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Floating Voltage? - "more me" monitoring setup

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Floating Voltage? - "more me" monitoring setup

Postby zigmond99 » Thu Jan 16, 2020 7:38 pm

Apologies for a long post but there’s a lot to explain.

I sing and play guitar and use IEMs. Live, in venues with decent kit, I take a DI from my guitar multi-fx straight to FoH (no guitar amp). Up till now, in smaller venues and at rehearsal I don’t use IEMs. The output of my guitar multi-fx goes to a clean guitar amp and I use floor wedges to hear my vocal or none at all if the room is small and volumes are low.

I prefer using IEMs and would like to be able to use them all the time. So I have been experimenting with a cheap, proof of concept setup that I can use for rehearsals and small gigs. In these scenarios there is enough ambient noise and spill into my vocal mic for me to be able to hear the rest of the band. All I need in my IEMs is my vocal and guitar.

Last night I tried this out. I split the output of my guitar multi-fx: one side to a guitar amp, the other to the monitor (line) input of a Behringer MA400 headphone amp. It has a mic in, a mic through and a monitor input. Experimenting with a friend’s I discovered that there’s just enough gain on the monitor input to take an input from my guitar multi-fx. I connected my mic to the mic input and another XLR from the mic through to the PA. At this point the guitar amp erupted in loud feedback.

My guitar multi-fx was running on batteries with no power adaptor connected. The Behringer MA400 has a wall-wart power supply (12v) with a 3-pin UK plug although the earth terminal is plastic i.e. no earth. At this point my guitar wasn’t connected to the input of my guitar multi-fx. I’d already connected it directly to the amp to set up a clean channel at a sensible volume (an earlier lesson learned) so the issue wasn’t the guitar amp.

The Behringer MA400 has a ground lift switch but toggling this had no effect. The only solution I could find (with the clock ticking and my band mates growing impatient) was to disconnect my guitar form the Behringer MA400 and run it direct to the amp. So I had vocals in my IEMs but no guitar. It was workable but not ideal.

When I’d tried this at home it worked perfectly. So my question is, what was going on here? It wasn’t classic earth loop hum. It was loud feedback, as though the amp was at max with my guitar connected (neither was the case). Was this a floating voltage? Did I put 100v DC though the front of the guitar amp?

I’d rather not have to hire the room again just so that I can fault-find. It may have been a faulty guitar amp or an other issue with the room wiring. My goal was to be able to eliminate a guitar amp and either run direct to the PA or use whatever amp is provided. But I’m now concerned that this setup isn’t going to work reliably across the range (and varying standards) of venues that I’m going encounter.

This was only a proof of concept setup and in that respect I’m happy that it will do what I want it to. I just need something more reliable. The Rolls PM351 looks like it would do the job as the schematic on the top cover shows that the instrument out is transformer isolated. If so, does anyone know if the ART MyMonitor II is the same as it’s quite a bit cheaper.
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Re: Floating Voltage? - "more me" monitoring setup

Postby Hugh Robjohns » Thu Jan 16, 2020 10:17 pm

Interesting conundrum!

Just for clarity it would help to know what kinds of cables you're using to connect the fx pedal to the ma400 -- I see it accepts stereo inputs.

And how are you splitting the output of the fx pedal?

I presume the guitar amp is properly grounded via its mains plug?

As neither the fx pedal nor the ma400 had mains grounds, it may be relevant that the problem noise occurred when the ma400 became grounded via the pa system when the mic-thru connection was made... But I'm struggling to think how at the moment.

As an experiment, it might be worth connecting the fx pedal into the ma400 via a transformer isolation box, just to separate the pa earth from the amp...

I'm intrigued that it worked at home -- as I'd expect it to (but presumably you didn't extend the mic-thru output to anything) -- but not when connected to the PA.

Hmmmmmm....... :think:
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Re: Floating Voltage? - "more me" monitoring setup

Postby zigmond99 » Fri Jan 17, 2020 1:01 pm

Hugh - you're right. When I tested it at home I didn't have an XLR from the mic-thru to anything so I've just tied that. I used an active speaker to simulate a PA. There was no issue and it all worked as expected.

I'm splitting the guitar multi-fx out using a plastic 1/4" TS jack to 2 x 1/4" TS sockets. I did consider using a stereo to twin mono splitter as the multi-fx output does accept a TRS jack. The manual for the multi-fx says it's +5 dBm with an output load of impedance of 10 kΩ or more or 20 mW + 20 mW into a 32 Ω load which I take to mean if you plug in a TS jack you get one, a TRS jack the other. I reckon the Behringer MA400 could take the latter but I'm not sure about my guitar amp.

Into the Behringer MA400 line input again I'm using a TS jack. The unit has a stereo/mono switch and it works as expected - set to stereo my guitar is in one ear only whereas set to mono it's in both.

I don't have a transformer isolation box that I can connect in between the guitar multi-fx and the Behringer MA400.

Plugging it all together today highlighted that the cables and adaptor I'm using aren't ideal as there were plenty of pops, crackles, and hum depending on how I jiggled things so I think as a next step I'll make up a custom cable with a TS jack on one end feeding a TS socket and a TS jack.

If I run I run into the same issue again I'll look to either the ART MyMonitor II or the Rolls PM351. On ART's website it says that on request they'll send a schematic so I've emailed them to ask for one for the MyMonitor II.
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Re: Floating Voltage? - "more me" monitoring setup

Postby Hugh Robjohns » Fri Jan 17, 2020 1:32 pm

zigmond99 wrote:When I tested it at home I didn't have an XLR from the mic-thru to anything so I've just tied that. I used an active speaker to simulate a PA. There was no issue and it all worked as expected.

Fair enough. I wouldn't have expected a problem. It would imply something amiss with the PA, or something not connected as you thought it was, at the gig.

You'll just have to try it again, allowing yourself a little more time for testing... ;)

I'm splitting the guitar multi-fx out using a plastic 1/4" TS jack to 2 x 1/4" TS sockets. I did consider using a stereo to twin mono splitter as the multi-fx output does accept a TRS jack.

I think that would be worth a go... It would mean that the MA400 and the guitar amp are being driven by separate buffered outputs, which might well help. And a TRS to 2xTS plug Y-splitter cable won't break the bank. They'll have a common earth, of course, but it doesn't sound like that'a problem.

The manual for the multi-fx says it's +5 dBm with an output load of impedance of 10 kΩ or more or 20 mW + 20 mW into a 32 Ω load which I take to mean if you plug in a TS jack you get one, a TRS jack the other.

Actually, you get the same thing regardless. It's just two ways of expressing the same signal. A line connection is interested in the signal voltage, which is +5dBu on both left and right outputs whereas headphones are interested in power which is 20mW into each side of a low-impedance headphone.

As a nerdy aside, dBm is actually the incorrect reference in this application because the spec states the need for a 10k Ohm destination impedance. The dBm term explicitly requires a 600 Ohm destination impedance! :ugeek: :wave:

I reckon the Behringer MA400 could take the latter but I'm not sure about my guitar amp.

+5dBu is a little on the hot side for a guitar amp, but not insanely so, and some guitars with hot pickups, when played enthusiastically, could generate that without too much trouble. You'd just need to keep the input level control at a low setting.

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Re: Floating Voltage? - "more me" monitoring setup

Postby BenWilesMusic » Fri Jan 17, 2020 4:16 pm

Considering you say your cables may not be in the best condition and (you imply) you were using the venue's guitar amp, I wonder whether the 'feedback' you heard was actually an oscillation caused by cable capacitance, or a faulty amp, or both? It sounds a lot more likely to me than earth hum, which, you know, sounds like earth hum. (It could be that a low level earth hum was driving that oscillation, however.)

I think you have offered your own best solution – an isolation transformer between the multi-fx and the MA400. Behringer make a stereo one for less than £20 - the HD400 'Hum Destroyer'. I keep one with me for problem solving with my keyboard rig – it doesn't always work (it is somewhat susceptible to EM interference when near power supplies!), but it often does. The other option would be a passive DI box and XLR-jack cable.

Either way would be cheaper than the Rolls or ART boxes.
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Re: Floating Voltage? - "more me" monitoring setup

Postby zigmond99 » Fri Jan 17, 2020 8:54 pm

Hugh - thanks again for the advice and explanations. I'm going to make up 2 Y cables: one a TS jack split to 2 TS outputs; the other a TRS jack split to 2 TS outputs. This will remove dodgy cables from the list of suspects plus give me another option to try, both at home and at troublesome venues. I'll make up the cables rather than buy as I need a jack split to a jack and a socket plus making them up means I can have them just the right length.

We've booked the same rehearsal room again so I can try out my improved cabling and also check a few other things, like that the guitar amp is properly earthed and I'll check the sockets with a socket tester.

BenWilesMusic - thanks for the recommendation of the Behringer HD400. I'd rather not add extra bits of kit but for £18.60 (currently on Amazon) it will be a cheap and handy problem solver.
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