Film-Tech Forum ARCHIVE: Help with JBL MPX1200 amps

This is topic Help with JBL MPX1200 amps in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Antonio Marcheselli (Member # 373) on 04-14-2003, 09:09 PM:

Hi everyone

I need help with two MPX200 amps driving a set of JBL subwoofers.
We replaced two MPX600 and now we have some strange behaviours:

1. As MPX600, the MPX1200 show only CH1 signal LED while in BRIDGE mode. We thought that it was normal but then we discovered on the manual that in BRIDGE mode both channel's LED should work. The problem came out because during louder scenes the CLIP led of the CH2 (that has no indication of any signal) flashes. How is it possibile? What does it mean? If the amp is really clipping, the subs would be already blown...

2. Suddenly an annoying HUM has started coming out from the subs. I found that it can be heard only with THX crossover connected. If I disconnect input cable from amps the hum disappears. If I disconnect output cable from Dolby with THX turned on the hum persists.
I connected the strip barrier at another amp (surround) and no hum can be heard. I connect the surround strip to the sub amp and the same hum can be heard (less noticeable). Earth is 0ohm between amplifiers.

The strip was removed from the old amps and attached on the new amps. No modifications.

Any advice?

Thanks for your help!!

Bye
Antonio

Posted by Gordon McLeod (Member # 33) on 04-15-2003, 10:51 AM:

Is there a ground isolate switch on the input to the amp also try disconecting the monitor lines from the amp output and see if the problem goes away

Posted by Antonio Marcheselli (Member # 373) on 04-15-2003, 01:23 PM:

Gordon:

1. To lift earth on MPX amplifiers I have just to leave alone the chassis ground pin and it is already isolated.

2. The monitor cable was already disconnected.

I've been told to disconnect the shield from the - at the THX crossover, they're linked right now.

What do you think?

Bye
Antonio

Posted by Antonio Marcheselli (Member # 373) on 04-15-2003, 07:41 PM:

Gordon,

I finally had an answer to many questions!!

The "-" from THX crossover was connected to both ch1 and ch2 inputs. This causes the mpxs to work abnormaly. Removed the "-" from the ch2 inputs (the amps are in bridge mode) and now both channels' signal led are working. More, the amps are working at +6dB efficency, so I had to raise the level accordingly.

The hum has almost completely disappeared but it's still there.

Here is our actual connections:

"+" from THX crossover TO MPX1200 barrier strip "+"
"-" from THX crossover TO MPX1200 barrier strip "-" and "S"

Shield is connected only to "-" on THX crossover.

Tried to connect the shield to amp chassis, no result.

Suggestions?

Bye
Antonio

Posted by Dave Macaulay (Member # 813) on 04-16-2003, 10:24 PM:

Generally, the shield should be connected to chassis ground only at one end of the cable, at the "input" end. That would mean the amplifier end.
I'm not sure what the "S" terminal is in your post. If it is supposed to be the shield, it's probably a ground and I would suggest disconnecting it from the "-" terminal first. If you still have hum try connecting the shield only at the amplifier end.

Posted by Antonio Marcheselli (Member # 373) on 04-18-2003, 05:16 PM:

Dave,

Yes, S means Shield. I'll try your suggestion asap.

I have another question. Since the connections were wrong and when restored the amp output was 6dB higher, I assume that also the two MPX600 had the same -6dB gain.
Can I also assume that with that type of connection the two 600 weren't working at their full capabilities?

Bye
Antonio