Where to plug?

[pusso9tails]

So, my drummer has all this home recording gear. Me and my rhythm guitarist/singer pay him a visit to test out the gear.

I brought my SWR350X to record with.

My intention was to grab the signal from the xlr output in the back of the amp and shoot it out to the board.

Instead he used a 1/4'' to get the signal from my speaker output. I played one song and stopped.

Of course, I'm thinking-there's gonna be too much signal shooting to the board.

Low and behold I was right, and so we went with the xlr. (No one ever listens to the bass player.)

My question is: Was/is it damaging to run my signal from the 1/4'' output speaker jack into the board. Considering that running a 2ohm load for a protracted period of time is not good for the amp, is running the amp into a board that has no ohms(?) bad as well.

[PJN]

He put 450 Watts into his mixer?

With an instrument cable?

 

Your drummer is a retard. I doubt that any damage was done to your amp, though. He probably thought that he was using the Tuner Out jack or something like that.

 

Don't ever let him forget this one.

What a dip shit.

[BenLoy]

I'm surprised the soldering points of his mixer's channel didn't melt. That's unbelievable.

[pusso9tails]

HAAAAAAAAAAAAAAA! No shit!

As long as my amp isn't effected. It wasn't connected like that for too long. He adjusted all the levels WAAAAAAAAAAYYYYYYYYY Down. But still, I love harrassing him so he'll never get away from that one.

[Soundcrafter]

I read that he used the speaker output and my stomach lurched(I'm a becoming sound techie)..

I'm not sure of the impedance on 1/4" inputs on a board, I know XLR is high impedance and 1/4" is low impedance but I can't give it in ohms. At any rate, putting that hot a signal in a board is insane! Does the channel he used still work?

[rumpelstiltskin.]

firstly, the drummer is a dip shit. let's get that straight away. even drummers have no excuse for connecting a speaker output to a mixer for recording. especially when he has a home recording setup.

 

secondly, let's talk about the physics of what's going on there, so we can all understand why it's bad to connect a speaker output to a mixer input.

 

the SWR 350x bass head is rated for 240W at 8Ohms. using our handy electrical engineering equations (really it's Ohm's Law), we see that P = V^2 / R. that's power equals the voltage squared, divided by the resistance. (i'm going to use the words "resistance" and "impedance" interchangeably in this discussion, because they are interchangeable for all practical purposes. very few people here would truly understand the difference, even if i explained it to them, so you'll have to take me on faith.) rearranging things, we see that V = sqrt( P * R ). that's voltage equals the square root of power times resistance. that means the voltage is the square root of 240W times 8Ohms, or is about 44V.

 

now, mixers use opamps, which are a circuit element that does a lot of useful things in circuits, like amplify signals and be a big part of tone circuits like a bass knob. most opamps operate on +/- 15V power supplies. in rare cases they use 18V or 24V supplies for more headroom. still, the 44V your amp is capable of producing is about double the highest, most rare supply voltages (24V), and about triple the average supply voltage (15V). even if you had your level at less than a quarter, you're still looking at quickly overdriving the input stage, let alone the other gain stages in a mixer, leaving yourself very little room to actually mix anything in the presence of such a powerful bass signal.

 

now for the really fun part. i hope your mixer has a high input impedance, because your amp can provide a lot more current than the mixer wants or needs, and the only way to stop it from frying things is the input impedance of the mixer. your drummer dodged a bullet in that the speaker output is .25" -- most mic inputs have a much lower input impedance than a .25" line input. for example, behringer's most pimpinest eurorack mixer, the UB2442FX-Pro has a line input impedance of 20kOhms, compared to a mic input balanced impedance of a high 2.6kOhms (good XLR inputs will be 600 Ohms lower).

 

let's revisit Ohm's Law now to see the lucky ineptitude with which we're dealing. Ohm's Law states that V = I * R. That's voltage equals current times resistance. in this case, we want to know about how much current the mixer's line input is going to draw from the amplifier's speaker output. therefore, we rearrange the equation to get I = V / R, or current equals voltage divided by resistance. that means the current draw will be a paltry 2mA (0.002A), since our voltage is still 44V, and the input impedance is 20kOhms. 44V / 22,000 Ohms = .002A. (if 44V were put into the mic input, the amp would be creating a very opamp-stressing output of about 250mW.)

 

so the good news is that the line input impedance is high enough that he probably didn't damage the board, although it probably sounded like utter shit. i'm sure the XLR output from your 350x sounded good and interfaced quite nicely with the mixer.

 

why don't you print this out and show it to your hoser, i mean drummer, with the message that he should stop pretending thate he knows what he is doing and actually learn about how things work before he buys more gear.

 

robb.

[Soundcrafter]

wups...seems I reversed my idea of impedance on XLR and 1/4" inputs.