How much heat sink is necessary?

[rosss]

Hi, I'm working on a pair of Aleph 2's, and was curious how much heat sinking is necessary. I know that more is generally better, but I'll have to get these things past the wife! I've got a bunch of 10" x 4 3/4" heat sinks, with 2" deep fins, as shown in the attached picture. I'm not sure if I could get away with 2 per side (resulting in a 10" high and 9 1/2 inch deep amp), or three. I'm planning making monoblock amps. One other thought was to use two per side, and one on the end. Of course, I also need to fit the transformer and caps inside too. Comments?

Thanks,
Ross

[rosss]

My second idea

[rosss]

Third idea. Oh, by the way, I had assumed that having the fins vertical would be better. Is that the consensus?

[Jean]

I think 3 per side might work for mono a2. You could then mount 2 devices per heatsink, for a total of 12.

[Jay]

There are formulas to calculate heat dissipation based on pre-determined power to deliver to speaker. The idea is you will heat up each one of the MOSFET to a reasonable degree. (If one knows how to do this, he might be an experienced builder. If he was, he would have known the required size only by quick glance at the heatsink).

Personally, I think your heatsinks are acceptable. People defined the “allowable hottest temperature” as if you just couldn’t put your finger on the heated heatsink for several seconds. The hotter the temperature the shorter it’s usefull life. That’s it.

To compensate for the heat, you can/must:
1. Blow with fans
2. Reduce current by altering the ccs setting (but the output power will be lowered too, just a bit)
3. Reduce power supply voltage or DC rails voltage (the output will be lowered too)
4. Install the MOSFET on the heatsink properly (with great care and tricks)
5. The heatsink must be exposed to open air (depend on the position, if the wind/air can access the heatsink better from front, then put it vertical )

[Jay]

I mean horizontal

[Brian Donaldson]

Go for 3 per side. That would make it almost exactly the same size as my A2s I'm building.

My sinks are 10.5 tall by 6.75 wide by amost 2 deep. I loaded one with 4 - 150 ohm resistors (aluminum cased sinkable 50W units) and hooked it up to the psu. Final temp was 155 F with 140 watts total. So I calculated that at 100 watts per sink, 400 watts total dissapation, I'll run 34 deg C over ambient. I also plan to add fan in chassis and drill 2 small holes in the base of sink between each fin to exaust air. If it drops temp enough, Ill up the bias to drive 200W at 4 ohm. Worst case, I'll have a std Aleph 2 without running the fan.

Anyway, 3 of your sinks are the same rough shape, size, and fin spacing as my 2 and I can dissapate 400 w per chassis, so yours should work fine as shown. Elevate it at least 1 inch.

[Brian Donaldson]

Thought I would show you gents a pic so you didn't thing I was all hot air. I like the reflection of the chop saw.

I have enough aluminum shavings on my garage floor to cast an aluminum small block chevy.

PS, I found a salvage yard in Houston TX with 12 sheets (Yes, 4' x 8') of 1/4" aluminum plate for $1.25 per pound. Cuts are an extra charge and with a torch.

[ultrachrome]

Brian, what did you use to cut those aluminum pieces?

I just picked up a 12' x 9" x 3/16" piece from Boeing Surplus. Unfortunately, not as thrifty as yours at $1.60/lb.

[Brian Donaldson]

Dewalt skill saw with negative raked blade for non ferrous metal.
Use a good strong straight edge and clamps, set the blade to 1/2 inch deeper than metal thickness, and for gods sake use face sheild. The finish cuts are straight and smooth. (with small chatter marks) Works great. cuts are to 5 /1000 or so. A saw with 13 amp min and no end play is needed.


The sinks were a 4' extrusion I cut on 12" chop saw with same type of blade.