Heatsink size adequate?

[rockgardenlove]

How's my heatsinking look for these LM3886? They run pretty hot I feel like.

Thanks!

[pacificblue]

Depends on the supply voltage, the load impedance, the ambient temperature and the volume at which you listen. The obvious flaw in the photo is that there are no holes below the heatsinks to let fresh air enter. If there are no holes above them either to let hot air out, there is bound to be trouble.

[rockgardenlove]

Supply is +/- 36 v and I'd say I'm planning to be using it at pretty loud levels a good amount. Ambient temp 75? And yeah, I should probably cut some holes, huh...
Thanks!

[infinia]

I would also add that based on experience, with driving the amp hard, your HS size needs to be at least triple sized with added airflow esp. since you are using the isolated tabbed version. An easy rule of thumb says the chip need to get rid of the same power ( heat) your speakers are running at, so 50-60W max. About the same as powerful CPU at full tilt. Consider using the the chassis itself for a HS with added ventilation inside.

[rockgardenlove]

Hm, I guess that might make sense to just bolt the chips onto the chassis itself...
I have a little 12v radioshack computer fan, only problem is it buzzes like heck when hooked up to a 12v rail I have going for the relays. Any way to solve this?

[infinia]

Fans and HQ audio don't really go together. Most pro gear uses a temperature sensor to control the fan speed, similar to modern PCs. Also 120 VAC fans are quieter than their DC counterparts. But consider when / if the fans fail, you would still need another form of protection too.
I think if you can use your chassis somehow that would be better.

[AndrewT]

Originally Posted by rockgardenlove

Ambient temp 75?

Centigrade or Fahrenheit?

What is the temperature of the air around the internal heatsinks. That is your ambient. For internal heatsink you could probably use Ta~40degC when the room temperature is <=25degC.

Have you used National's datasheet for sizing your heatsink?
Do that and then double the size of the sink.

[rockgardenlove]

Fahrenheit. The thing is it's a bit theoretical thus far as I'm in Oregon now and I'll be using the amp in Southern California so the weather conditions are a bit different...

I think I'll probably use the case. That's the most feasible as I'm leaving in a few days and don't really have time to source heatsinks :-/

[pacificblue]

You should use the case and the heatsinks together.

[Andrew Eckhardt]

Wow, all that aluminum plate and no heatsink.

Yeah, mount the sinks on the other side of the plate and put the chips in the middle of the sink mounting area. May as well put the sinks as far apart as possible as well. If you do that you probably wont need any additional sinking or forced air even at 36 volts. Some vents in the enclosure (assuming you'll put a top on it) will make a beneficial difference.