Film-Tech Forum ARCHIVE: Sound Rack Power Down Procedure

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Posted by Steven Ferguson (Member # 5911) on 02-17-2011, 05:01 PM:

I have a QSC MX 2000a Dual Monoaural Amp.
When shutting down for the night, do I turn this off with the switch on the amp, or can I power it down with the master switches that power down the sound rack in it's entirety?

I've seen it done both ways, but I've got a couple staff who swear adamantly it's way better for the amp to shut it down first.

Thanks

Posted by Joe Redifer (Member # 3) on 02-17-2011, 05:09 PM:

Doesn't matter. Either way it is cutting power. The caps in the amp may take a while to drain so the lights on the amp may stay on for a bit, but there is no harm in shutting everything down at once from the master breaker.

Posted by Steven Ferguson (Member # 5911) on 02-17-2011, 05:23 PM:

Thanks!

Posted by Ron Lacheur (Member # 1126) on 02-17-2011, 05:29 PM:

Is there a master breaker for the rack there? If there is and everything shuts down at once, I don't think it would be an issue. I think QSC amps mute the output on power down anyways.

We shut the amps down first then breakers off, because all the outlets in the sound rack are individual breakers and the processor turns off before all the amps are off.

Posted by Louis Bornwasser (Member # 3063) on 02-17-2011, 06:02 PM:

The amp switch is not designed for daily use. You will be replacing it.

That said, there are some inferior systems where the processor "pops" on shut down. (Sounds like a cannon in the auditorium.) Louis

Posted by Steven Ferguson (Member # 5911) on 02-17-2011, 06:04 PM:

Ahhhh that makes sense.
Yeah, we have three switches for the rack, and all can be flicked off simultaneously.
And there is no 'pop' during power down, so it sounds like I'm all good.

Posted by Sean McKinnon (Member # 612) on 02-18-2011, 12:10 AM:

I always liked the racks (big sky I think) that come with a built in contactorso that the entire rack can be switched on or off from the projection console. Very elegant IMO.

Posted by Carsten Kurz (Member # 5396) on 02-18-2011, 05:56 AM:

We have four Amps in our Rack. When we switch them all on/off with the rack main switch, our fuses blow. We switch them off individually so we need to sequence them manually on power on after the main switch. An automatic power sequencer would be handy, but would also be another possible point of failure.

Whenever possible, I would always switch off an amp individually before attached gear is switched off. These amps have huge capacitors and may still deliver bad spikes from the inputs. You may not always hear them, and you may not always have the same situation on attached systems. A blown speaker is a blown speaker.

- Carsten

Posted by Bill Yacey (Member # 6102) on 02-18-2011, 01:12 PM:

This is good advice. I used to be a partner in a live sound touring production company, the power amps were always the last thing to be powered up, and the first thing to be powered down, with the input gain controls always reduced to minimum.

This eliminates any possibility of signal thump from other devices powering up or down while the power amps are still working.

Powering up multiple high power amps simultaneously creates a very large initial current draw until the filter caps charge up. Powering down isn't a concern. The filter caps tend to absorb any inductive kick from the power transformers should the line voltage be shut off.

Posted by Barry Floyd (Member # 385) on 02-18-2011, 02:57 PM:

At our church, we have a automated power sequencer that steps through the "powering on" and "powering off" of the various equipment in the sound, lighting and video booth. The mixing consoles, rack components, etc. are powered on first, the crossovers and equalizers are powered on next, and the amps for the auditorium are powered on last. When "powering off" the system, one push of a button and it automatically does everything above in the reverse order.

Our amps are located in a closet upstairs behind the stage, so it's remotely hardwired from the booth.

Ours is actually activated with a key lock switch as opposed to a button - mainly to keep "the people who say they know how to run a sound system" from messing with it.

Posted by Ron Lacheur (Member # 1126) on 02-18-2011, 05:20 PM:

quote: Louis Bornwasser
That said, there are some inferior systems where the processor "pops" on shut down. (Sounds like a cannon in the auditorium.) Louis

CP45 does this and powering the amp off first has put an end to the woofers over excursion and failure.

Posted by Marco Giustini (Member # 4544) on 02-19-2011, 06:32 AM:

As far as I know it depends on the amp. Some amps charge the capacitors slowly and avoid breakers tripping. Some other don't. DCAs for example should be safely shut down using the main breaker, QSC call the feature "Active Inrush Limiting". Another cinema I used to work for had Crown amps and you couldn't turn them on using the breaker or it would trip (they actually did for years, trying several times every day when the breaker would trip. Eventually the breaker... broke and an amp went up in magic smoke).

I have always turned the amps off by their switches though.

Posted by Bill Yacey (Member # 6102) on 02-19-2011, 10:28 AM:

Amps that have a switching power supply can have a ramp up time to charge the filters slowly, but more amps are of the older brute force power supplies, and these are the ones we need to be concerned about.

Posted by Gordon McLeod (Member # 33) on 02-19-2011, 11:12 AM:

The charging of capacitors is only part of the problem
Many amps use large torroidal power transformers and they have very high inrush currents until the core saturates
I do all our soundsystems with all processing equipment left on all the time and just the amps being powered off
On the large systems the entire thing is left powered up 24-7

Posted by Ron Lacheur (Member # 1126) on 02-19-2011, 09:14 PM:

That's how it was when I worked for Famous Players. The racks including the amps were left on 24/7.

In the day and age of " going green ", doesn't your clients have an issue with the power bill? Or does idling amps draw next to nothing?

Posted by Bill Yacey (Member # 6102) on 02-20-2011, 02:20 PM:

Pretty much all commercial amplifiers nowadays are class AB2 or some variation of it. They draw very little current on idle. Even a 2000 watt amp doesn't draw more than around 1 amp @120V at idle. This amounts to about 120watts of power dissipation at idle, which could add up to quite a bit of power if there are multiple amps per theater in a multiple theater establishment.

Posted by Jonathan Seaman (Member # 6150) on 03-06-2011, 04:49 PM:

The theatre I work at has QSC amps in one of the screens and we switch them off using the switch on the front of the amp, then the main breaker for the sound cabinet. We have been doing this for around 10 years+ and the switches have never been replaced.

On another note, it is probably best that you switch the amps off using the switches on the front as if you were to leave the amps switches to on, when you come to power the rack up the following morning if you switch the breaker on there is going to be a large in rush of power as everything powers up it could blow fuses, breakers or even damage equipment.

Posted by Louis Bornwasser (Member # 3063) on 03-06-2011, 07:47 PM:

That will not happen if the system is properly engineered. I do agree it happens; just that it is a sign of a "work-around." Louis

Posted by Mike Blakesley (Member # 26) on 03-06-2011, 11:29 PM:

We have all the amps plugged into a power strip and we use the switch on that to turn them all on at once. (QSC amps)

Posted by Jake Spell (Member # 5294) on 03-07-2011, 12:49 AM:

Ours is super easy, turn on QSC DCM, JSD-80, XD-10 and reverse to power down. [Big Grin]

Posted by Brad Miller (Member # 2) on 03-07-2011, 01:03 AM:

quote: Mike Blakesley
We have all the amps plugged into a power strip and we use the switch on that to turn them all on at once. (QSC amps)

A power strip??? How many amps do you have and what model?

Posted by James Wallor (Member # 6154) on 03-07-2011, 02:14 AM:

I could kind of see it working on a power strip. Ours are power by a 2-pole...30 or 40 amp breaker, and we have six amps. So, essentially three amps on a single 40 amp breaker. So if they had three amps setup like L/R, C/S, RL/RR for a 5.1 setup, then perhaps.

Posted by Louis Bornwasser (Member # 3063) on 03-07-2011, 06:47 AM:

Not to be nasty. . but. . if an amp has a failure in its power cord, it will take a 40 amp load to kick the breaker?? Louis

Posted by Scott Norwood (Member # 30) on 03-07-2011, 07:57 AM:

I'm with Barry: just get the power sequencer (or, more likely, two or more, depending on the power of your sound system) and be done with it. I have no idea why these are not popular with cinema systems. Most recent installed sound systems in non-cinema venues use them.

And add the CP50 and CP200 to the list of processors that make a nasty and potentially speaker-blowing "pop" when powered on.

Posted by Mike Blakesley (Member # 26) on 03-07-2011, 08:29 AM:

quote: Brad Miller
A power strip??? How many amps do you have and what model?

Brad - I was half asleep when I made that post, apparently. They are NOT plugged into a power strip, but there IS one wall switch that turns them all on at once.

Posted by Jonathan Seaman (Member # 6150) on 03-07-2011, 09:40 AM:

I have to say that I do partially agree with you Louis but I would have put our comment slightly differently. I would have put 'This should not happen if properly engineered.' Electricity is a dangerous and mysterious thing.

Law in the UK requires us to have a system in place where if the fire alarm sounds then film sound is cut. This is done by a remote relay which is set to trip and cut power to the whole rack not just the amps.

This is why we turn the amps off individually as when we switch it on in the morning we have to throw the breaker for the sound rack to on and then reset the relay. If we left amps on then when we fire the sound rack up the following would all come on at once.

Dolby CP65
Dolby Digital Processor
5 QSC amps
Hairing Aid Loop Amp.
CD Player

It is not a great idea to fire this all up at once. But each cinema has it's own start up and shut down sequence.

Posted by James Wallor (Member # 6154) on 03-07-2011, 12:32 PM:

quote: Louis Bornwasser
Not to be nasty. . but. . if an amp has a failure in its power cord, it will take a 40 amp load to kick the breaker?? Louis

I don't remember the breaker size off the top of my head. it could only be 20amp. I was just using a value to explain the setup.

Posted by Jack Ondracek (Member # 1466) on 03-07-2011, 03:50 PM:

Barry's setup sounds really great, but seriously, people. Don't you think just about everything out there is built with the reality that the building might suffer a simple power outage? Nobody would buy your stuff if it couldn't handle that.

At most, I could maybe see turning stuff on in a particular order. Not everything is quiet on startup. For off... kill the breaker & go home.

Oh... hopefully, your place was built with switch rated breakers, or you'll eventually have the same problem Louis was talking about with front panel stuff.