QSC series one model 1400 is loud, noisy fan?

[buggyboy]

I'm planning on moving the mixing desk out of a glass room and onto the main sanctuary floor. I've noticed an issue that might be a problem. My amp (QSC series one model 1400) is awfully loud. It make a humming noise that is really rather obnoxious. In the booth it doesn't matter as the walls and door muffle the noise, but if I take that amp out into the sanctuary it might be a serious distraction from the service.

Is there a way to muffle amps? Is this normal for this model?

I suppose I could put the amp in a room by the stage, but then I'd be on a different electrical circuit.

thanks for the help.

[[Director] drew ryan]

I don't know if your amp has a naturally loud fan or not, but I would seriously think about leaving the amp where it is if you can. Line level signal is usually quite a bit less expensive than speaker level cable.

[buggyboy]

I hadn't thought of that. good point.

This is a rather strong argument for keeping the amps close to the desk, is it not?
Is that standard practice in the industry?

[waynehoskins]

Standard practice, in general, puts the amps as close to the speakers as possible to minimize speaker cable runs. Usually amps are offstage, sometimes in a dedicated amp room. Amps for lobby speakers are often placed at FOH if that would significantly shorten the cable runs; if the difference is negligible or there are other considerations, they'll often end up in the amp room if for nothing other than simplicity.

On large stadium shows, it's pretty common to have three sets of amp racks: one on each side below stage (for each hang of main array), and if using wedges, one at Monitor Beach for monitors. Sub amps may have their own racks depending on how the soundco sets up their racks.

[buggyboy]

has anyone heard of locating the amps above the ceiling? This would put them very close indeed to the speaker cluster.

The point, I believe, is to that the run from the desk to the amp using shielded balanced mix cable pics up less noise than that same run using speaker cable. correct?

[buggyboy]

Question, of the two sub-par options, which would be better.

Going from desk to onstage amp via multichannel snake (same one the main mic lines come from the stage to the desk)

Or, keeping amp near desk, and running speaker cable (unshielded, unbalanced) to stage speakers.

(both terrible ideas, but the best I've got at the moment.

[waynehoskins]

Originally Posted by buggyboy

has anyone heard of locating the amps above the ceiling? This would put them very close indeed to the speaker cluster.

Above the ceiling is a poor choice, presuming you're talking about a ceiling (I've seen them on service truss before, but that's an entirely different thing altogether). Access above the ceiling for maintenance is problematic, as is cooling them in the summer.

Originally Posted by buggyboy

The point, I believe, is to that the run from the desk to the amp using shielded balanced mix cable pics up less noise than that same run using speaker cable. correct?

With proper equipment, both are reasonably immune to noise. The chief reasons we aim for short speaker cable are cost and resistance. Speaker cable costs more than drive cable, so shorter speaker cable means less cost. Speaker-level is (moderately) high-power, so there is resistance loss and voltage drop in the cable, which means the cable dissipates power as heat. This is reasonably negligible in reasonable lengths of properly-sized cable, but it is a consideration.

Originally Posted by buggyboy

Question, of the two sub-par options, which would be better.

Going from desk to onstage amp via multichannel snake (same one the main mic lines come from the stage to the desk)

Or, keeping amp near desk, and running speaker cable (unshielded, unbalanced) to stage speakers.

(both terrible ideas, but the best I've got at the moment.

Amps on stage, drive lines on main snake. That's standard practice in small PAs. The only difference in larger PAs is the drive lines are sometimes on their own snake (or in installs, in their own conduit), or they may use a digital interconnect between stage and FOH.

[buggyboy]

I finally took the time to take a recording of the amp in question.

the setup is a sm58 about 5 inches away from the front of the amp while the amp is turned on and off a few times.

file should be attached.

thanks

[snedecor]

Man, that would be a perfect sound effect for a Dr. Who episode!


Doesn't sound like fan to me, sort of sounds like a transformer hum...

Good luck!

[buggyboy]

That comment just officially made my day!!! 8D
I had to listen to it again...I could see it as a piece of gear on the Tardis. Sweet

Another thread I posted about some hum in this amp's output signal has me looking for a few possible causes, including transformers and rectifiers.

thanks

[buggyboy]

Here is a new recording of the amp with cover removed.
and a pic of the caps.


There was, as expected, a load of dust inside.
The capacitors look fine, no bulges etc.
The fan mount is solid, but the rotor core slides back a forth a bit...I doubt that is the hum i hear.
No leaking or burned spots as far as i can tell.
I disconnected the fan and powered the amp up for a bit and the fan noise went away, but the low grade hum remained (very low).

I'm not sure how to troubleshoot a transformer hum.
I could find the rectifiers, or didn't recognize them.


In the recording you'll heard 4 sets of 4 pulses.
With the cover removed I rigged up my trusty sm58 and went "fishing" for the mysterious hum.

the pulses were just so I could find the right section of audio in my editor.

the first set of four are passing the mic over the main transformer and a distance of about 2 inches. i was worried about the magnetic field harming my mic if i stayed still for long.

the second set of four is the mic being set over the capacitors and lifted away again 4 times. it has a crackly sound.

the third set of four is the mic passing over the fan. it is much lower in volume. at least some of the noise is from the fan because you can hear the fan oscillating as it speeds up and slows down over time.

the last set is the mic passing the rear of the amp, near the fan grating.


I'm about out of idea for this amp. I might just have to live with the hiss and hum. If only I could get a teensy weensy bit of funding. ah well.

thanks fellas.

[daver2]

The full-wave bridge rectifiers look like the two black (grey!) square blobs with the holes in the centre on the lower right-hand side of the image (by the fuses).

Incidentally, I found the schematic diagrams for the QSC 1400 on the website! These will be invaluable for what you are doing if you have not found them already!

http://www.qscaudio.com/support/tech...rt/schems1.htm

Dave