Commercial Gainclone kit- building instructions - Page 38 - diyAudio
Go Back   Home > Forums > Commercial Sector > Vendor Forums > Audio Sector

Audio Sector Kits & PC boards from AudioSector

Reply
 
Thread Tools Search this Thread
Old 10th January 2009, 09:55 PM   #371
diyAudio Member
 
Coreyk78's Avatar
 
Join Date: Sep 2008
Location: Farmington, MN
Quote:
Originally posted by Peter Daniel


I assume you want to use 2 LM3886 in parallel? Just follow 'red option' in this schematic: http://audiosector.com/lm4780%20amp.pdf

Yes 2 in parallel, thank you Peter.
  Reply With Quote
Old 12th January 2009, 11:38 AM   #372
diyAudio Member
 
Join Date: Dec 2008
A clarification question here.

I assume the Gainclone is ok to drive 4 ohm speakers, so long as the rail voltages are down at 22V (as per the graph that was posted towards the start of this thread.)

Anything else I should be aware of when driving 4ohm loads?

Thanks
  Reply With Quote
Old 12th January 2009, 12:05 PM   #373
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
double the dissipation of the heatsink cf National specify.
Page 14 table shows Rth s-a=3.8C/W for Ta=25degC
or
Rth s-a=3.2C/W for Ta=40degC.
Double these values to 1.9C/W or 1.6C/W for each channel.

eg. An internal heatsink for a 4ohm+4ohm amplifier needs 0.8C/W to cool the two channels.
__________________
regards Andrew T.
  Reply With Quote
Old 12th January 2009, 12:25 PM   #374
diyAudio Member
 
Join Date: Dec 2008
Ok, that's not so bad - I've got two 0.7 heat sinks being delivered (one per channel), so it looks like I should have adequate cooling properties, even if they do run a little warmer than the chips with an 8 ohm load.

Thanks
  Reply With Quote
Old 12th January 2009, 12:31 PM   #375
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Rth s-a = 0.7C/W per channel is enormous. They will appear to run cold (=good).

You could probably increase the supply voltage to +-25Vdc with sinks that big. Maybe even stretch to +-28Vdc.
__________________
regards Andrew T.
  Reply With Quote
Old 12th January 2009, 12:56 PM   #376
diyAudio Member
 
Join Date: Dec 2008
25Vdc would be the most I'd want to go to to start with. Running the figures through the formula's in the heat sink thread, then 25Vdc rails with 0.7 K/W would give a heatsink temperature of 61c assuming TA is 40c.

Ok, highly unlikely we'll ever get 40c days (in fact almost guaranteed in the UK), but there have been hot summer days last year where my room was approaching 30c (it's a bit of a sun trap), so on those days the heat sink would be 51c.
  Reply With Quote
Old 12th January 2009, 01:22 PM   #377
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
no.
Look at the table again.
I know that 0.7 is off scale but you can estimate that somewhere between Ta=80degC and Ta=100degC would be the recommended max for a 0.7C/W sink when running a 4r0 load at maximum power from +-25Vdc supplies. This is after the supply rails have sagged to +-25Vdc.

Now look at the Tc value. It is 114degC.
Lets assume the maximum allowable ambient is 95degC. Subtract actual maximum ambient from this value leaving 65Cdegrees. Now subtract this deltaT =65Cdegrees from the predicted Tc and you get actual Tc=49degC. The heatsink will be about 10Cdegrees cooler than this i.e. about 39degC.
This is the maximum temperature if the heatsink is still rated @ 0.7C/W for this low deltaT=9Cdegrees. But, that is irrelevant.
The sink should never get to that temperature since you are reproducing music where the average output power <=20dB below maximum power.

The big difference is that the Tj value when Ta=~95degC will just reach 150degC on transient peaks. But, we now know the whole amp runs cooler than this by ~65Cdegrees. So Tjmax<=85degC.
We get more output current passing Spike protection by using a big sink than by using the recommended sink. The result is cleaner sound.
__________________
regards Andrew T.
  Reply With Quote
Old 12th January 2009, 02:27 PM   #378
diyAudio Member
 
Join Date: Dec 2008
That's my problem with heat sink calculations. The National tables predict that a much smaller heat sink is required than the formula calculations explained here

So which do I use - on the one hand the Heat sink thread says not to trust published tables, and on the other hand you are saying that the National table is a reasonably reliable source as the basis to work out a heat sink that is double what National publish.

I'm confused.
  Reply With Quote
Old 12th January 2009, 02:36 PM   #379
diyAudio Member
 
Peter Daniel's Avatar
 
Join Date: Jul 2002
Location: Toronto, Canada
Send a message via AIM to Peter Daniel
Show me the heatsinks you have and I'll tell you if they'll work.
__________________
www.audiosector.com
“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
  Reply With Quote
Old 12th January 2009, 04:36 PM   #380
diyAudio Member
 
Join Date: Dec 2008
Its's the first one at the link below (SK47) and it's 100mm wide. The graph shows 0.7 K/W so it'll be up to the job.

http://www.farnell.com/datasheets/75797.pdf

The question was more about heat sink calculations in general and whether to trust the specs that the manufacturers say are good for the chips. Other threads seem to suggest otherwise.
  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
Commercial complete Gainclone kit for a beginner? gychang Chip Amps 365 4th October 2011 08:19 AM


New To Site? Need Help?

All times are GMT. The time now is 05:08 PM.


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