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Does a bass preamp fry a channel on an analog board?

My bass player just fried two channels on my board. He used a preamp compression pedal, into a passive DI box into an XLR channel. My formerly beautiful board smoked. It was clipping. We changed channels, DI box, cables and the second channel started to smell too. I disconnected the bass player and ran him thru my Roland keyboard amp. Please help me understand what happened! The board was only two months old and never had a problem with bass, to di to board. Is it the preamp?


juli k Sun, 01/14/2018 - 21:13
The pedal is BassInnovations bass compressor. The mixer is Heath & Allen. The DI box is Whirlwind IMP 2. It was set to lift. The channels have a condenser on them. I'm not sure what a dynamic is but the whole channel was shot after the smoke. I can't even mute the channel. When the bassist switched to another channel, the clipping light for the channel was flickering and it started to smoke. That was with a different DI box (same brand) and cable. I stopped him before he could kill another channel. Thank you so much for any advice!!!

paulears Mon, 01/15/2018 - 07:50
Smoked as in real smoke from burned components? This seems to be almost impossible as in the bass causing it. If a component actually fried it means a current flow through to to overheat. The pedal when DI'd goes through a transformer to go to the mixer via the mic cable - so even if it had fault current on it somehow - a serious pedal fault, the isolation would have prevented this getting through. When you used the Roland - with the compression pedal or without? This worked fine so anything in line seems to be fine. Even if you were using phantom power, the resistors limit current to levels unlikely to fry. It's probably the DC power rail in the mixer somehow going down to ground, and this would have the current to burn a resistor up.

The only way I can see the bass being responsible would be if your DI was not transformer coupled, but maybe a differential type device that could, possibly have volts on the signal maybe pin 2, and has pin 3 grounded and linked to 1? Your mic level input would then be hot - but even at PSU typical levels - that might be 9-12V typical, and most mixers would have DC blocking capacitors on the input - because of the phantom powering. I suppose you could use the next working channel and put a voltmeter across it's XLR and see if there is any voltage present there. It is possible the mixer PSU has put voltage on the ground - and the mixer is floating high, until you connect to something else that is grounded and whatever components bridge the gap are frying. You'll need to open the mixer up and find out what has burnt - this should also point you to the problem - but prodding a meter around the mixer ins and outs and the compressor/DI ins and outs should lead you in the right direction.

Good rule would be that if you smell burning, switch off - never start unplugging and re-plugging because if there IS a fault you could find your hand and heart in the signal path with dire consequences. If it's strong enough to burn a resistor, then it is a real fault, so you stop.

Boswell Mon, 01/15/2018 - 10:56
The OP still hasn't said what model of A+H mixer it is.

The IMP-2 DI box is transformer-isolated, and with the ground lift switch in the lift position, the should be no d.c. electrical path between the guitar/pedal/amp input side and the XLR cable going to the mixer. Naturally, that doesn't stop an AC signal, so if there were 110V mains voltage at the input, that would transfer as less than a volt at the output - no damage possibility there. On the other hand, if the DI box were wired up back-to-front, it would be a different story (15KV, anybody?)...

pcrecord Tue, 01/16/2018 - 05:32
juli k, post: 455055, member: 51100 wrote: My bass player just fried two channels on my board.
Must have been a very warm tone.. lol ;)

juli k, post: 455055, member: 51100 wrote: The board was only two months old and never had a problem with bass, to di to board. Is it the preamp?
I could understand it fried channels if he used the speaker out of an amp instead of a line out. I checked the pedal, it isn't a preamp, just compressor, I can't find the specs and output level to see if it could be too high for mic input... Unless you had the gain set too high on the board but even then most mixer will distort but not fry. Do your mixer have a pad on the input ?
Like the others, I wonder how it could happen if the Di was used correctly.

Davedog Sat, 01/20/2018 - 00:58
I own 3 Imp-2 s. You can't possibly screw up using them. Transformer based DI. The compressor is just a compression stompbox. MXR if I remember. Putting it first in-line and then the DI would be appropriate usage and non-destructive whether the PP was on or not. Even if it was, and most A&H boards have separate PP on every channel via a switch, the signal would "ignore" the 48volts and simply operate, much like an SM58 might react to PP. There's something else going on here that can't be determined without more info. As an update: The newer ZED A&H mixers have a universal PP. It also has a warning that the XLR connectors should not be used when PP is present to patch in another mixer/keyboard/electronic circuit.....The part that doesn't make sense to me is using the DI would make this input 'see' a mic level signal....It's what a DI does.....Also the A&H 's are pin 2 hot...

DonnyThompson Sat, 01/20/2018 - 02:45
juli k, post: 455058, member: 51100 wrote: 'm not sure what a dynamic is but
He is referring to a particular type of mic. Dynamic mics, like Shure SM57's, 58's, SM7B's, EV RE20's etc., do not require voltage (phantom power) to operate like Condenser mics do. Ribbon mics also do not generally require additional voltage to operate, either.
There was a time when sending PP (phantom power) to mics that didn't require it could damage them, but most newer mics and cables (within the last 20 years or so) dynamic and ribbon mics these days are usually not damaged by having the phantom power on for those channels - as long as everything is wired correctly and there aren't issues with the console channel or the cable used.
It's still a good idea to turn the PP off on channels where those types of mics are used, but the caveat there is that there are mixers with global PP across the board.
I'm still trying to figure out how a simple 1/4" DI fries a channel on your mixer ....not to mention TWO channels...i dunno, maybe I'm missing something here, but it's certainly not supposed to happen. 1/4" DI's are fed to consoles all the time. It's a standard thing, we don't even really think about it.
Oh...wait. I just read your post again and saw you were getting his signal into the board using XLR, not 1/4" instrument input...
Did you use an adaptor of some kind between the box and the mixer channel input, or use an inline preamp, or lift the ground somewhere on the device or cable?
Does the bass stomp his have a Line Out jack? If so, Did you use it?
If you used XLR to XLR, was it your cable or the bass players?
If it was yours, was it a good cable? You haven't repaired it recently or altered it in any way?
I'm just pecking at possibilities here... You should always default to Boswell's advice though, for anything electronic, he's the man.

DonnyThompson Sat, 01/20/2018 - 03:01
Pal, could a very low impedance device, sending a +4 level, actually cook a channel input?
I would think it could definitely light up the overload indicator if there was no pad used and the trim pot was opened up...but could that type of input level at a low impedance actually fry a mixer channel? And if so, could it be immediate, or would it take a continual level of that signal hitting the input hard over a period of minutes to eventually do damage?
Or do you suspect that there's something else occurring, perhaps even something in addition to what I asked above? A faulty ground? Maybe Hot pins on the cable switched?
Just curious...

Davedog Sat, 01/20/2018 - 03:05
Donny...The IMP2 is 1/4 in/parallel out to XLR out. It is transformer based so the XLR out would be a mic level output regardless of what it sees at its input side. Like I said I have three and you can't cause this kind of damage with that device. The bass compressor device is just a compressor. At least this what the poster said. It's the MXR series and is either an M87 or an M81 model. The M87 is basically an 1176 in a foot pedal and the M81 is a preamp with EQ. If the chain is as described with the bass device in front of the DI and then into the board, there really can't be a problem there. The DI would buffer all input to mic level. But the info about the "clipping" immediately when it was hooked up, makes me think we don't know all the particulars.

DonnyThompson Sat, 01/20/2018 - 04:16
Thanks Dave...
I'm not nearly as knowledgeable on this stuff as guys like you and Bos are, so i appreciate the clarification.
I'm really curious now to find out what happened.
I know I've taken DIs from bass and guitar amp/head outs, DI box outs, Guitars and Keys using your "everday" DI boxes, both active and passive, for more times than I could ever even remember, and I've never encountered anything like this. At first I thought maybe it was a faulty mixer channel...but two?
I dunno... but I am watching this thread closely now. ;)
Let me ask...
If an XLR cable had a faulty ground - say a cold short, or perhaps pin 1 was bent or out of alignment so that the ground wasn't making contact first as they're supposed to, and the channel was also sending phantom power, wouldn't the damage be more likely to the device, and not to the mixer channel?

TAO Wed, 01/31/2018 - 05:08
Davedog, post: 455263, member: 4495 wrote: My last A&H was a WZ3. Individual PP on each channel. The poster indicted it was a 'new' board so I assume this would be a ZED. They are universal PP. And thus the warning in the USER MANUAL....Which people should read before driving

In other words, you hit a certain switch and under certain circumstances the thing smokes. That's BRILLIANT design. Universal PP is a crappy area to skimp on. If adding a few switches keeps the smoke in, it is worth the few extra cents.

paulears Wed, 01/31/2018 - 10:01
On all the mixers I have ever used that had universal phantom power, they were all designed to cope with all manner of cable issues - shorting of pin 2 to ground, pin 3 to ground or 2 and 3 to each other. In every case, a fault on one channel has never caused a problem. When I have fed phantom to a device that didn't want it - as in a sub-mixer, feeding into a couple of channels of another mixer - the unwanted phantom has never caused anything to die - ever. Mixer inputs have to be designed to cope with phantom power getting to where it isn't wanted. going from a Yamaha mixer to the output of a Soundcraft with phantom attached made all the LEDs on the sound craft instantly go full. The other way around, the Yamaha just went into oscillation - a nasty high pitched whistle. The Behringers I have used totally ignored it, apart from the input channel meter just went to full. If smoke comes out of a device then a LOT more than current limited phantom caused it. The chain in the original post should be re-connected, and a meter stuck on the output. I'd stab a guess that this will show nothing, and it's more likely that somehow that chain of kit was live - possibly the supply it was connected to miswired, or even a stray miswired IEC, and there were volts on the ground, and when the two things connected serious current flowed which could cause smoke. If you ever abuse a resistor, it usually fails with minimal smoke.

There is a missing link here and I bet it's nothing to do with phantom. 10mA of current at 48V if they use 1/4W resistors, it's very unlikely they'll fail.

dvdhawk Wed, 01/31/2018 - 17:23
Since the OP hasn't been back in 2 weeks, we may never know, but I'm in agreement with Paul on this. My thinking from the start has been that mic inputs are meant to tolerate all manner of defective cables and screw ups, but reversed AC mains can cause havoc in the form of smoke and small scale pyrotechnics.

I was a short-term sideman for a band of slightly older guys in the early 80's. They would pull a 16-channel snake to the back of the room to their stunningly-terrible-by-today's-standards (Kelsey) sound board, but never bothered to pull a 100 ft. power cable from the amp rack to the FX rack and soundboard, to insure both racks shared a circuit and ground. They relied on plugging the Kelsey and FX rack into any available receptacle at the back of the room. Neon beer lights be damned. Most nights this wasn't a huge problem, but when you get into some of these clubs where Bob the barfly did some high voltage wiring for them (probably paid in beer while he was working), there's a distinct possibility you'll find a percentage of mis-wired receptacles. I can tell you as a first hand observer that the snake can complete a short circuit from the front of the room to the back of the room. And under the right/wrong circumstances when Dennis, the unsuspecting soundman, plugs the soundboard / FX power into the wall last, it can result in a brief shower of sparks and urine nearly simultaneously. I have no idea how someone using proper wire gets hot and ground reversed, but that was the case on the receptacle he tried to plug the mixer into, as I discovered in the postmortem.

The moral of the story: 1) A simple 3-LED receptacle tester can prevent sparks, fires, and wet denim. 2) There are no shortcuts in doing it right.

I've gotten some dirty looks from professional electricians when I test their outlets at a new install right in front of them, but I don't care, because every once in a while I find one with the hot/neutral reversed. Then their dirty looks turn into embarrassment. If I go to a new venue, I test all the stage receptacles. If it's a place we've worked before, I will skip that step. Open ground is the most common fault I find. Hot/neutral reversed is a fairly small percentage. In the 35+ yrs since the Dennis incident, I've only found hot/ground reversed 2 or 3 times. I've also seen small sparks between the XLR shell and XLR panelmount, and felt current on a church install where phantom power was coursing through their homemade snake and stagebox due to mis-wired XLRs. I fixed that one with a hacksaw (after the power cables were unplugged).