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QSC PLX resonance problem

B

bassbloke

Feb 26, 2002
174
1
UK
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#1
I was having a problem with a loud, ugly/uncontrollable sustain on my new rig (SWR Grand Prix/QSC PLX 1602/various cabs. I was puzzled about what was causing this, at various times thinking that the cab or even my bass was to blame. Eventually I realised the problem disappeared if I took the power/pre-amp off the cab. The problem existed with whatever cab I used and by a process of elimination I realised the power amp was responsible.

Symptoms are a loud, aggressive, grating sustain particularly noticeable on the low E, Eb and D on the B string, but also noticeable on some other notes. It seems to be some kind of resonance problem. I'm not technically minded and am reluctant to open the amp up in case it invalidates my guarantee.

I've spoken to the retailer (Soundslive in the UK). They have spoken to Shure (UK distributers) who have offered the preliminary opinion that the magnetic field from the bass cab is causing the problem. This seems an odd response to me because it suggests they think the amp is unsuitable for use in a bass rig. Given the number of bassists who use them, that seems surprising. They have offered to look at the amp under the terms of the guarantee, but I understand I'd have to meet shipping costs and given their attitude so far I don't have a lot of faith that they aren't going to turn round and say there is nothing wrong with it.

Anyone have any comments or experienced similar problems?
 
#2
Try hooking up the rig without the power amp on top of the cab. This way you can see if it does the same thing without the amp in the vicinity of the magnetic field from the speakers. Good luck...
 
#5
To rule out their theory have you tried operating with the power amp right next to your cab but isolated from the vibration of the cab ? That way it is close enough that a magnetic interference problem would still show up but vibration wouldn't.

Also, try with a different amp if you can find one. See if that one has the problem when stacked.

I ,and many others like you said, have QSC amps stacked on top of our cabinets without any issues. I think the amp is defective personally.

Matthew
 
#6
I tried with the power amp on the cab but the pre not - problem persists. With the pre on the cab and the power amp not - no problem.

Hmm, thinking about that now I can see how that it doesn't completely eliminate the possibility that the problem is in the pre-amp.
 
#7
I'm not clear on what the problem is--are you saying it's a mechanical vibration, like something is loose within the amp?

I was just at Shure two weeks ago. I can understand they would be reluctant to blame the amp because unless there's something rattling within it, what you describe just doesn't sound like any sort of an amp problem.
 
#8
"To rule out their theory have you tried operating with the power amp right next to your cab but isolated from the vibration of the cab ? That way it is close enough that a magnetic interference problem would still show up but vibration wouldn't."

Yes, I pretty much had to to that because the leads are short. No problem when just adjacent as opposed to being on top.

It's difficult for me to try another power amp in my neck of the woods - not sure I known anyone who's got one - bass heads are the rule round here.
 
#9
"I'm not clear on what the problem is--are you saying it's a mechanical vibration, like something is loose within the amp?"

I've described it as best I can - I think the noise is not inconsistent with it being some kind of resonance problem with something loose vibrating at around those particular frequencies and that seems to me the likeliest explanation, although, as I said, I'm not technically minded. On the other hand there could be other explanations, and I thought at one time some kind of particularly ugly overdrive might be the problem, although that seems much less likely now (or why would it disappear when the amp is adjacent to the cab?) I played a gig with this set up on Friday and totally loved the sound, but I knew I couldn't put my amp on my cab.
 
#11
Bob, that seems by far the likeliest explanation but the truth is I don't know.

What do you make of Shure's comments that the amp is a PA amp and in effect the problem arises because I'm using it for a purpose it's not designed for by having it in a bass rig? I was told the sheilding would be better if it was intended for bass players. The guy at Soundslive was frankly sceptical when I said a lot of bassists use these amps.
 
#14
The PLX is a range of general purpose amps that can be used for PA, bass, DJ, or whatever. The magnetic field from the speaker won't affect it. If there's something mecahnically rattling inside the amp, that can be fixed; sometimes the air duct buzzes a little from vibrations, but that can be alleviated with rubber or foam pads. You would be able to hear the noise coming from inside the amp.

If the "sustain" sound is coming through the speaker, you've got something else going on. Maybe feedback, maybe an EQ thing. It's impossible to tell without knowing what sort of noise it is.
 
#15
Appears from what you say that it is a form of microphonic pickup from some part in the amp.

The magnetic field thing seems unlikely on the face of it...what would the pickup means be? And speakers have a pretty constant magnetic field, the voice coil field does not leak out that much usually.
I tend to think that they didn't (think, that is).

On some of those amps, I think the top is the part the main board is attached to....maybe I'm wrong, haven't looked at one for a while. If so, pressing it would sure cut down on vibrations of the internal PC board....

Anyway, its kinda surprising that a power amp would have enough gain to feed back like that. I'm thinking something inside must be loose bigtime...like a busted solder joint on an input.

Try folding up a towel or two loosely and setting the amp on that on top of the cabinet...that might "decouple" it enough to not do it, but it would still be in range of whatever mysterious rays Shure thinks are the issue. its worth a try...they could be right....maybe....

Also, try hitting the amp while its on and hooked up, with the heel of your hand or your knuckles on top, side, bottom, try all over (but don't lam on it too hard). If it is a vibration pickup you should hear a sound in the speaker...if so, it virtually certainly has a warranty-type problem they need to fix.
 
#16
Thanks for your input guys, it is much appreciated. I really am ignorant when it comes to these technical matters.

One question which occurs: if the problem is microphonic, does that mean it is more likely to be in the preamp? I had convinced myself that the preamp wasn't the problem, because it is the power amp being placed on top of a cab that causes it, and pressing down on the power amp decreases it. But could a microphonic tendency in the pre-amp only become manifest when the power amp is on top of the cab? I'd like to be 100% sure the power amp is the problem before I return it to sure, and I don't have easy access to other pre-amps or power amps so I can compare.
 
#17
just some thoughts, bassbloke:
what kind of speaker leads are you using? if it is sheilded maybe it is becoming capacitive...
are you running your preamp and power amp on the same circuit? you may be experiencing earth loops if you're not.
have you checked your rig using different basses? there may be the possibility that the grounding on the bridge of your bass may not be correctly coupled to your b-string, however this IS highly unlikely.

just advice from previous experience. good luck!

Lexy
 
#18
Hello fellow UK TB'er

I have a similar setup as a rehearsal rig (SWR Grand Prix pre amp and a Berhinger Power amp).

If you are any where near Preston / Manchester you are more than welcome to try out my pre-amp and any number of my other PA power amps (Various Crown MA1200's, Peavey PV2600, Behringer EP1500)

If you are not in this area you must know another band who owns a PA with a power amp ?

IF not maybe the shop should be able to lend you something to try out overnight.

Try every avenue before sending the amp away.

Like I said if you are near me I would be more than willing to try some amps / pre amp combination.

Best of luck,
Dave
 
#19
I have a PLX2402 in a RoadReady 4 space rack and it rattled too. My solution was a piece of 1.5 inch (or 39cm) the same dimension as my rack case. Problem solved.
 
#20
I have to use foam or a couple beach towels to isolate my rack when it sits on top of a sub. My QSC was not buzzing; it was the top cover of my SWR IOD making all the noise.

I don't think all that pounding is very good for soldered connections or preamp tubes. Best to move it off the cabinet altogether.
 

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