My first LM3875: check my parts please?

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meaghers

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Joined 2006
Will I be able to use the aluminum chassis at a heatsink for both ic's?

the dimensions of the chassis are 10"x6"x2" (l x w x h) and 1.0mm thick.

How do I calculate heat dissapation required and heat dissapation provided by x material for x volume?

I realise as i write this that i haven't searched extensively for this information yet, but I will right now. As always help is appreciated. I hope everyone is well!
:D
 
M

meaghers

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Joined 2006
CarlosT said:
Who's PCB board kit are you using and how many volts you got going through that LM3875? How many ohms are you going to drive...8 ohms or 4 ohms? That last one makes a big difference for power and power dissipation.
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Hi Carlos,

I'm using peter's kit from audiosector.com. I'm looking at one 44VCT transformer per channel, so that's 22V before rectification, 31V after rect., and i'm going to use this amp with 8 ohm loads only, which may be a limitation in the future, but I'm okay with that right now.

thanks carlos, I hope you can give me some more info here!
 
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CarlosT

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Joined 2006
You should really read the LM3875 datasheet which has some really good charts in the end and a decent writeup of heatsinking.

http://www.national.com/ds/LM/LM3875.pdf

This is all kinda voodoo to me still but just looking at the datasheet charts, it looks like you'll be making about 42W and will need to dissipate about 20W through either the chassis and/or a heatsink.

I'm using a heatsink that dissipates about 20W in still air but I'm running a more severe scenario since my speaker driver is a 4 ohm driver. I will be running about 25V +- which at 4 ohms is suposed to put me up there in terms of output (and heatsinking requirements). I'm hoping that the combination of the heatsink and the Hammond aluminum case I'm using will be enough.

Another nice writeup that I'm making my way through is here

http://sound.westhost.com/heatsinks.htm

Although this writeup is also not quite "Heatsinks For Dummies" :D

The heatsink I will try to use.

http://rocky.digikey.com/WebLib/Wakefield Engineering/401-K.jpg

Wakefield Engineering 401-K standard extrusion

Good luck.
 
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CarlosT

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Joined 2006
One other thing to consider is the physical layout of the heatsink. In my case, I was lucky because switching to the Techdiy kit, the LM3875 has more room around it to accomodate the heatsink fins of my particular heatsink. The AudioSector kit is much more compact.

The standard extrusions catalog from Wakefield should give you some ideas. I initially was going for the low profile 621 series but I had the hardest time finding it through the usual suppliers. Because of the Techdiy's more leisurely layout, I think...think...hope the 401-K works out.

Here's the Wakefield standard extrusions.

http://www.wakefield.com/pdf/extruded_heat_sink.pdf
 
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meaghers

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Joined 2006
geeze man, I thought i read that datasheet thing, but I obviously never READ it. That certainly does help a lot with transformer and heatsink choice doesn't it? It sure does take a while to wade through those equations though, makes me feel like I'm back in class.

thanks carlost!

So after reading all that crud, I'll move up to the 60V ct transformer for a power output of closer to 40W.

As for heatsinking, I still don't get it, I'll have to search for more info on that... but I think I need something with 4.62 C/W does that sound right?
 
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CarlosT

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falcott said:
I had some confusion about heatsinks. When I asked, I got some good advice that might help you, on this thread:

http://www.diyaudio.com/forums/showthread.php?postid=1065570#post1065570

Read the following few posts for answers. :)
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Except that I don't think that JackinJoisey ever answered my point about LM3875T versus LM3875TF. I actually still got time to order some nekkid Ts versus the plastic clad TFs that come with the AudioSector kit.

So yes...a nekkid LM3875T has the better thermal je ne sais quoi but when you add the kapton or silpad or whatever you need to sandwich between the chip and the heat sink to keep it from frying electrically, doesn't that negate the goodness of the nekkidness? I think that the answer is YES :D

Actually as good of a reader of the LM3875 datasheet as I purport to be, I got no clue where the hard numbers are for the thermal stuff for the T versus the TF.
 
M

meaghers

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Joined 2006
CarlosT said:
I think that you're OK right now...why step up to a higher volt supply?
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After rereading the lm3875 datasheet and seeing the example on page 16 I think that using a power supply closer to 30VCT to give +-38V rectified is a sound idea. Those are the numbers that are used in the example to get a 40W output.

I'll check my math again tomorrow, but if you see something weird, let me know.
 
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CarlosT

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Joined 2006
Here's a cool heat sinking subtopic...screwing down versus clipping the chip to the heatsink. There would seem to be a lotta pluses to clipping.

Pros of Clipping TO220s:

1- Even clamping pressure over a greater surface area of the chip

2- No doubleguessing about how tight to screw things

3- No additional concerns over the screw making contact with the metal and shorting the chip

4- No screw and nut to figure out

5- No holes to drill into a nice virginal heatsink

6- Quick and easy...easy to undo

Cons of Clipping TO220s:

1- Maybe...not enough pressure to make good thermal contact?

2- Maybe...clip may come off under jarring or shock to amp chassis?

3- ...

I'm grasping for cons here...
 
M

Minion

Member
Joined 2006
meaghers said:



After rereading the lm3875 datasheet and seeing the example on page 16 I think that using a power supply closer to 30VCT to give +-38V rectified is a sound idea. Those are the numbers that are used in the example to get a 40W output.

I'll check my math again tomorrow, but if you see something weird, let me know.
Click to expand...


30V AC will give you closer to 44V+ without a Load so you would be better off with a 2X28v (Or center tapped) Xformer....

I tried my LM3886 Based Guitar amp with a 30v CT Transformer and it got VERY hot very quickly and when the Transformer first start up there is a Large Current/Voltage inrush which will put the Voltage much higher for a couple of seconds unless you have a soft start curcuit, The inrush caused my Chip to make bad Popping and staticy noises for a couple second after I turned it on when useing the 30v Transformer and I was useing an EI transformer which has a Much lower inrush than a Toroid...

I would go for the 28v AC over the 30v one just to be safe and a 28v one will be easier to find .....

Cheers
 
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CarlosT

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Joined 2006
Going back to the LM3875 datasheet...

I guess the part I don't understand is that...are all the charts in the last few pages based on the "typical application" which has a gain of about 20? Could the resistor values not be messed with to reduce the gain to keep things more copacetic?

Let's say that the charts show that my supply voltage of about 28V+- puts the chip right up there with a 4 ohm load, couldn't I simply reduce the gain to let's say 15 and be OK?

Maybe I'm just not getting this...
 
falcott

falcott

Member
Joined 2004
CarlosT said:

So yes...a nekkid LM3875T has the better thermal je ne sais quoi but when you add the kapton or silpad or whatever you need to sandwich between the chip and the heat sink to keep it from frying electrically, doesn't that negate the goodness of the nekkidness? I think that the answer is YES :D
Click to expand...


It seems likely that the pad betwixt chip and sink for the T type will be thinner than the plastic casing of the TF type... hence better heat transfer. My guess. ;)
 
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