Speccing power transformer for several speaker loads - 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 21st June 2005, 11:31 AM   #1
diyAudio Member
 
Join Date: Jun 2005
Location: In the back of the Greta Garbo Home for Wayward Boys and Girls
Default Speccing power transformer for several speaker loads

Hi there. I just wanted to check the results of my head-scratching session with some of you guys (I'm a solid-state newbie).

I want my amp (LM3875 kit) to be able to drive different loads, depending on the cab I hook up: 8 or 16 ohms. If I run through the P=V*I (etc) equations, I find that 56W into 16 ohms is 29V at 1.9A and the same into 8 ohms is 21V into 2.6A (in theory, though I'm not sure if the ChipAmp would struggle to deliver all 56W into 16 ohms?).

So if I want the flexibility of being able to use both loads, I guess that I need a power transformer that can deliver at least 2.6A at 29V? (80VA-ish - per channel)

If so, do I need to over-rate these two factors to add a safety margin?

Any other stuff I need to watch out for?

TIA
  Reply With Quote
Old 21st June 2005, 11:50 AM   #2
Mark Kravchenko --- www.kravchenko-audio.com
diyAudio Member
 
mwmkravchenko's Avatar
 
Join Date: Jan 2002
Location: Perth Canada
Default Check this out

Check out the attachment and it will help.

Mark
Attached Files
File Type: zip lm3886 spreadsheet.zip (22.2 KB, 54 views)
__________________
Mark
  Reply With Quote
Old 21st June 2005, 03:02 PM   #3
diyAudio Member
 
Join Date: Jun 2005
Location: In the back of the Greta Garbo Home for Wayward Boys and Girls
Thanks Mark. That's a cool tool - someone put a lot of work in there.

Trying an 8-ohm load and 30V predicts 42W (which would be acceptable to me).

But the spreadsheet's built-in checks don't allow for 16ohm load calculation of the Vpeak, Ipeak and Output power. And it's password-protected, so I can't reverse engineer that maths (which I'm not sure enough of to work out for myself!).

What do I do next?
  Reply With Quote
Old 21st June 2005, 09:23 PM   #4
Mark Kravchenko --- www.kravchenko-audio.com
diyAudio Member
 
mwmkravchenko's Avatar
 
Join Date: Jan 2002
Location: Perth Canada
Default To bridge or not to bridge

A 16 ohm load is limited by the voltage the amp chip can handle. It doesn't particularily like it. If you look over to the bridged section you will find that you can indeed use this chip to power a 16 ohm load in a two chip or bridged configuration. The bridge allows double the voltage swing and more power into a 16 ohm load.

Mark
__________________
Mark
  Reply With Quote
Old 22nd June 2005, 08:41 AM   #5
diyAudio Member
 
Join Date: Jun 2005
Location: In the back of the Greta Garbo Home for Wayward Boys and Girls
I see. So a 16ohm load driven by an unbridged LM3875 GC is limited in output power to a lot less than 56W... hmm - time to do some reading on bridging GCs.

I really appreciate your help, Mark
  Reply With Quote
Old 22nd June 2005, 01:28 PM   #6
Banned
 
Join Date: Aug 2002
Location: New York, NY
Send a message via AIM to yldouright Send a message via Yahoo to yldouright
privateer
The 3886 really prefers to see a lower ohm load because of its current delivery optimization. An 1875, 3875 or a 4780 might be better suited at those higher rails but I would look at some of the other manufacturers op-amps too.
  Reply With Quote
Old 22nd June 2005, 01:38 PM   #7
Mark Kravchenko --- www.kravchenko-audio.com
diyAudio Member
 
mwmkravchenko's Avatar
 
Join Date: Jan 2002
Location: Perth Canada
Default To limit or mot to limit

WHen you go higher up in impedance it is not the current that is starved but the voltage. All the chips from NAtional have a similar maximum voltage. THere are chips from ST Microelectronics. THe 100 volt 100 watt amps that are optimised for the higher impedance loads. They have a good app note on them with their own pcb to boot!

The other option is to bridge as has allready been noted. Ther are plenty of PCB designs floating around the forum. SOme of the members are even selling quality de3signs at good prices. BrianGT is one and jackinnj ( sorry jack can't remember your moniker ) Any way check them out. If you ned help you know where I'll be.

Mark
__________________
Mark
  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
Any point in putting dummy loads on unused speaker outputs on headunit? eRiCdWoNg Car Audio 2 6th May 2004 08:12 AM
? HT speaker loads and amp power 2Bit Multi-Way 2 8th January 2004 10:49 PM
Power output into small loads Wingfeather Solid State 9 6th April 2003 06:30 PM
Best amp choice for driving NASTY speaker loads stokessd Pass Labs 8 25th March 2003 11:32 PM
Question about Pass a40 and speaker loads Eric Pass Labs 14 24th April 2001 10:30 AM


New To Site? Need Help?

All times are GMT. The time now is 07:47 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