the main level up a little to try get the M20d to notch out the most obvious feedback frequencies.

Rarely do we get feedback during a performance and we don't run anyone "out front" tweaking things. I have my ipad on my side of stage and the M20d is on the opposite side near our guitarist.

Even if people are in the room, we typically follow the same routine. Indeed the whole playing back the recording often piques the interest and we've had people stay on to watch us after hearing the soundcheck, people who normally leave before the bands start!

 

Scenario 2 (Small pub), 2x L3m, 4 vocals, 2x sax mics, and guitar (mic'd from a small fender combo). Drums acoustic. Bass backline only.

In this scenario our only option is to do a live soundcheck. The guitar, bass and drums have a quick riff to get their backline balanced and for the guitarist to get his fender to an appropriate level for his on-stage needs. We then trim the guitar combo mic (I use an Audix D4). Vocal mics are identical for each of us every outing so trim levels are retained from previous outings, rarely needing to be changed. Likewise, my sax mics are radio mics so their level simply doesn't alter from one gig to the next.

 

We have two songs we sound check to (I have two saxes so do one song for each sax)

As I'm on radio, I typically go "out front" to listen whilst we play through each of the two songs.

 

Typically, we use a saved setup which is a (scenario 1) mix we know was great. Masters are simply brought up to level that balances my saxes, the guitar and vox against the backline. Don't need to touch anything related to drum or bass channels as the inputs aren't present.

 

 

 

I'll not pretend that it was easy at the outset. Most of my/our issues at early outings was created by overpowering a room. Learning when to drop to one sub and even when you don't need subs is a difficult lesson, but one which once learnt will ease some of your issues. I don't bother with subs for small venues (up to 125'ish people) - typical UK "pub". One sub rolls out when we're pushing 125-200 capacity and the second sub only typically comes out for venues over 200 capacity.

 

The L3t/m speakers have remarkable bass handling capabilities and are more than capable on their own in small venues. Perhaps your issue is the sub being present though your low level hum style feedback isn't one I've ever experienced. If you turn the main level (on the sub itself) down whilst the hum is present, does is go? Worth trying. Though beware that when you have an L3s in your chain, everything below 120Hz is routed to the sub and turning it down will kill your bass. The L3m/t will not be receiving those low frequencies. Better to unplug the L3s from the L6Link.

[ncockerill]

Awesome feedback and suggestions.  Thanks so much for your time.  

[silverhead]

Of all the good info from SiWatts69 I think the most important thing he said for your situation is this: "you MUST soundcheck at full pelt". Otherwise you are not actually checking your sound. You are checking some other sound and essentially wasting your time. It's almost certain that the Auto-trim Settings you generate during sound check are NOT the correct settings for your performance. That's most likely the reason for the feedback.

[ncockerill]

Agreed.  I think my inexperience here is the culprit.  I thought that the trim control was used to detect and set the level of an input signal into the mixer --- and had NOTHING to do with ultimately how loud we turned up the mains.  That is, I thought since most of our inputs are direct from digital sources (electronic drums and guitar processors) -- these can be set consistently from show to show (e.g. I always set my output volume from the Helix at 1 o'clock -- I assumed this would result in a near identical input trim level from show to show).  I mean, I am sending the same level on that channel every night, right?

 

But where the issue really lies is in the vocal mics.  That's where we are hearing the low rumbly feedback from.  Wondering --- should we Auto-trim the vocal mics one at a time while the whole band is playing at gig volume, or will that not matter?  The mixer doesn't set the Trim relative to the other channels, right?

 

Personally, I think we should ditch the L3s for most of our shows.  We play relatively small bars and I suspect that sub is doing more harm than good.  

 

Christ -- am I making any sense?  LOL

[silverhead]

....  I thought that the trim control was used to detect and set the level of an input signal into the mixer --- and had NOTHING to do with ultimately how loud we turned up the mains.  ......

 

 

 Wondering --- should we Auto-trim the vocal mics one at a time while the whole band is playing at gig volume, or will that not matter?  The mixer doesn't set the Trim relative to the other channels, right?

 

Personally, I think we should ditch the L3s for most of our shows.  We play relatively small bars and I suspect that sub is doing more harm than good.  

 

....

 

You're right that trim control is used to detect and set the level of an input signal into the mixer, and it is independent of the volume of the mains unless the room acoustics deliver the sonic reflections from the mains back off the walls and into the mics. If the bars you're playing in are very small this may be contributing to your problems.

 

But I think the main problem is this: it's really important to set the trim input levels while you are performing at the levels that you will be performing at during the show. As I understand it you are now going through your sound check at relatively lower levels and that is when you are setting the trim levels. That means, for vocal levels in particular, that your singers are singing/speaking at lower levels than they are when they are really wailing during the show. If you set your auto-trim at that time then the mics will seem way too hot when the louder levels of the show kick in. When you use auto-trim you need to have your vocalists singing with the same intensity that they will perform with during the show. No mumbling little 'test 1... test2... test3...'. That's fine for checking that the mic is actually working but that's not a sound check.

 

I suggest that for the sound check you select one of your louder songs and play it in its entirety as you will during the show. Have your sound guy apply the auto-trim feature at the loudest section of the song when all mics are being used. After setting the trim levels he can then capture a 20-second recording during the same song. You don't need to have the mains on when doing this - it can all be done using headphones in the M20d. When the song's done the band takes a break and the sound guy does any final level tweaking still using headphones during playback of the recording (no EQ or FX changes at this point - headphones sound different than the mains). Repeat until satisfied.

 

You don't need to set trim levels for each mic one at a time. Just make sure all mics are being loaded at gig volumes.

 

I don't use the L3s at small bar venues - just for bigger halls and larger assembly-type rooms.

[SiWatts69]

I thought that the trim control was used to detect and set the level of an input signal into the mixer --- and had NOTHING to do with ultimately how loud we turned up the mains.

What auto-trim is trying to do is set the signal level into each channel strip as close to 0db WITHOUT "clipping", ie distortion.

Ultimately it DOES affect the output volume stage for the channel, but it does so early in the chain. Distortion that is then supressed by a lower output level doesn't disappear.

 

When you auto trim, if you are not giving it enough welly (ie if you are not performing exactly as you will during the performance) auto trim will set the initial gain stage higher than it should be. You then kick off your performance, and your sound guy is battling a "hot" mic.

 

Imagine it this way...

If I stood next to you and sang, the chances are the volume my voice would generate would be different to the volume you generate. You may be louder than me, I may be louder than you.

If we both sang into an SM58, the mic would create more signal from the louder voice than it does for the quieter voice.

When the signal from the mic reaches the M20d, the "trim" stage (or gain stage) aims to make those two voice levels the same. It might pre-amplify the louder voice by 3dB but need to pre-amplify the quiter voice by 10dB

By doing so, the signal strength in the two channels is now equal. Trim is there to equalise the input signal strengths.

 

That is, I thought since most of our inputs are direct from digital sources (electronic drums and guitar processors) -- these can be set consistently from show to show (e.g. I always set my output volume from the Helix at 1 o'clock -- I assumed this would result in a near identical input trim level from show to show).  I mean, I am sending the same level on that channel every night, right?

It is true that fixed level output devices are unlikely to need trimming every time. Indeed, many of us don't bother to trim vocal mics every time, BUT we do so knowing that the trim level was set correctly on a previous occassion, and nothing in the chain has changed; same singer, same (exact) mic. You can only do this though if the same singer uses exactly the same mic, processed (pre M20d) in exactly the same manner every time.

 

I'm not sure how the output stages of your guitar boards work, but I'd strongly suggest that they get trimmed, at least a few times. If they've never been trimmed the signal could also be too hot. Get them trimmed. Then at the next outing, trim them again, and again the time after. Once you are confident that auto-trim is not having to adjust the trim level, you can stop trimming them, but you should still do it every now and then just for checking.

 

Your TC-Helicon vocal boxes have likely got Mic-Pre's built in. I use a TC-Helicon voicelive solo pedal on my sax, but before hooking it up, I check that the mic-pre level is fully down. You need to check that the mic-pre levels on your TC boxes aren't up and if they are, consider turning them dow somewhat before trimming. Not certain which TC boxes you have, but if they have external analogue mic pre level controls, they are very easily nudged in transit.

 

Auto-trim will show you the change it makes. It'll be either +XdB or -Xdb. The figure displayed is the amount of adjustment from the existing level.

 

But where the issue really lies is in the vocal mics.  That's where we are hearing the low rumbly feedback from.  Wondering --- should we Auto-trim the vocal mics one at a time while the whole band is playing at gig volume, or will that not matter?  The mixer doesn't set the Trim relative to the other channels, right?

Trim level is distinct to the channel. It will happily auto-trim multiple channels at the same time. But, as Silverhead has pointed out, auto-trim needs to be done at the loudest section to be truly effective.

 

 

Personally, I think we should ditch the L3s for most of our shows.  We play relatively small bars and I suspect that sub is doing more harm than good.

The only way you'll find that out is by daring to run out without the L3s!

 

Don't be scared of taking it and not plugging it in.

Don't be scared of reigning your volume in a little (too many bands play too loudly IMHO)

 

If it puts it into context, I have two subs. One of them gets used around five or six times a year. The second one gets used ten to twelve times a year. We run out without either sub around 20-25 times a year!

[Rewolf48]

Some great advice here, but can I ask the dumb question?

 

Do you have the Highpass Filter enabled on the Input of the vocal channels (deep tweak) and set to a suitably high value?

 

If the low frequency noise is below the frequency range that you want to hear from the vocals then it will reduce the level significantly - it won't work so well if you have low range voices such as Bass or lower Baritone of course, but generally for amateur singers setting the filter higher works well as it also cuts the proximity effect from when they try to eat the mic in order to get the volume up, and moves the resulting vocals out of the mud range

[ncockerill]

Once again, huge thanks to SiWatts69 and SilverHead for all of the insight and education.  I am humbled by your willingness to help a fellow musician; it’s what makes the Line 6 Community so great.

 

We have a gig in 2 weeks, and I am going to religiously follow all the advice given.  It’s a small venue, so I’ll ditch the sub, and setup very early to leave time for a proper sound check at full volume.  I’m not so naïve to think that everything will be perfect now, but I am confident that your direction has set me on the proper course.

[ncockerill]

Some great advice here, but can I ask the dumb question?

 

Do you have the Highpass Filter enabled on the Input of the vocal channels (deep tweak) and set to a suitably high value?

 

If the low frequency noise is below the frequency range that you want to hear from the vocals then it will reduce the level significantly - it won't work so well if you have low range voices such as Bass or lower Baritone of course, but generally for amateur singers setting the filter higher works well as it also cuts the proximity effect from when they try to eat the mic in order to get the volume up, and moves the resulting vocals out of the mud range

 

ReWolf48 – thanks for joining in.  It’s actually a great question.  I don’t have the board fired up right now, but it is something I intend to check.  I believe that I did set a low cut (high pass) filter on the vocal mics --- somewhere around 100Hz.  But it is certainly possible that I missed this in my latest Setup.

 

In general, is there a low cut value range you suggest (we are all male vocals, none with low range/baritone voices)?

[SiWatts69]

One last suggestion then...

 

Start again FROM SCRATCH!

 

Start with a blank new setup. Rebuild your inputs (you can do this before getting to the venue) and then load standard presets with nothing whatsoever tweaked.

 

That way, if your lovely (but inexperienced) sound guy (or anyone else in your band) has inadvertently introduced any gremlins in your attempts to sort it in the past, they'll be gone. Clean slate.

 

 

Let us know how you get on!

[ncockerill]

One last suggestion then...

 

Start again FROM SCRATCH!

 

Start with a blank new setup. Rebuild your inputs (you can do this before getting to the venue) and then load standard presets with nothing whatsoever tweaked.

 

That way, if your lovely (but inexperienced) sound guy (or anyone else in your band) has inadvertently introduced any gremlins in your attempts to sort it in the past, they'll be gone. Clean slate.

 

 

Let us know how you get on!

SiWatts69 – you read my mind.  I definitely think we need a “clean slate†approach.  I’ll rebuild the stage with standard presets and start from there.  Undoubtedly I have tinkered way too much with our previous Setup and introduced plenty of Gremlins for sure.

 

Our sound guy is a great guy, a family friend, who graciously donates his time to help the band.  His only compensation being he’s on the band’s bar tab.  We are typically set up and (poorly) sound checked before his arrival.  I think it’s clear now that I’ve been handing him a board to manage that’s a bit of a mess.

 

Oddly enough, the response from the venues/patrons regarding our sound has been overwhelmingly positive (they typically don’t listen as critically as we musicians do).  All things considered, he’s done a fantastic job of fighting through issues due to my poor setup.  I need to fix that for him, and the band.

 

I’ll let you know how the next gig goes.

[Rewolf48]

In general, is there a low cut value range you suggest (we are all male vocals, none with low range/baritone voices)?

 

100Hz for the -3db point is barely cutting anything audible.  

 

I am not saying that this will be suitable for your situation, but double checking what I have set in reaper for recordings for our band for the male vocals I have it set up at 400Hz (can't remember if the M20d low cut cut can go that high, but if not then the other EQ can).

 

This means that it starts cutting at about 600Hz and is -12db at about 200Hz - it lifts the not particularly high vocals out of the main contention range (mud).

 

Good singers wouldn't necessarily need that treatment, but we aren't great singers with a high level so tend to have to get closer to the mics and the proximity effect has to be attacked hard. Don't be afraid to cut hard in the lower end on everything that doesn't have to be there which is typically just Bass and Kick. 

 

The professional settings on the M20d defaults do need to be tweaked to suit the less professional bands.  I seriously suggest if you aren't doing it already getting a 32GB SD Card and recording all inputs from rehearsals and gigs and then to practice trying to get a good live recording whether just using the M20d or a DAW (I use Reaper which is very reasonably priced) - the EQ settings that you make for those live recordings can be replicated into your band set-up or into preset adjustments for your instruments. If you record the main outs as well then you will be able to hear what the PA was putting out when playing live and you can sit back and listen critically a few days later and decide whether you are happy with the sound.  The ability to record without additional gear is one of the best features of the M20d, and there are loads of "tutorials" on the web that will help you with the basics of Live Sound mixing.

[ncockerill]

Reporting back after last night’s gig.  Huge thanks to SiWatts69, Silverhead and Rewolf for the advice.  I followed all of your guidance, and very happy to say that we had ZERO sound issues last night.  No squeaks, squeals, hums, or gremlins.  All accounts were that things sounded great.

 

Our sound man had a family commitment and could not come, and I was left to do the setup, sound check, and mix by myself.  As advised, I started from scratch with a brand new Setup (completed the night before with untweaked presets).  We got our equipment set up in plenty of time, and had the club owner cut the house music for twenty minutes or so while I trimmed everything, then set levels, and finally took a few recordings of the whole band performing various songs from our setlist.  Got things dialed in at gig volume with an iPad at front of house and we were all set for the night.  Outside of the tiniest tweak to one vocal mic level in between sets, no other changes required.

 

Ditched the L3s sub – no need for it in this small venue and the L3t’s had plenty of bass to fill the room.  I do think that helped a lot.

 

While I still have lots to learn, the process is definitely repeatable, and feeling much more confident.  No more cutting corners or compromising.  Do it right, and it sounds right.  Thanks again, guys.

[silverhead]

Great to hear! Thanks for reporting back. This thread may be helpful to others in future.

[SiWatts69]

Brilliant :-)

 

Long may it continue ;-)

[Rewolf48]

:)

[CollettesKaraoke]

i was reading some of the chat & we had some hum noise coming through our system when we tried to isolate it it was coming through the mains power outlet where we were connected to a TV & aerial was plugged in once we took the aerial plug out of TV no hum I have been running a Karaoke show so had to access Pubs TV which had an interconnecting AV & aerial line into the TV we were using we had a HDMI

  

[SiWatts69]

i was reading some of the chat & we had some hum noise coming through our system when we tried to isolate it it was coming through the mains power outlet where we were connected to a TV & aerial was plugged in once we took the aerial plug out of TV no hum I have been running a Karaoke show so had to access Pubs TV which had an interconnecting AV & aerial line into the TV we were using we had a HDMI

Quite possibly, the venue has a powered attenuator or amplifier on their aerial signal.

When you plug into the mains "in the room" it's unlikely that you're on a different mains circuit (except in big venues with multiple circuits), but the power for the attenuator/amplifier is quite possibly on a different circuit entirely and it is this (to my understanding) that creates the ground loops that give you the hum/buzz.

[eklynx]

My personal troubleshooting hums list (not feedback, just hums).

 

1) mute channels one at a time until the hum goes away.  That'll confirm which channel is producing the hum..  if the hum gets quieter but it still exists, then there's most likely more than one channel humming.

2) Once you've isolated the channel(s) producing a hum, verify all cable connections are tight.

3) If hum persists, swap cables with known good ones.  use short test runs (i keep a known good 3 ft cable in my pocket for this).  this is to isolate the problem to the cable.  

4) if hum is gone, run another cable along the length.   if hum comes back, get cables with better shielding or run the cable a different way to avoid what's causing the interference.

5) if hum didn't go away, double-check the device.  If it's a microphone swap with a spare (you do carry a couple spares, right?) If it's a DI or other device, hit the ground lift.

6) If that still doesn't solve the problem, put an isolation box in the run (i.e. http://www.audiopile.net/ISOBOX-1)

7) if hum is still there, then you most likely have something wrong with a device itself (bad connection on a jack, dying device, etc). good luck :P