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Old 12th February 2005, 12:02 PM   #1
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Default can I disable van of my PA-amp?

I've got a PA-poweramp, that has a van mounted on top of the cooling rib. On the back of the unit, I've mounted a switch to turn the van off. It makes noise, and can be quite arritating when listening to music at low sound levels.
With the van on, the amp stays at a very low temperature. When it's of, it still doesn't get much warmer. Is it harmful for the amp, even if it doesn't get warm, to have the van turned off?
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Old 12th February 2005, 12:18 PM   #2
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Monitor the temp over a long period of time. Say two or more hours of normal playing.
When the heatsink only gets slightly warm, there's no danger to turn off the fan.
It's mainly there to cool the amp under heavy PA conditions.
But if you plan a party...

/Hugo
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Old 12th February 2005, 01:16 PM   #3
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You are better of reducing the fan's rpm by soldering a resistor in series to lower the current.
Any fan can be derated and will produce significantly less noise.

Good chance that you could have the fan on say half speed, giving sufficient cooling to the heatsink, and produce a noise level that is acceptable.
Even better would be adding a temperature switch that puts the fan on nominal speed when heatsink temperature is too high.
Just a temperature switch could do the job.

Another option would be switching to a low noise fan. I have used low noise level Papst fans with reduced speed that could only be heard running when no other noises were present.
Quite often those Papst fans can be had on the web, like marktplaats.nl , brandnew for a few bucks.
A Papst 8958 80* 80 mm does 27 dBA at 1800rpm
A papst 4890 120*120 mm does 28 dBA at 1450rpm
Both 220V.
With reduced rpm noise level could drop to next to 20 dBA.
At those levels the only reason for not using a fan is the highest quality amplifier.
Fan quality on amplifiers is often rockbottom, with noise levels of +40 dBA on nominal speed.
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Old 15th February 2005, 04:59 AM   #4
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You could set up a temperature comparator circuit using a couple of op-amps and a thermistor mounted to the heat sink, preferably close to the outputs. Set up a voltage divider with the thermistor to the +V and connected in series to a resistor, connected to the -V. The voltage divider goes to one input of each op-amp, the other inputs have set voltages(resistor voltage dividers). One is slightly above the other so the fan turns on at a slightly higher temperature than it turns off. Place a pot in one of the voltage dividers, say 10% of resistor, so one switch point is slightly higher than the other and can be tweeked for the situation. This keeps it from going: on off on off on off... ect. Use some simple TTL so that one op-amp turns the fan on and the other turns the fan off. If the fan is AC line driven, just use a relay(or SS relay), and use this circuit to control it.

I once made a circuit like this and it worked real good. I don't remember the exact circuit off hand, but this is basically what I did. Still there is probably many ways to accomplish the same goal.

Once you work out the right turn on/turn off temperatures right for the amp, it's automatic. No fan noise untill you crank it but who would ever hear it then?

Or you could keep it real simple and put a toggle switch on it. Just don't forget to turn it on if you crank it!
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