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Old 3rd July 2008, 10:09 PM   #1
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Default Looking for an amp that can survive

Hey,

Im looking for a high powered amp for PA bass use, I want about 500 watts. My challenge is that I will be using this in my church and the band guys like to have a lot of bare wire all over the place so I am sure speaker wires will short at some point in time, no question about it. I keep talking to them about it but it goes in one ear and out the other, very frustrating.

So I need something that has very good short circuit protection. I have seen many designs but none of them have short circuit protection.

I have 10 pairs of MJL4281A and MJL4302A which I hope to use. I originally wanted to build the ESP sub amp but it has no protection.


Anyone know of a good amp that can survive abuse???




Thanks

Lawrence
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Old 3rd July 2008, 11:28 PM   #2
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500 watts into what impedance?

It is possible to add protection to the P68 sub amp. VI limiters need to be added, output clamping diodes and a current limiter for the VAS or the VAS will blow up when the amp tries to current limit. It is a bit hard though to design it so it will protect the amp under a short/near-short and still allow reactive loads and transients without using an overkill output stage. Of course, an overkill output stage will have better reliability too so maybe isn't a very bad idea.

Some amps use more complex VI limiters that use capacitors so that the current limit will be higher even around 0V when the amp is playing a high-level output signal.

And then some Crown amps have an analog multiplier which calculates instantaneous dissipation that feeds 3 RC-networks and a temperature sensor on the heatsink to simulate internal transistor temperature. Those are cool but it is a bit complex to determine the thermal impedance of the transistors/heatsink assembly.
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Old 4th July 2008, 12:43 AM   #3
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another route you could go is a relay circuit. it would trip for DC offset OR overcurrent. the detector circuit uses RC time constant models of the output transistor's SOA (safe operating area) curves, and kicks the relay out if you are operating the devices outside of their safe limits, and will also kick the relay if you have DC of more than a volt on the output. circuits i've seen also have a turn-on delay, but de-energize iinstantly with loss of AC power. the turn-on delay gives about 5 seconds for the amp to settle to 0V, and eliminates turn-on thump. they are relatively simple circuits and like the VI limiters, can be retrofitted to existing amp output stages. another useful circuit is a load impedance indicator, which lights an LED if you are driving a load impedance below a preset value (most trip at 5 or 6 ohms, but can be set at 3 ohms for instance). the SOA circuits are sensitive enough to trip instantaneously if the amp is severely overloaded, but contain a time delay as well (equivalent to the output stage's thermal time constant) to allow the amp to continue if you are pushing the amp to the edge from time to time (playing a short loud passage in the music).
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