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Debugging Pops
Old 4 weeks ago
  #1
Here for the gear
 

Debugging Pops

Hi everyone,

I am currently using a secondhand Rane SM-82 to mix together a few synths before I send it over to my band's mixer (a Behringer X 18). I just got this setup running a couple days ago and as best I can recall it worked fine on the first day.

Today as we were playing I noticed a weird popping sound every time I played a note - it sounded similar to a very harsh/distorted envelope click. This happened on every channel I tried and every synth I was playing. Running the synth into the band mixer directly fixed the problem.

Could this be some sort of ground loop problem? Or is it more likely to be an issue with the submixer's electronics itself? Anything I should try to debug/fix this?

Thanks for any/all ideas
Old 4 weeks ago
  #2
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Ground loop? No.

Ground loops are responsible for hum and electrical shocks. When one piece of gear with a better connection to ground, all the pieces of gear you plug into it with audio wires try and use that better ground. This sets up an AC voltage flow between units through the shielding in the wires connecting them which in turn generates magnetic flux in the signal wires which creates a hum.

The fix is to simply use the same AC outlet.

Given the fact your audio system seems to work find and the problem occurs when adding the Rane, its obvious your issue is coming from the Rane, its power supply or any additional cables you're using to connect it.

First thing to do is rule out the easiest possibilities swapping cables to be sure they are good. Boosted line level signals carry allot of current compared to what the instrument alone might produce so weak cables can "break up" a signal when the connections aren't really good. Cheap connectors use rivets to hold the connections together and over time they typically loosen up as the plastic between them shrinks from pressure. Military Quality connectors on the other hand have a single piece build which is designed to allow a tank to drive over them and still work.

Your symptom in this case suggests you are either overdriving the signal, Components are failing from heat or lack of proper voltages.
With a device like this you typically want to get a 1:1 level signal in and out. What goes in is the same amplit8ude as what comes out except you've combined multiple line in's and mixed them down to tow channels out.

The reason you want a 1:1 signal is so you don't overdrive you audio system which is obviously working properly at line level.
According to the schematic the circuit is very simple consisting of a buffer, plus an input and output op amp. Not a hell of allot that can go wrong besides maybe a cracked solder joint. The Manual says it can produce a boost of 26dB max so you shouldn't have to run the channels more then 50% and simply makes sure your keyboard volumes are set where they would be without the Rane. After that you simply vary the volumes a little either way.

Where I think your problem actually lies is with the –10 dBV to +4 dBu Shifter switch. Most modern gear will need to have that switch set for -10dBv. Its highly unlikely you'd ever use the +4dBu setting unless you have some vintage recording gear with passive components that require decibel levels that high. Most modern PA mixer inputs will all be -10 to -20db. If it was +4 you'd be needing an attenuator for your keyboards going straight into it too.

I suspect you accidentally kicked the switch from -10 to +4 and its overdriving the PA mixer. Set it back and get those gains so they are the same as when the keyboards are running direct. Once its right mark your settings down on a cheat sheet. Put a piece of tape over that switch too so you don't accidentally nudge it again.
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