My first LM3875: check my parts please? - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd December 2006, 07:42 PM   #21
diyAudio Member
 
Join Date: Mar 2006
Location: Vancouver BC
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!
  Reply With Quote
Old 2nd December 2006, 08:09 PM   #22
CarlosT is offline CarlosT  United States
diyAudio Member
 
Join Date: Oct 2006
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.
  Reply With Quote
Old 2nd December 2006, 08:21 PM   #23
diyAudio Member
 
Join Date: Mar 2006
Location: Vancouver BC
Quote:
Originally posted by CarlosT
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.
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!
  Reply With Quote
Old 3rd December 2006, 04:20 AM   #24
CarlosT is offline CarlosT  United States
diyAudio Member
 
Join Date: Oct 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"

The heatsink I will try to use.

http://rocky.digikey.com/WebLib/Wake...ring/401-K.jpg

Wakefield Engineering 401-K standard extrusion

Good luck.
  Reply With Quote
Old 3rd December 2006, 04:37 AM   #25
CarlosT is offline CarlosT  United States
diyAudio Member
 
Join Date: Oct 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
  Reply With Quote
Old 3rd December 2006, 05:24 AM   #26
diyAudio Member
 
Join Date: Mar 2006
Location: Vancouver BC
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?
  Reply With Quote
Old 3rd December 2006, 05:53 AM   #27
Nordic is offline Nordic  South Africa
diyAudio Member
 
Nordic's Avatar
 
Join Date: Sep 2005
think its closer to 2.5
  Reply With Quote
Old 3rd December 2006, 04:45 PM   #28
diyAudio Member
 
falcott's Avatar
 
Join Date: Jul 2004
Location: Wherever I hang my hat...
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/showt...70#post1065570

Read the following few posts for answers.
__________________
"Everything should be made as simple as possible, but not simpler." - Einstein
  Reply With Quote
Old 3rd December 2006, 06:08 PM   #29
diyAudio Member
 
Join Date: Mar 2006
Location: Vancouver BC
Quote:
Originally posted by falcott
http://www.diyaudio.com/forums/showt...70#post1065570

Read the following few posts for answers.
Thanks for the pointer finger dude! That's a real help.
  Reply With Quote
Old 4th December 2006, 12:10 AM   #30
CarlosT is offline CarlosT  United States
diyAudio Member
 
Join Date: Oct 2006
I think that you're OK right now...why step up to a higher volt supply?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Intrest Check: 120w @ 8ohm Tube Buffered LM3875 Amplifier rhysh Chip Amps 192 16th March 2013 03:19 PM
WTB: lm3875 chip amp parts RobPhill33 Swap Meet 4 4th July 2006 02:56 PM
Could you check my parts list? maurycy Chip Amps 7 28th February 2006 06:01 AM
Picking LM3875 gainKlone parts 900mhz Chip Amps 3 2nd April 2005 02:12 PM
PARTS PARTS PARTS!!! Infocus Powerview 820, triplet, ballast, light, power supply!!! marioinla Swap Meet 1 31st August 2004 04:22 PM


New To Site? Need Help?

All times are GMT. The time now is 07:45 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2