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Amp problems, (I think) Ampeg, QSC

yamaha

yamaha

Apr 7, 2006
1,264
14
57
Montreal
#1
Hi friends,

This is a new ''problem'' for me. My newest amp rig is a SVPCL into a QSC PLX 1804. When I turn up all my volumes, I get a strange Static ish sound. Low on noise, but speaker cone movement is pretty intense. When I take volumes lower, (either on the amp or the preamp), the problem seems to disapear. Only happens when a bass is plugged in. Bass not plugged, no problem. Tried a few diff cables, but still does it.

Any idea what's the deal ????
 
#2
Yeah, you're trying to blow out your speakers. You're pushing them as far as they can go without blowing. When you've reached the absolute excursion limit (Xmax), speakers will often make a staticky sound. Anything past that and they'll blow. You're actually lucky you still have speakers. Calm down with the volume and realize that just because the knobs go up all the way doesn't mean your cab can take it.

Oh, also it could be you're overdriving the power amp and causing it to clip as well. But from the description, it sounds more like you're reaching xmax with the speakers.
 
#3
Thanks Jimmy, but NO, that's not it. Everything happens without playing a single note. Bass just plugged in, not being played. And detail, Not every volume is at max. Just tried variations (ie pre at 4, master 10, qsc 10) then (pre at 10, master at 10, qsc at 4) etc....

Before I got the SVPCL, I tried the QSC with a Shuttle 6.0 as a preamp, and a LM2, and it didn't do it (or at least, I didn't notice).
 
#4
Additionnal detail, the frequency of the speaker movement seems to be very low (10 to 25 Hz), so virtually no sound, just back and forth movement. It is not (the movement) close to Xmax, but it is quite noticable. The sound seems like low rumble with static ish sounds at (maybe 400Hz).
 
#5
Well you saying that if you back down your volume it goes away still makes me think its a volume issue, but I must admit that with the new info, and provided it really isn't you turning up too loud for your cab, I'm stumped.
 
#6
Sounds like some kind of low freq. oscillation or high dc offset.

Does it do it with just an instrument cable plugged into the input with no bass?

I'm thinking you're picking up something environmental with the bass pickups, or there is something wrong with the preamp that goes away with the input grounded (no cable plugged in).
 
#8
Sounds perfectly normal to me. If you put all your controls at maximum anything that's connected to the input jack will be amplified to the maximum that the amp is capable. The slow pulsing of your speaker cones is sub-sonic crud that you never want going through your amp. If your QSC has a high pass filter engage it. If it is variable, like my Mackie 1400's set it for about 20-30Hz if you have a B string or about 30-35Hz if you don't.

Paul
 
#9
With a bass, turned up, at some point the speaker can vibrate the bass strings (feedback) and you will get low note noise........

if the gain is higher, or the bass closer to the speaker, it might howl like PA feedback.

I'm still not clear on the actual issue and conditions, so this might not apply.
 
#10
Update.

All tests are done, sometimes with bass plugged in, sometimes not, but NEVER being played.

With LM2 as a preamp into QSC, barely any noise, and no low Hz important cone movement. With SVPCL into LM2, much more noise (but still not extremely noticable, just shouldn't be there), but not as much as the Ampeg into the QSC. The QSC being about twice the power of the LM2 might be the difference. Now, I try to put the SVPCL gain at 75%, output at 100%, and QSC at 15%, and there the noise is really not that bad. I'm thinking (after these tests), that it's the SVPCL that is causing the noise, and as long as the s/n ratio is high, the noise is managable. But as I lower the preamps output in favor of the power-amp, s/n ratio is lower so more noise. I guess??????

Next test would be to re-tube the SVP-CL

What are the stock tubes for this.

Thanks
 

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