"Interesting" QSC RMX amp "issue"

Evan Kirkendall

Evan Kirkendall

Senior
Jan 10, 2011
903
4
18
Abingdon, MD
www.harfordsound.com
Here's an odd one. I've got a random QSC RMX850 in a rack that just gets used for misc stuff. Kind of the "bitch" amp that goes out whenever I'm short on gear. Usually it just runs stereo. Today I was doing some shop work and decided to bridge it so I could make some presets for my SLS920's. Switched the DIP switches around, tossed a banana plug on the binding posts, and boom, no noise. Well, it was very fucked sounding. Very distant, almost like someone flipped the polarity on one of the amp channels. Oh wait, channel 2's gain knob is still up. Ok, turn that down- about halfway down it starts sounding normal, and then at 0% the signal light on channel 2 goes out. What? Sounds normal enough now, but the signal light is out. Turn the gain back up, signal light comes on, and it sounds fucked. Very strange.

Now, if I recall correctly, in bridge mono, channel 2's gain knob is irrelevant, and it shouldn't matter where it is, right? Also, the signal light should not be affected, right?

The amp seems to function "normally" when I have channel 2's gain knob all the way down, but it only shows signal/clip on channel 1. I can confirm, it gets way louder when the amp is bridged, so something is working correctly. I'm just kind of baffled with the gain knob/polarity issues. Anyone seen this before?

On an unrelated side note(well maybe not)- the difference in tone quality between a bridged RMX850 and a PL6.0 stereo is pretty amazing.





Evan
 
Re: "Interesting" QSC RMX amp "issue"

""On an unrelated side note(well maybe not)- the difference in tone quality between a bridged RMX850 and a PL6.0 stereo is pretty amazing."

Just wondering what you heard? Which was better?
 
Re: "Interesting" QSC RMX amp "issue"

Hi Evan.

If you look at the back of the amp, there is a small table on the right side with required settings for the different operating modes. For bridged mode you need to set Channel 2 gain to minimum and disable the limiter and filters of Channel 2 as well.

I once had the schematics of the RMX on my desk, and there you can see that the input controls of channel 2 are in the audio path for bridge mode as well. I just don´t remember the details now.
 
Marcus Baeumler said:
Hi Evan.

If you look at the back of the amp, there is a small table on the right side with required settings for the different operating modes. For bridged mode you need to set Channel 2 gain to minimum and disable the limiter and filters of Channel 2 as well.

I once had the schematics of the RMX on my desk, and there you can see that the input controls of channel 2 are in the audio path for bridge mode as well. I just don´t remember the details now.
Click to expand...

That is correct. Had a similar problem at a small installation once years ago. When in bridged mode the RMX channel two gain and filters must be turned off.

For us it sounded OK until is was driven hard and the got very crunchy quickly. We thought it was damaged speaker components until we did a swap and the problem remained.

A call into QSC afterward confirmed it as well.

Jason
 
Re: "Interesting" QSC RMX amp "issue"

When you bridge an RMX amp, you must have channel 2's gain control turned all the way down OR have the inputs separate (i.e., in stereo). I always suggest doing both (unless you specifically need the inputs paralleled) to minimize the likelihood of user error.

In bridge mono, channel 2 gets an inverse-polarity feed from channel 1, but if the inputs are paralleled and channel 2's gain control gets turned up, you'll also be summing what gets fed into channel 2's input (via the parallel inputs) with that inverse-polarity signal. If you turn the channel 2 gain control all the way up, you'll actually null out the normal-polarity and inverted-polarity signals, and channel 2's output will be zero. And then it will just act as a big virtual ground, sourcing and sinking the current from channel 1.

However, there's also a bridged mono protection circuit that looks at the midpoint of the channel 1 and channel 2 outputs, which should be mirror images of each other. If that midpoint strays much from zero, it's likely that something's wrong in one of the channels and the circuit will mute the signal. (I don't think that's what's happening in the OP's situation; I just added it for informational purposes.)

From Evan's description, it seems like the amp was not actually in bridged mono (or at least there was no feed from channel 1 to channel 2), but the inputs were parallel and the loudspeak(s) was connected across channel 1's and channel 2's hot outputs (i.e., the two red binding posts or 1+ and 2+ on the upper Speakon output); that's the only scenario I can think of that would cause such behavior.
 
Re: "Interesting" QSC RMX amp "issue"

No, the PLX has some additional op amp stages in the audio path, and the bridged mono feed from channel 1 is isolated from the filters and the clip limiter's OTA. Therefore, bridged mono setup is simpler and more straightforward, esepcially the original series (PLX1202, PLX1602, PLX2402, PLX3002, and PLX3402). On those models (as well as in the sibling ranges CX, DCA, PowerLight 2, and PowerLight 3) you simply set a switch to bridged mono, and the amp ignores channel 2's filter, clip limiter, and gain. I wish all the bridgeable amps were like this.

In the PLX II series (PLX1802, PLX1502, PLX3102, and PLX3602), signal from channel 2's input stage can still pass through, so you would have to turn its gain control all the way down.
 
Top Bottom