amp stops sound for some seconds

leosep · #1 12-02-2012, 12:18 AM

I have this pa system head with 300 watts rms total, a bass preamp di box BDI21, and my bass with some Quarter Pound™ for P-Bass® SPB-3. Im using all this with a 4x10 500 watts cab connected to one channel or speaker output 8 ohms of the head, i guess its 150 watts per channel. The 4x10 is wired series/parallel. 125 watts rms each driver 8ohms.

The thing is that sometimes when im playing loud, the head or the drivers stop the sound for some seconds and comes back. 1,2 ,3 or 4 times or more. The PA head is new, is it an amp problem or am i doing something wrong?

What do you think?

figuredbass · #2 12-02-2012, 01:18 AM

It sounds as though the head is going into protection mode for a few seconds, either due to excessive heat or excessively high signal gain. The clue is that it happens when you play loud, and that it comes back after a few moments. Your setup sounds like it should theoretically be OK to use, but keep in mind that you may be running it too far "in the red" if you're trying to get it louder than the 150 watts it was meant to put out safely. If you're hearing any distortion at all it may be warning that you're red-lining - remember that PA's are invariably designed to be run clean.

Also check to make sure the head is sufficiently ventilated, particularly if it is passively (no fan) cooled. I had a situation a few years ago where my passively cooled combo shut down thermally because of a tablecloth (restaurant gig) I didn't notice was draped over the cooling vents.

Aside from all of this there is of course also a chance the head may be faulty.

evilt70 · #3 12-02-2012, 01:37 AM

The head is going into protection mode. I had that happen for a while with a band I used to jam with, but it was the sub amp that would cut out (in the PA) if I was jamming something hard. If its your head, maybe check the vents for dust bunnies or other things similar that will allow heat to build up. Heat is your enemy in this situation. Or get a bigger amp.

tabdog · #4 12-02-2012, 02:41 AM

If you have the option of running the
amp bridged, it should increase the
headroom and that could allow your
amp to run cooler.

Tabdog

craig.p · #5 12-02-2012, 04:02 AM

Agreed with everyone so far. Just want to mention that you have to be careful with power ratings. You need to pick them apart like a detective. Even the RMS ones. For example, you could have a head that's rated at 150 watts per channel, but what sort of power is it putting out at BG frequencies? That's the big question.

There's also the question of duty cycle. A small powered mixer's designers probably envision a duty cycle far less strenuous than the one you're throwing at it.

In view of that, and to figuredbass's point around cooling, I'd recommend pulling the amp cover and vacuuming the inside, especially if it's fan-cooled. because fans draw in crap that accumulates over time. You are going to need every bit of cooling you can get.

You can also try some external fan assistance. A clip-on personal fan works if you can find something to clip it onto. If the amp already has a fan, aim the external one to work with the amp fan's airflow direction, not against it. If no fan, aim the external fan to work with the case vents and with heat's tendency to rise. Typically this would mean aiming the fan at the bottom vents so hot air gets forced out the top vents. A look inside will help you decide.

leosep · #6 12-02-2012, 05:44 PM

Oh thanks for the responses, i didnt know about that protection mode, and yes i play with the meter in red, specially when i use the DI Preamp BDI21 to boost the volume. I dont really know if this pa head can be bridged, how can i tell if this amp can be bridged?. Using a multimeter or something? And if its possible, the speakers outs are for 1/4 cable plugs. And if i bridge it, will the out be 4 ohms?

I dont know if this helps to know if its bridgable , but in the amp box says 300 watts at 4 ohms, and on the back of the amp each 1/4 jack out channels says 8ohms. If its bridgable how to i bridge with 1/4 plugs?

craig.p · #7 12-03-2012, 02:14 AM

If there is no bridging switch, then that option is not available to you. At least not safely/reliably. Special precautions have to be taken by the circuit designers if bridge mode operation is envisioned.

leosep · #8 12-03-2012, 06:28 PM

It doesnt has a switch but in the description of the amp, in the manual says (Speaker Out: Each output is rated at 8 ohms either two speaker cabinets at 8 ohms each or one at 4 ohms). And like i said the box says 300 watts rms at 4ohms. Any ideas of what this means?