Heatsinking for power amps (Avondale 260z etc) plus internal component layout

[RichardH]

Posted this as I started having a private message conversation with Quickie, but thought it might be of more general interest.

I'm building one of Les's dual mono power amps, and (being me) decided to do it the hard way - built the PCBs myself, and also the case. See "Should I spray" thread for progress on that front, but the case is looking good - painted black and ready to roll.

There have been a couple of comments re the Naim power amps not being sufficiently cooled (and I must admit, my 250 gets warm just on standby). As my case is basically a copy of a full width Naim case, I've started wondering whether I ought to add some more cooling, and if so, what and where.

Easiest thing would to be to run some heatsinking comb section down the sides of the amp, I guess, though most of it seems to be the wrong dimensions to suit. However, that would make the amp wider, and a bit of a joggle to get into the rack.

Another thought I had was to attach a heatsink to the alu heatsink brackets the modules sit on, within the case. This would mean bringing the modules closer together to allow space for the heatsink, and also I'd need to drill some holes in to top of the case to allow hot air to escape.

This brings me on to another point. My amp will contain a big fat transformer, 2 pairs of caps, 2 NCC modules and a soft start module (SSM). My case doesn't have the double skinned front like Les's ones, so I can't easily bolt the SSM to the front as per Les's layout - unless I use the sticky plastic pillar things - I wonder if they're man enough for the job though. I thought it might work if the SSM sat at the rear of the case between the NCC200 modules - short power cable runs and all - but I don't know if that will cause problems having that power circuitry so close to the NCC modules.

Any thoughts, anyone?

 

[mcai7et2]

RS do small heatsinks which clamp to the top of a TO-3 transistor, part number 402-989.

This should help to get the heat out of the transistors and into the atmosphere, I would then mount a small computer cpu fan throttled back to be silent in the back of your amp to remove all this hot air. This should help a lot, and is much cheaper and neater than heatsinking the entire case.

Not sure about layout though

Cheers,

Ed

 

[RichardH]

I'd considered the fan route - perhaps a 12v fan running at 5v (usually as silent as a pricey Papst at 12v, in my experience). I'd also looked at the T03 sinks, and have been musing whether they would fit - the two T03s are quite close together on the bracket.

 

[Andrew L Weekes]

The problem with sticky things is they often soften and lose their stick when they get hot.

For something mounted on a panel, this means it's likely to fall off with time and maybe short out - tricky one - maybe you can make a small bracket of some description?

For the heatsinking, the additional surface area to the case may help by adding an external sink, but ideally you want the modules close to it, ideally bolted to it.

Every interface between the modules and the heatsink adds thermal resistance, which means a larger temperature gradient between the modules and the case - i.e. the case may be cool, but the modules may run quite a bit hotter.

The simple rule of thumb is to get the modules as closely coupled to the outside world / heatsink as is practical.

Something worthy of consideration - on the NAP150, for example, there's a very large block of alloy that the o/p devices mount to. It's the full height of the case, and has heatsink compound applied to top and bottom so that it makes excellent thermal contact with both halves of the case, improving thermal performance.

Certainly worked last night after several hours of use at 3/4 volume

Was bloody hot afterwards though...

Andy.

 

[RichardH]

For the heatsinking, the additional surface area to the case may help by adding an external sink, but ideally you want the modules close to it, ideally bolted to it

I think that's the key - if I did have heatsinks on the outside of the box, there would be a long path from module to sink, so not efficient. Had a look at those TO3 sinks Ed pointed out - neat, as they go over the TO3, and I don't have to remove the transistors from the module.
As I said above, the other alternative is a sink on the vertical of the bracket - this way it's close to the transistors, and I can get a bigger section on there without too much hassle.

You've confirmed my feelings re the sticky things, Andy. I'll get my thinking cap on.

 

[mcai7et2]

Looking at the design of your case (big thick aluminium side pieces), you may be better off bolting the heatsink hard against the side of the case with thermal compound. The sides of your case will be an excellent heat sink and will radiate the heat directly into the atmosphere, rather than just heating the air inside the case more effectively (of course, with a fan, this is no bad thing itself).

One thing to remember about heatsinking is that if the case is being used as part of the heatsink, then you want it to get as hot as possible, which can seem a slightly illogical goal initially, but remember, the heat has to go somewhere and better the case than it stays in the device.

Cheers,

Ed

 

[RichardH]

Maybe I'll just get it running and see how it performs. The back panel can be removed, so I can always whip it out for fan installation is required at a later date...