Switch Puzzle

[Skervald]

I've got a puzzle I'd like to throw out to CB world. The details and background on this are long, boring and confusing so I'm going to distill it down to just the relevant information.

The desire:
A portable, cart mounted sound system (CD, mp3 playback only) that gets used for rehearsals. It has two powered speakers mounted on it and functions as a self contained unit wherever it's needed. In addition, the cart contains two amps and an output that can be used to feed a set of passive ceiling mounted speakers in one of the larger rehearsal spaces.

The rub:
The power draw of the powered speakers on top of the power draw of the amps is too much for a 20 amp circuit. One or the other needs to be turned off before the other is turned on. Seems simple enough but someone who doesn't know better is going to mess this up, trip a breaker, and I'm going to get a phone call at home. I'd like to avoid that.

My question:
Is there an off the shelf product (a switch?) that would automatically cut power to one when the other is powered up?

Yes, there are other ways this problem could be solved (i.e. mounting the amps elsewhere, using powered speakers in the ceiling, splitting the systems entirely, using passive speakers on the cart, splitting the power feed to the cart in two so it could be fed from two circuits, etc.) but in our situation, each has it's own set of problems. For this reason I'm on a quest to find this magic piece of hardware. I feel like it should exist and be pretty easy to find but I'm stumped. Has anyone ever run across something like this?

(It may be that this question is better suited to another forum but since it involved audio, I posted it here. Please feel free to move it.)

 

[JohnD]

It seems you need a single pole double throw switch (and just to complicate things a break before make switch to complicate things.

For house wiring this is usually called a three way switch, I have no idea why?
The simplest way might be to use a dual gang outlet box with a switch and a duplex outlet, the duplex outlet has to be the kind in which there is a tab that can be removed so that the two outlets are fed the hot line separately. Which way the switch is thrown indicates which outlet is energized. OBLIGATORY SAFETY NOTICE: This should all be done by a qualified electrician.

 

[avare]

The rub:
The power draw of the powered speakers on top of the power draw of the amps is too much for a 20 amp circuit. One or the other needs to be turned off before the other is turned on. Seems simple enough but someone who doesn't know better is going to mess this up, trip a breaker, and I'm going to get a phone call at home. I'd like to avoid that.

This is highly unlikely. Power amplifiers draw a fraction of the rated power. Model number would help to determine the real power requirements.

Andre

 

[Skervald]

Thanks @JohnD. Great information. I've wired three way switches before and that would be the simple solution but I was looking for something off the shelf in order to skip the whole qualified electrician/liability thing.

@avare - one piece of information I left out (to keep things simple) was that this project is also being worked on by an audio contractor. They told me the power draw would be too much and haven't been able to find an appropriate off the shelf switch. The equipment in question is listed below:

CDI 1000 Crown Amps 700W (two, one to drive ceiling speakers and one to drive subs)
JBL EON612 powered speakers mounted on the cart (two)
I don't have specifics on the ceiling mounted speakers yet but they're looking at hanging pendants and two hanging subs

The room itself is 50x70' with 20' ceilings

Thanks for your thoughts!

 

[JD]

This is highly unlikely. Power amplifiers draw a fraction of the rated power. Model number would help to determine the real power requirements.

Andre

Agreed. Program power is usually 5 times as high as the actual wattage draw as audio has to have massive headroom to take care of transits. Figuring a 20 amp circuit can provide 2400 watts of power for non-continuous durations, almost 10,000 watts of program audio could be powered off of this. In practice, it would be less and you would have to sequence the turn-on of the power amps as not to pop the breaker on inrush. My point (even if exaggerated a bit) is that it is unlikely that such a cart system would even be close to this kind of draw. Generally, the current draw labels found on amplifiers lists the power draw under the most extreme situations, such as full power, full load sine wave outputs. So, unless you are trying to pump large volumes of organ music out while running the amp at clipping, I doubt you will hit those kind of numbers.

EDIT: Just caught the follow-up post. You should have no problems running all of that off one 20 amp circuit.

 

[FMEng]

As usual, JD is on the mark. If the system trips the breaker at initial turn-on, then turn on the speakers and amps one at a time. 20 Amps should power the thing just fine unless you are running it quite hard or are running 2 ohm loads on the amps.

 

[Skervald]

I don't see an immediate need for large volumes of organ music but if I've learned one thing, it's never say never! I'm sure there are times this system is going to get cranked pretty loud but I guess it's hard to quantify that. Sounds like the audio contractor is playing things very cautiously. If that's the case, I'll just need to convince them to ignore their engineer's recommendations and do what I want. They're currently working on other solutions so I may see what they come up with before I go down that road.

Follow up question: This system is going to get used by a lot of people with little to no audio experience. I'm planning on putting locking covers on the amps so those levels can't be adjusted. Are there any other precautionary measures you would employ?

Really appreciate everyone's input on this. You've all been very helpful!

 

[TJCornish]

I don't see an immediate need for large volumes of organ music but if I've learned one thing, it's never say never! I'm sure there are times this system is going to get cranked pretty loud but I guess it's hard to quantify that. Sounds like the audio contractor is playing things very cautiously. If that's the case, I'll just need to convince them to ignore their engineer's recommendations and do what I want. They're currently working on other solutions so I may see what they come up with before I go down that road.

Follow up question: This system is going to get used by a lot of people with little to no audio experience. I'm planning on putting locking covers on the amps so those levels can't be adjusted. Are there any other precautionary measures you would employ?

Really appreciate everyone's input on this. You've all been very helpful!

I agree with the other posters - you're not going to have a problem tripping the breaker.

The best user-proofing that can be done is a signal processor setup by your audio contractor that can't be changed - either because the DSP doesn't have a user interface, or that it is mechanically prevented from being accessed in some way. Some DSPs are sophisticated enough that they can replace a mixing board, and you can wire up some simple knobs or switches to control which input is active and levels, while still under the protection of the DSP's limits. The DBX ZonePro series can do this, as well as the BSS Soundweb series, and others.

Whether this is necessary or not depends on your budget and needs, but stuff like this is done frequently when wanting to keep users within a tight fence is desired.

 

[rwhealey]

I can see where the contractor is coming from. The CDi1000 1/8 power pink noise spec is 7.8A at 70V. Crown describes 1/8 power as "Typical of program material just at clip", so if you are expecting heavy use you could theoretically that current draw which comes out to 15.6A from the amps alone.

Rather than build a high voltage switch, I would use a relay controlled sequencing outlet strip which are available from Furman, Middle Atlantic, Atlas, among others. Then the circuit for your switch would be low voltage. Safer and no electrician required.

 

[Skervald]

I would use a relay controlled sequencing outlet strip which are available from Furman, Middle Atlantic, Atlas, among others.

I've seen these before and they seem handy for a simple power up/power down sequence. I was under the impression that they would only do one sequence at power up and then reverse that same sequence at power down. Am I just looking at the wrong models? I would need multiple sequences to make it work.

 

[TheaterEd]

I've seen these before and they seem handy for a simple power up/power down sequence. I was under the impression that they would only do one sequence at power up and then reverse that same sequence at power down. Am I just looking at the wrong models? I would need multiple sequences to make it work.

Can you install two different sequencers. Turn Key A for one mode, Turn Key B for the other?

 

[Skervald]

Can you install two different sequencers. Turn Key A for one mode, Turn Key B for the other?

It would still be possible to turn both on at the same time though, correct? I'd also have to separate the components needed for all operations (cd player, etc.) onto a third power strip.

 

[avare]

Thanks for the model info. As others have written after my first post, one 15A circuit will be adequate. If the contractor feels that 2 circuits are necessary, why not just doit and avaoid things like selectable (read another potential problem) power switching.

Everything that you described as being in the cart can be powered by one 15A circuit.

Have a great success and congratulations on researching first!

Andre

 

[JD]

Lots of people?
I would get a rack mounted limiter with a cover plate.
Although I don't think you will have enough of an inrush to trip anything, a 1-2 on, 2-1 off switch system would help. You always want the amps on last and off first. Most all amps have a turn on delay but you can still have a nasty turn-off thump if everything gets its power cut at the same time. Getting the amps off before the head end stuff will help preserve cones and eardrums.

**Mod edited to correct power sequence per subsequent posts.**

 

[rwhealey]

I've seen these before and they seem handy for a simple power up/power down sequence. I was under the impression that they would only do one sequence at power up and then reverse that same sequence at power down. Am I just looking at the wrong models? I would need multiple sequences to make it work.

Middle Atlantic MPR series would let you do it since it has one relay per duplex outlet module. You would either need play around with a digital controller or design an analog circuit that let you use a switch to do the either/or since the MPR controller provides sequencing only.

 

[RonHebbard]

Lots of people?
I would get a rack mounted limiter with a cover plate.
Although I don't think you will have enough of an inrush to trip anything, a 1-2 on, 2-1 off switch system would help. You always want the amps on first and off last. Most all amps have a turn on delay but you can still have a nasty turn-off thump if everything gets its power cut at the same time. Getting the amps off before the head end stuff will help preserve cones and eardrums.

John; Did you really mean what you wrote when typed: "You always want the amps on first and off last." If I take you literally, you're suggesting I turn the amps on first so they can then amplify any / all power up transients from any / all of the front end gear and then to keep the amps on until the bitter end so they can also amplify any/all turn off transients. Personally; I've ALWAYS gone with: Amplifiers, Last On, First OFF ALWAYS! (And all of my speakers still contain their cones in working condition.
Toodleoo!
Ron Hebbard.

 

[JD]

John; Did you really mean what you wrote when typed: "You always want the amps on first and off last." If I take you literally, you're suggesting I turn the amps on first so they can then amplify any / all power up transients from any / all of the front end gear and then to keep the amps on until the bitter end so they can also amplify any/all turn off transients. Personally; I've ALWAYS gone with: Amplifiers, Last On, First OFF ALWAYS! (And all of my speakers still contain their cones in working condition.
Toodleoo!
Ron Hebbard.

Nope! You caught my Dyslexia in all its glory! Amps on last, off first! :/
By the time I noticed, the edit was locked out! Thanks for the correction!

 

[Chris15]

Your local neigbourhood moderators are here to help in these situations

 

[Evans Poulos]

I like the 2 sequencers idea. Could you install a single key switch for both? Turn left for one setup, right for the other and center off.

 

[JD]

Having both amps on even if only one is loaded is not a problem with solid state amps. (Still is with Tube amps, so watch out guitar players!)
The bigger problem is the turn-off thump. It's a hard one to deal with as the operator has to actually turn the power amps off first, as compared to just killing the power or pulling the plug.
Best solution that I can find is the furman http://www.furmansound.com/pdf/datasheets/PS-8_PS-8R_datasheet.pdf

Automation helps remove the "idiot factor" when a large number of people are using the equipment.