My theatre uses CROWN COM-TECH amplifiers and we have had many problems with channels going out.

My manager thinks that they might be getting too hot because they are stacked on top of each other, but I didn't think it mattered since there is no ventilation on the top or bottom; just on the front.

Our booth is at a proper temperature, so I don't think that is the problem.

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As you can see from our photos, I am only one rack space high, where as Ken Layton is at least four rack spaces high.

At another theatre we used to use an air compressor and blow air in the amps and hideous amounts of dust would fly out the other side. I'm sure that dust could be an amp killer.

However, I'd check your speaker hookups for the proper impedance.
I know of several theatres that have their speakers (especially the surrounds) hooked up either all in series or all in parallel. This can cause a borderline overload (or sometimes an "underload") on the outputs which will cause the amps to overheat and fail prematurely.

What you should do is check the specs of the amplifiers and make sure the impedance of the speaker hookups matches the amps' specs as closely as you can manage. This means that you'll have to hook the speakers up in some kind of series-parallel arrangement. Yes, you'll have to get out your calculator and a pencil and paper and do y'erself some good ol' fashioned cipherin'. If you have an odd number of speakers you might only be able to get the impedance to come out to 6.667 ohms but that's a whole lot better than some wierd value like 3.334 ohms.

Even though some amps say they can tolerate loads as low as 2 ohms I don't think they mean that they can do it all the time. Besides they'll run a whole lot cooler and they'll probably sound a little bit better as well.

PS: Why can't this board parse the ohms symbol? (?) -- Just wondering.

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I have to repair quite a few of the Crowns based on that platform (Macro, Micro and Com-Tech)....their failure mode can be rather nasty...if the output devices fail...the will often take out the main module as well. Crown also tends to beta test in the field...if you have an early version of their amplifier then they are still in the testing phase...If you ever look at their service manuals....there are often several revisions until the settle on the final version.

Crown also often will settle for a marginal componet in their amplifiers...that is, if the theoretical wattage for a resistor is .497 watts....they will just use a 1/2 watt resistor...I'm sure some bean counter can show the cost savings and the amplifier will make it out of warranty before it fails.

As to impedance...the Com-Tech should deal with 2-Ohms just fine btw.

Randy, you are incorrect in that many amplifiers nowadays will run just fine into 2-Ohms...in fact, the QSC DCA-1222 will run just fine into 1.6 Ohms...all day long! Looking at the spec sheets you can often tell if it really can handle 2-Ohms...if they are willing to give a 20-20KHz spec on 2-Ohms it can probably handle it just fine...if they only like the 1KHz spec, it won't necessarily catch fire but you are bound to be heating it up a bit.

As to surround speakers, you should ALWAYS avoid series connections, IF your amplifier can handle the load. A series connection will introduce a sloppy bass response since this series bass will be very signifigant to the damping factor (a normally worthless figure in amplifiers). An experiment to try at home is to take an 8-Ohm resistor and put it in series with your speakers...listen for the difference in the bass response (some people actually prefer the sound).

Steve

Still, I stand by what I said earlier. You should at least check to be sure that you don't have an impedance problem.

Darryl;

Yes, I was in Salisbury just yesterday. I talked to Don. He had nothing but good things to say about you... no kidding! I was telling him that there are certain theatres that I NEVER hear from and that when I pick up my phone and see the name of one of these theatres displayed I say to myself, "Oh, Sh**! What's wrong?!" Don said that when Bob D. comes to your theatre it's almost like a vacation for him because there's nothing for him to do!

I totally agree with him on this, especially if you are having failures on the surround channels. I have found many an improperly set up surround channel sometimes that went under 1 ohm! These channels even if configured properly can be overly taxing on an amplifier and hardly ever have a constant impedance curve. You may want to be sure that its wired correctly and that there are no blown tweeters. Blown tweeters that have shorted can cause alot of amplifiers to become unstable and eventually fail. I used to sell alot of Crown amps and also had a number fail and I stopped selling them. They do make a few neat, handy modules though(PIP-RPA for instance) that are useful for PA purposes and or background music in theater lobbies and I continue to use them for that. Have not had a failure there. They are all 70 volt systems though.
Mark @ GTS

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I agree with the others on checking the impedance (or even just the resistance can tell you pretty much what is going on)...a proper impedance bridge can let you see what is going on by frequency though.

Another thing to remember when running surrounds at these low impedances is to have large enouch wire to handle the current load and not contribute significantly to the overall resistance of the system. If your typical surround speaker can take 100 watts (at 8-Ohms) then there will be at maximum just over 3.5 amps on the wire...Your wire needed be very large to handle the current load though you don't want it so small it acts too much like a resistor. However, if you take advantage of these new amps and run say 400 watts at 2-Ohms (4 100-Watt speakers in parallel) then the current leaving the amplifier jumps to over 14 amps! This puts you at 14awg for current considerations. If you use the QSC DCA-1222 with 5 8-Ohm speakers you now are up to nearly 18 Amps...and 12awg minimum.

The above should point out why one should use "home-run" lines rather than a series/parallel in the theatre. Power though the wire is expressed as current^2 x Resistance of the wire. Since current causes the power loss in the wire itself to move with the square of the current, it is best to have the current in the wire as low as possible since it is the dominent factor.

Steve

And the resistor thing is just what I am always bitching about. The bean counter wants 1/2 watt to save pennies when a good design will use 1-watt resistors minimum. Like in the Simplex XL tube systems.

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