Heat sinks for LM3875TF

[monitoraudio]

I am hoping for some advice and help

I have a pair of gainclone dual mono amplifiers, with a separate power pack

Each mono has its own case, approx 17 x 10 x 5 cm, of metal construction

I have noticed after extended use the case gets relatively warm

I was thinking of sound deadening each case, which could make heat dispersal worse (?)

Should I be thinking of fitting heat sinks to each LM3875TF ?

If so, how do I go about selecting a heat sink and is it just a case of selecting my desired heat sink and using the thermal adhesive tape supplied ?

Here is hoping for some help

 

[lanchile]

Can you post pictures?

 

[monitoraudio]

What specifically do you want to see pictures of ? General pictures of the amplifiers or of the circuit boards ?

 

[lanchile]

of where those IC's are mounted! are you using the chassis as a heat sink?
I build those amps long time ago from Audiosector and the IC's did not get hot at all, but again I used big heat sinks.

 

[monitoraudio]

They are mounted on a pcb, which is not in contact with the chassis, it has a fastening screw coming through the bottom of the casework which raises them approx 1cm from the bottom of the case

 

[No 4]

So your saying the ic has no physical connection to anything in the enclosure aside from the pins into the pcbs.

 

[monitoraudio]

You are quite correct, no physical attachment to anything else in the enclosure, besides pins and pcb

 

[lanchile]

They are mounted on a pcb, which is not in contact with the chassis, it has a fastening screw coming through the bottom of the casework which raises them approx 1cm from the bottom of the case

What? are you saying those IC's are "floating" and nothing on the back to dissipate heat?. if this is right....YOU MUST PUT HEAT SINKS TO THE IC'S!!!!

 

[monitoraudio]

Hence my original post

What type of heat sink should I use and should I attach it directly to the chip ?

 

[AndrewT]

Look at the National datasheet for heatsink guidance.
Then I suggest you double their suggested sink size.

 

[lanchile]

Hence my original post

What type of heat sink should I use and should I attach it directly to the chip ?

Take a look of some amps using different heat sinks.
http://www.diyaudio.com/forums/chip-amps/79303-chip-amp-photo-gallery.html

 

[monitoraudio]

Thanks for the advice

Would something like this work and do I directly "attach" it to the chip ?

eBay - The UK's Online Marketplace

 

[lanchile]

Too small! I would use bigger heat sinks.
if the IC is TF just use heat sink compound on the back of IC and bolt it to heat sink. make sure it is tight and flat to heat sink. that way it will "transfer" all heat to the heat sink.

Large Aluminium Heat Sink Power Amplifier/ Power Supply/ Transistor/IC/FET/PA | eBay

 

[ptempel]

You could use 2 older pentium ii heatsinks:

pentium ii heatsink | eBay

Saw someone doing that here and on some other sites. If you have a local computer or electronics surplus store, then you could save a few quids on them.

 

[michal]

I was thinking of sound deadening each case, which could make heat dispersal worse (?)

Why do you need to sound deaden the case?

 

[monitoraudio]

Because I can feel the vibrations from the speakers by touching the case

They are on focul feet, on top of a granite plinth, which is on focul feet and then in a target hifi rack, one speaker is approx 80cm away

 

[scid714]

try a 6" x 3" x 1/2" chunk of aluminum. maybe drill some holes in it add a cheap 12v fan ran at a lower voltage to quite it down. just my two cents worth . it would be cheap and easy to procure. Better than floating the chip. at least bolt it to chassis.

 

[pacificblue]

So, you don't have heatsinks yet, but your amp works without trouble? That means you only need a small heatsink, because you obviously use an easy load as speakers at moderate volume and with low supply voltage.

If your case is all metal, it may be sufficient to connect the IC thermally to it by means of thermal grease a screw and maybe some additional piece of metal, if the IC is to far away from the cases superfices. The fact that the case becomes hot even now, shows that it is a good heat conductor.

If you want to add a heatsink, it makes sense to calculate its size, not guess at it.

 

[ChristianThomas]

PB, that's just what I was going to say, though you beat me to it by 6 hours.

Even if the case is not that sturdy put a heatsink on the other side, on the outside of the case as well. Do make sure the chip is completely electrically isolated - as you case will be at Earth potential (I hope) and the back of the chip is not. You won't need a mica insulator if it's the TF chip, but it is worthwhile having the proper plastic washer for the screw else you could go through the coating.

I have to say that I'm impressed that your chip has stood up so well so far. You probably should tell National!

I like the idea of those pentium II heatsinks - and get the processor thrown in for free!

 

[monitoraudio]

I have fixed the chip to the base of the casing, then added the heatsink and earthed correctly

Ran them for a good 1.5 hours this afternoon and the case was much cooler to the touch

I do think that they cant be that taxed as my speakers are relatively easy load at 92db/8ohms