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Username: glocke

Post Number: 35
Registered: 9-2002
Posted on Saturday, April 30, 2005 - 5:57 am:   Edit Post


I need some advice for running my demeter preamp and QSC RMX 350 power amp.....

The problem is that no matter how I set the levels, clipping occurs, and the power amp cuts power to the speaker for a few seconds...I am running it in bridged monon mode...the clip limiter in back is switched to off...
Also, what is the best way to handle the level settings on the pwer amp? Should I keep it really high and keep the pre amp level down, or vice/versa ?
If anyone has any suggestions I'd appraciate it...

Username: gregbump

Post Number: 11
Registered: 4-2005
Posted on Saturday, April 30, 2005 - 7:30 am:   Edit Post

I'd check your speaker cable for a short.

You might also want to check the filter settings on the amp (30 vs. 50 Hz, On vs. Off). I've had crackling at low volume, as a string dies out for instance. I'd turn it off, at least for the purposes of troubleshooting.

Good luck!
Advanced Member
Username: dnburgess

Post Number: 400
Registered: 1-2003
Posted on Saturday, April 30, 2005 - 2:47 pm:   Edit Post

I had a customer with a similar problem with a demeter pre. The output was so hot he couldn't have the level past 1 or 2 without having to severely attentuate the power amp.

We were looking at getting a tech to mod it to drop the output by about 20dB - but he decided to get an Alembic F1X instead.
Username: dfung60

Post Number: 80
Registered: 5-2002
Posted on Saturday, April 30, 2005 - 11:34 pm:   Edit Post

A couple of various thoughts:
1)Can you test these three pieces (preamp, amp, speaker) separately? Do you know if they work properly when paired with other equipment? From the description here, I'd look most closely at the power amp. If the sound cuts out, then that's probably the protection circuit tripping and you might suspect the speaker cable or speaker for a short as gregbump mentioned. But, if you're hearing distortion prior to it cutting out, that's more indicative of a problem in the power amp. The best test here is to try another power amp that works and see if the problem goes away. If that's not it, then swap, speaker and preamp until you find the source of the problem. No, this isn't easy unless you have access to a lot of gear, but it's the only way you'll isolate the problem.

2) Try running in regular mode instead of bridged. When you run in bridged mode, you'll get more power, but there's more opportunity for problems and overheating (which will lead to the problems you describe). It also has the effect of halving the impedance of your speaker (if you have an 8 ohm speaker, it will look like a 4 ohm speaker to the amp in bridge mode). Lower impedance=more power, but will easily overheat your amp leading to distortion and triggering the protection circuit. Doing this may be easy or hard depending on the kind of cabling you're using (e.g. banana plugs will be easy, modifying a 1/4" jack or Speakon less so).

3) Be sure to check the easiest thing - make sure the cabinet is working properly with another amp. Steal your guitarist's amp for this test - it doesn't have to sound great, but you do want to run it pretty loud to see if it's causing the amp to misbehave. But be careful not to kill your guitarist's amp if the speaker isn't really the problem.

4) It's possible that the preamp may have a problem as well - failed power supply regulation might cause DC to come out on the output which would freak out the power amp and cause another shutdown.

5) For the least noise at output, you basically want to set the preamp level relatively high and use the power amp gain control to control overall volume. This will minimize the amount of noise when you're not playing. But if your preamp tonally sounds different at different output levels (varying levels of distortion for instance), then set the preamp gain to the tone you want, and adjust the master output level with the power amp gain. The power amp level setting won't have any appreciable difference in the output tone for most solid state amps.

Good luck,

David Fung
Username: 0vid

Post Number: 15
Registered: 4-2003
Posted on Sunday, May 01, 2005 - 2:39 pm:   Edit Post

I have had this problem with a stewart power amp. I doubt this is your case, but in mine, it was becuase I was using a transformer to step down from 220-240 V to 110V. (I was in NZ) The step down transformer was not powerful enough in current rating to handle the power amp's demands though it worked with the various pre amps. When I got a heftier stepdown for the power amp alone, all was fine.

It is entirely possible you may have a faulty power transformer.
Username: jure_the_second

Post Number: 32
Registered: 9-2002
Posted on Monday, May 02, 2005 - 4:19 am:   Edit Post

Demeters have an input sensitivity knob inside the chasis. Perhaps it is set to max.


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