As some of you know I have a pair of oldschool orion gx series amps that I am planning on using in a new install. These things don't run cool by any means and I plan on using several fans but i'm not sure what would be the best setup. I planned on removing the bottom plates and mounting the amps on an amp rack with fans blowing air into the amps. Would I benefit any from running these fans in a push pull configuration? Or only blowing air out of the amps? Would it be more worthwhile to use the fans to blow air across the heatsink? I'm sure any method would help but I was just wondering what everybody thought about this subject.
You need air flowing over the fins of the heatsink. The metal tabs of the transistors transfer heat to the heatsink. The fins on the heatsink provide additional surface area to help dissipate the heat. The amp was designed to have the majority of the heat dissipated through the fins. That's where you need the most air flow.
Forcing air into the inside of the amp isn't generally much help unless you're going to abuse the amp. Sometimes, forcing air into the inside of the amp can cause problems with class AB amps. This is a class B amp and probably won't have any problems with having air forced into the bottom of the amp.
Forcing air into the inside of the amp isn't generally much help unless you're going to abuse the amp. Sometimes, forcing air into the inside of the amp can cause problems with class AB amps. This is a class B amp and probably won't have any problems with having air forced into the bottom of the amp.
Just curious what kind of issues it may cause. Blowing air across the heatsinks will require a few less fans than I was originally planning so that helps a bit.
Class AB amps require a critical amount of bias current. There is a bias compensation transistor that's adjacent to the output transistors. It, in effect, monitors the temperature of the output transistors and adjusts the biasing to compensate for the change in the electrical parameters of the outputs (due to a change in temperature).
In some amps, the transistor has nothing to clamp it to the sink and it's often floating slightly above the sink (only the temperature of the air to influence its temperature). If the bias transistor is significantly cooler than the output transistors (due to cool air being forced over it), there will be insufficient compensation for the increase in temperature of the outputs and the idle current can increase significantly enough to cause the outputs to fail.
In some amps, the transistor has nothing to clamp it to the sink and it's often floating slightly above the sink (only the temperature of the air to influence its temperature). If the bias transistor is significantly cooler than the output transistors (due to cool air being forced over it), there will be insufficient compensation for the increase in temperature of the outputs and the idle current can increase significantly enough to cause the outputs to fail.
You might notice that some amps like older PPi and what not have fans inside them causing a draft to be pulled through the amps case and across the boards. PPI buries the temp sensing diode at the PCB edge under a pile of heat sink compound. So the effects of drafting the amp have little effects on their design.
If you think you might be going to have the same issues with temp controlled bias then you might try what they did, and or fabricating some sort of shielding so the transistor or diode used to sense bias temp compensation will not be drafted by your fans.
These older amps either had the fans or not. Perry is correct in his assumptions about drifting the bias by excessive fan cooling. The only way I know to compensate is by shielding and or burial of the temp compensation sensor... Hope this helps...C
If you think you might be going to have the same issues with temp controlled bias then you might try what they did, and or fabricating some sort of shielding so the transistor or diode used to sense bias temp compensation will not be drafted by your fans.
These older amps either had the fans or not. Perry is correct in his assumptions about drifting the bias by excessive fan cooling. The only way I know to compensate is by shielding and or burial of the temp compensation sensor... Hope this helps...C
I see some amps newer MTX iirc use a little piece of black foam rubber (like thick weatherstrip) under the clamp to hold the temp sensor on the sink, with sink grease under it too but the foam covers it from the air. Some there is no clamp there is a dab of grease on it and the weatherstrip is stuck on the sink to hold it/cover it.
What I used mostly is 1-2 PC case fans about 12" away from the amps, it would blow air on most or all the sink. Seems if you move the air even slowly over the whole sink it takes a lot of heat away. In trunk mount I would hang the fan from the rear deck where it could not be seen easily, or hit with things in the trunk. Maybe warmer air up there, but it certainly cooled them down compared to no fan, and the slower PC case fan is very quiet and low draw. I used to run them off the remote wire but told not to do that with newer HUs.
How about the little fans for amps on ebay now, anyone use one? I'd rather make my own. I would assume maximum cooling would be a fan on the sink like a PC CPU cooler. It would react faster to rapid heating say from SPL use I would guess. Could be ugly though.
I found a squirrel cage style fan once (at RS) about the size of a PC fan, it is quiet and works really nice but blows a maybe 3/4" beam of air out the end. There are the slot fans that go in the back of PC cases similar to that, also I was interested in the larger diameter fan used in the sides of PC cases....depending on your install.
Also Perry has a couple of fan thermal controllers that are interesting.😉 The case fans are so low draw and unobtrusive I just let them run with the remote, if I remembered I unhooked them in the winter. I was not abusing amps, they were old amps that ran hot and some I was running at 2ohms and not sure they were supposed to. You want the ball bearing fans if you can, they don't wear out near as fast, and you can find PC fans cheap if you dig around. I still have some sleeve bearing ones I bought for $1 each. If you are really cool get the LED lit ones...😀 The high rpm ones are noisy, don't forget that, is the reason I was looking at a larger diameter case fan.
I also saw some really tiny square fans at some place I forgot. If you didn't have a bias issue they might be nice to mount on the end of an amp for inside cooling. Don't know rpm/noise levels for those.
Another cheater cheap way to run fans is a snapdisk control for a furnace fan. Could just mount one above/against an amp and get basic turn on at an ambient temp. They are crude but durable and some adjustable. Graingers has them. For example if you wanted them to run only on hot days, just run an ignition switched wire to a snapdisk hidden next to an amp.
What I used mostly is 1-2 PC case fans about 12" away from the amps, it would blow air on most or all the sink. Seems if you move the air even slowly over the whole sink it takes a lot of heat away. In trunk mount I would hang the fan from the rear deck where it could not be seen easily, or hit with things in the trunk. Maybe warmer air up there, but it certainly cooled them down compared to no fan, and the slower PC case fan is very quiet and low draw. I used to run them off the remote wire but told not to do that with newer HUs.
How about the little fans for amps on ebay now, anyone use one? I'd rather make my own. I would assume maximum cooling would be a fan on the sink like a PC CPU cooler. It would react faster to rapid heating say from SPL use I would guess. Could be ugly though.
I found a squirrel cage style fan once (at RS) about the size of a PC fan, it is quiet and works really nice but blows a maybe 3/4" beam of air out the end. There are the slot fans that go in the back of PC cases similar to that, also I was interested in the larger diameter fan used in the sides of PC cases....depending on your install.
Also Perry has a couple of fan thermal controllers that are interesting.😉 The case fans are so low draw and unobtrusive I just let them run with the remote, if I remembered I unhooked them in the winter. I was not abusing amps, they were old amps that ran hot and some I was running at 2ohms and not sure they were supposed to. You want the ball bearing fans if you can, they don't wear out near as fast, and you can find PC fans cheap if you dig around. I still have some sleeve bearing ones I bought for $1 each. If you are really cool get the LED lit ones...😀 The high rpm ones are noisy, don't forget that, is the reason I was looking at a larger diameter case fan.
I also saw some really tiny square fans at some place I forgot. If you didn't have a bias issue they might be nice to mount on the end of an amp for inside cooling. Don't know rpm/noise levels for those.
Another cheater cheap way to run fans is a snapdisk control for a furnace fan. Could just mount one above/against an amp and get basic turn on at an ambient temp. They are crude but durable and some adjustable. Graingers has them. For example if you wanted them to run only on hot days, just run an ignition switched wire to a snapdisk hidden next to an amp.
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