Dual CT Transformers with LM3886?

[Hugh M]

Hi Guys
I ran a perfboard version GC with 2 3875's mounted to a PII heatsink with good results but really wasn't using that much power. My latest project is making my Heresy surround speakers active with 1-3875 /speaker and I plan on using 1 PII heatsink for each amp and I'm thinking it will be fine. We'll see.
Hugh

 

[SpittinLLama]

When you guys test for heat problems how do you test? What is the output power, load, input frequency or music (or pink noise as JBL told me they use), and supply voltage rails? I'm sure you're all aware that with a Class AB amp the peak power dissipation does not occur at peak output power. So testing at full power is not as harsh of a test as testing at lower output power levels. There are formulas in the datasheet to find the peak based on supply voltage and load impedance.

SL

 

[Nuuk]

When you guys test for heat problems how do you test?

I just monitor the temperature for a few days playing different music at different levels. Not very scientific I know, but it works!

 

[BrianDonegan]

I just got this heatsink for a pair of BrianGT 4780's:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7505811186

It will be the top of the amp chassis, with walnut front/sides/back, and a thin aluminum bottom plate. Probably overkill, but I'll never worry about it, and it was only $9.

This guy gets more of these from time to time. Send him an email if you're interested.

 

[SpittinLLama]

Wow! That is a kick A price on such a nice heat sink. If you are using the whole thing for a single LM4780 then it will be over kill but it NEVER hurts to keep the IC cooler (not talking using coolers to pull temp below ambient). I would probably cut that sucker into 3" sections so the cooling fins run vertical and make 4 heat sinks from the hole thing. One of those 3" sections would probably be just fine for a single LM4780, depending on exact operating conditions. Thanks for the link W@H.

-SL

 

[BrianDonegan]

It's actually for two 4780's. I am going to be giving this amp to someone, so I want to be sure it's bullet proof. I'll post a pic.

Just noticed he's got some smaller ones for $5.50:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4660&item=7508015970&rd=1&ssPageName=WDVW

 

[blip]

Wow! Thanks for all the replies... Didn't expect so much information. A lot of great practical information here.

I was thinking about using something like this or maybe this. I think that the second one should work the only question is do I need to do one for each chip or can I get away with both on one?

 

[SpittinLLama]

I'd get the HS-111 one. I don't know if the milled out part will cause you problems mounting tho. Using TF parts I wouldn't plan on running 4 ohms with out some sort of protection kicking in unless you put fans on those sinks. Fans usually cause some noise problems either from radiation or in the supply lines so best to avoid them.

I like the one WorkingAtHome linked on ebay. Only $5.50 and if you cut it in half it will be a better heat sink than those other ones, me thinks. cheaper over all.

-SL

 

[steenoe]

I think that the second one should work the only question is do I need to do one for each chip or can I get away with both on one?

I would go for two, of the second type. Mount them with the fins vertical and in a manner so that the air can freely flow along the fins. Something like this:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=49619&perpage=10&pagenumber=7
Holes are drilled in the bottom of the chassis along the body of the heatsinks to ensure the air can travel freely.

Steen.