Film-Tech Forum ARCHIVE: QSC Amp Intereference

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Posted by Steve Scott (Member # 630) on 05-11-2005, 10:00 AM:

I have two QSC MX1500a amps in one of our sound racks that are getting a pulsating distorition whenever they both have their gains turned to full. One amp powers the sidewall surrounds and the other powers two sets of subs. This problem just began to occur after years of running both amps at full gain. Though the surround speakers are at a nominal level, now that they've been turned down, my question is, do these amps have issues with being run at full gain in a rack where they have only a single unit of rack space between them?

Posted by Mark Gulbrandsen (Member # 72) on 05-11-2005, 11:10 AM:

No, not really. They can be installed that way. Sounds like you have either a component failure in your power amps or processor. This could also be a ground loop issue. Ground loops become even more of an issue as gain is increased.

What type of processor and how are the amps wired to the processor? Was any work at all done lately in any of your booths, in any of the theatres? Preshow equipment installed, etc?

Mark

Posted by Steve Scott (Member # 630) on 05-11-2005, 01:29 PM:

The processor is a CP500D. The processor's amp outputs are wired using that green terminal block. It's a standard wiring with the CM35 monitor and the amps, nothing special. No work has been done to this rack since it was installed in 1997.

Posted by Steve Guttag (Member # 268) on 05-11-2005, 03:17 PM:

On the MX-1500 amp, how are the inputs connected (list what wires are going where)?

Posted by Sam D. Chavez (Member # 1841) on 05-11-2005, 09:08 PM:

It is also important to carry a ground from the Amp speaker negative back to the monitor along with the hot lead. You may be breaking into some subsonic oscillation.

Just a hunch.

Posted by Steve Scott (Member # 630) on 05-12-2005, 12:05 PM:

On the Main Surround amp, the left and right surround channels from the CP500 are wired with one triple conductor Belden cable for each channel, and are hooked to the amp with spades into the terminal inputs. The Sub amp is the same, only it is operating with one input channel (same cable type) in parallel mode. Each channel is connected to the grounding terminal of its respective amp using the third conductor with another spade. The surround amp is daisy-chained to the back surround amp (an MX700) with spade plugs and the same Belden via the terminals of each amp. Speaker outputs are via binding post using a larger gauge cable.

The monitor is hooked up as follows: the processor inputs are derived from a dual conductor Belden coming off each output from the CP500, and the amplifier inputs are derived from a two conductor Belden wired into the positive output of each channel. Bare wire connections.

Sam, are you saying I should add a ground to the amp-to monitor connection?

Posted by Sam D. Chavez (Member # 1841) on 05-13-2005, 02:05 AM:

You should definitely carry one ground from the black binding post back to the monitor.

You cannot rely on a perfect ground between the monitor chassis and amp chassis.

I have seen whole sound systems take off into oscillation because this connection was left off. Usually it only takes one ground from to amp of your choice back to the monitor to eliminate a chance of having this problem.

Lastly, the ground of the speaker terminal is not always the same as the amp input ground.

If you ever want to see a Hafler issue white smoke, put a clip lead from the black speaker terminal and the input minus.

Posted by Kenneth Wuepper (Member # 1174) on 05-13-2005, 06:39 AM:

Steve,

Do you have the owner's manual for this equipment?

I think you referred to the single input connection as being "PARALLEL". The manual refers to this as "MONO BRIDGED" configuration. I suggest that you check the connections against the manufacturer's design and be certain that the output leads to the sub speaker system are properly connected.

It is possible to use only one channel of the two internal amplifiers (mono)or to double the output power by "BRIDGING" the output sections of the amplifiers. The ground terminals of the amplifiers are used differently in those two configurations.

KEN

Posted by Steve Guttag (Member # 268) on 05-13-2005, 08:19 AM:

I'm not going to get too much into the methods of grounding but how and where really depends on the equipment being used.

On the input side of things, the CP500 is an unbalanced device on its output and its chassis ground and signal ground are not bonded. They fixed that mistake on the CP650. Before you go too much further...try taking any of the ground terminals on the phoenix conenctors and make a small jumper to the chassis ground tabs...see if your problem vanishes (with some hum leaving as well).

Another thing to try after that is to connect the "-" terminals to the ground terminal at the amp input. On some amps (not normally the MX-1500a though) if you don't they will oscillate...Ashleys are notorious for this when driven with unbalanced sources). You may need to reset levels after this.