[PSP]

I plan to build a minimalist chip amp (I plan to use the LM3886) to drive a pair of monitors (I might use my GR Research Paradox 1s or maybe the Parts Express BR-1) at the family cabin. 

I plan to take care in the layout and construction of this amp, but it is not intended to be an all-out no-compromise amp.  So, for this chip amp, with 30v rails, mostly for moderate listening levels, how much heatsink do I need?  While the no-compromise heatsink might look like the grill on a '52 Nash, I would like to build this amp into a reasonably small enclosure.  What do I need to do a good job?

Many thanks,
Peter

[art]

First, you have to tell us how much power you intend to dissipate, and how high above ambient you are willing to let the case to be. Then we can tell you.

If you have a rough idea on those 2, you can look through heat sink catalogues and see what the thermal resistance is. Expressed in degrees (C)/watt.

So, if you need to get rid of 50 W, and you are only willing to accept a 25 degree (C) rise, then you better find something less than 0.5 degree/W. Less, because there are some other factors that are part of the exact calculation. But that will get you close enough for hobby purposes.

Edit: I found the ancient text that gives max dissipation for a Class B output stage. It is:

V_s^2/(20*R_L)

In this case: 60^2/160 = 22.5 W, for 8 ohms. Add another few watts for the idle dissipation. So, plan on at least 25 W to get rid of.

I did the hard part; you have to decide how hot you are willing to risk.

Pat

[PSP]

Pat,
Thanks... some good clarifying questions.  You have been very helpful.

For the BR-1 (85db/watt) most listening would be between 0.5 - 2 watts.  Most of my listening is classical and jazz and I rarely listen really loud, so if we say the maximum would be ~95db steady state = 10 watts, is that correct?

The next step is to decide on max temperature rise.  Since I would like the amp to run as cool as possible, I think I will look at the various heatsink geometries that I can find and then balance heatsink size vs. chassis dimensions to get the best compromise between size and temperature rise.

Besides Digi-Key and Mouser, where can I look for suitable heatsinks?

Thanks again,
Peter

[art]

Part of the problem is that dissipation is max at around............1/4 power, if memory serves. It is not at max power.

You have 30 volt rails. Does not really matter how high the volume usually is. You may have less current through the outputs at a lower power, but the voltage is high. And therefore.......it gets hot when you think that it wouldn't.

I guess that you could go to The Key and Mouser to see what is available. Not sure they have a large enough selection. In the past, we bought an entire 6-10' extrusion, then cut it down to size. Not sure how I would proceed in your case. Maybe a surplus store.

I may have some heatsinks sitting around in the Junque Boit that might work. Send me a PM, and I will look sometime tonight and see what is taking up space. I may not have the data for them, but can probably come up with a guess for anything that I would find.

Pat

[randytsuch]

Hi Peter
I built a 3886 chip amp a while ago, used a couple of surplus heatsinks that are maybe 4x5x1 (can measure them if you are interested).  No idea what their thermal coefficient is. 

I was driving a pair of tyler monitors, about 89db efficient, in a pretty large room, and they barely got warm.

These guys have heatsinks
http://apexjr.com/Sinks.htm
and here is another place
http://www.surplussales.com/Heatsinks/HeatSink1.html

I have bought from both, with no problems.  Apex has more then is listed on his website, so you can call and ask what other heatsinks he has.  They are local to me, so I buy more from Apex, and they are easy to deal with.

My experience with the 3886 is that they didn't generate that much heat, so you may not need as much heatsinking as you think.  It's not like a class a amp.

Couple other things.  Most people think they sound better at a little lower voltage.  I was running mine with a plitron rated for 18VAC.  It ran a little higher, because I was not loading it much versus it's 6.25amp rating.  You should still have enough power to drive some monitors for you cabin.

And when you actually build it, the chips vary from one to another, in terms of DC offset.  Some are kind of high.  I bought a few, so I could select and test for lower offset.

Or, you could just buy the one that somebody is selling right now, if you have not already collected the parts.  The price he is asking is pretty reasonable, the parts cost add up pretty quickly, when you factor in a chassis to put it in.

Randy

[Kevin Haskins]

I'll give a shameless plug too.   The heatsinks I use for my multichannel Class D builds are pretty nice and you won't find an off the shelf extrusion cut to size for this price let alone a pretty blue anodized one.   aa

The size is 5" long x 3" tall.   The fins are just around 2" deep and there are two threaded holes in the bottom that allow you to mount it to the chassis with just a pair of #8 screws.    I use an adaptor plate to mount amps to it but you could drill right into the extrusion.    It has plenty of area for a chipamp.

 
http://www.diycable.com/main/product_info.php?cPath=140_141&products_id=551

[Occam]

Gents -

Lets keep our shamelsss plugs in PM (Private Messages) I realize these exchanges are ideally, mutually beneficial, but I try to keep even the appearance of commercial interest out of the Lab Circle, as we (apparently) have the Critic's Circle for that  , in addition to the Marketplace. I realize its a fine line.

Thanks,
Paul

[sts9fan]

checkout ebay.  there is a guy selling nice heatskinks for $18 a foot plus shiping.  he also will cut to any lengths you want. I  had him cut me four 3 inch sections.

[PSP]

Gentlemen,
I've been occupied for the last few days and have not had time to respond.

This info (even the shameless bits) has been extremely helpful, and I thank you all.

Peter