Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 17th December 2001, 03:17 AM   #1
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
Default LM3886 power suply...

Well, I've decided on an amp to build, the LM3886 based on ESPs project 19. My questions is what type of power suply will be required, I'll obviouly want the most power I can out of these at 4ohms (I seem to recal reading somewhere that at 4ohms you have to have a lower voltage psu then if it will be running 8ohm loads). My main questions is, if I will be building a multiple of these (7, one for each rear speaker, one for center channel, and two for each main side speakers (bi-amping) ), I would be best off using multiple power suplies am I not? If so how many would be ideal? Could anyone point me to a resource for information on this? The circuit for the amp seems rather easy and am confidednt I woun't have any problems with it, but the power suply part still puzzles me, what type of transformer will be required, hoe many, what type caps will I need and again how many... etc etc
  Reply With Quote
Old 17th December 2001, 05:05 AM   #2
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
Hey, JoeBob, I replied to your other thread, suggested 8 ohms might be okay too.
So much depends on what you can find in the way of transformers and heatsinks. Look around for surplus stuff first. Enclosure, transformers, heatsinks and capacitors will probably make up 90% of the cost of your amp, so it's worth your time to hunt around. The 3886 can run off a wide voltage range so transformer voltage isn't critical.
As for caps, the larger the better, buy what you can afford.
Multiple supplies are better, but also more expensive. If you get one huge transformer for all 7 channels it would be cheaper, but you'd probably at least want to run individual wiring to each amplifier. As amperage levels go up, the wiring gets more difficult - larger wires, etc.
Also consider packaging: everything in one big box? Separate power supply & amp boxes? Multiple boxes? If I were building it, I would probably use 1 power box + 1 amp box for the front biamped + center speakers, and one box (power and amps) for the rears. Total of 3 boxes.
By the way, I bought the ESP boards for this project. Haven't built them yet but the boards look good.
  Reply With Quote
Old 17th December 2001, 05:21 AM   #3
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
Thanks alot for both posts. I'll take your advice on 3 boxes. As for boxes I'd like to build them myself, the same goes for heatsinks, I've been building/designing my own computer cpu heatsinks for a while now so one for an amp shouldn't be much trouble .

While I understand how the amp circuit works, I'm not sure how the psu works. Is it transformer then caps then +/- on the amp circuit? I'm not clear on that and everywhere I look seems to take for granted I know such things... Basically the majority of the cost of this for me will be the transformer I wouldn't mind using two or more (depending by how much exactly it raises the costs).

Another point on the transformer, if the amp circuit needs +/-25v or +/-35v, my transfor won't be 25 or 35v will it? Will I need 25v or 35v from the transformer for each amp circuit? Or 25v or 35v shared among them and only the current drawn will be added up to total what I need from the amp?

If that's too much to answer, a link explaining such a thing (as well as what exactly the caps do in the psu and the relationship to their values on effects) would also be apreciated.

And again, thanks for your help...
  Reply With Quote
Old 17th December 2001, 06:47 AM   #4
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
Join Date: Oct 2001
Location: Queensland, Australia
If you use multiple transformers per power supply for this amp then they must be all be identical and must carry the full voltage rating and have the same number of windings... it is the current that is shared ... not the voltage. For instance if calculations suggest you need 18Vac @ 20Amps you could get this by using 2 transformers each rated at 18V @ 10Amps or 4 rated at 18Vac @ 5Amps.

You will want the supply rails to be +/-28V and dont try and push the amps above 50W of output per chip or the protection circuit will activate.

For the front power box you will need a transformer preferably toroidal (ie. donut shaped.) with 2 secondary windings of 25-28Vac and 35Amps per winding. As above this can be done by using say 5 transformers or any other number for that matter. Then use a capacitance multiplier circuit to stablize this to +/-28V.

For each of the rear amps you will need a transformer preferably toroidal (ie. donut shaped.) with 2 secondary windings of 25-28Vac and 7Amps per winding. Then use a capacitance multiplier circuit to stablize this to +/-28V.

The capacitance multiplier circuit can be found at http://sound.westhost.com/power-supplies.htm

Use 50,000uF of capacitance for the front power box and 10,000uF for each of the rear boxes.

3 really good links explaining about power supplies are:-
http://sound.westhost.com/power-supplies.htm
http://www.passlabs.com/pdf/powersupply.pdf
http://www.passdiy.com/pdf/a75p2.pdf
  Reply With Quote
Old 17th December 2001, 12:26 PM   #5
diyAudio Member
 
Join Date: Dec 2001
Location: UK
There's a design to do exactly this at www.linkwitzlab.com/xo_eq.htm (towards the bottom of the page). I'm using this for 10 channels, though I'm having noise problems (see thread on Charging Spikes if interested). There's nothing wrong with the design, but you will have to be careful about layout, particularly grounding.

If you don't want the lights dimming every time you switch on, and you want to preserve the expensive power supply parts, you might also consider Rod's soft-start circuit. I've done an adapted version of this (powered from my pre-amp, also switches the mains to the power amp) which works a treat.
  Reply With Quote
Old 17th December 2001, 02:00 PM   #6
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
I see. I always thought it was current shared, but I thought I'd ask anyways. I guess it's time to read through those links, I hopes they learn me good.
  Reply With Quote
Old 17th December 2001, 07:48 PM   #7
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
Quote:
Originally posted by AudioFreak
...you will need a transformer preferably toroidal (ie. donut shaped.) with 2 secondary windings of 25-28Vac and 35Amps per winding.
This looks high to me. A 1750VA transformer (25V * 2 windings * 35A) to get 250W of audio?
The rule of thumb I've seen is: calculate your DC power requirements and double them to get a transformer VA rating.
From the LM3886 datasheet, at 50 watts into 4 ohms using a +/-30V supply, the chip dissipates 45 watts, say 50. That makes 100 W per channel (50 for the speaker, 50 for the chip). 5 channels is 500 W, so a 1000 VA transformer should be enough.
Even that is pretty big, that's why you might consider multiple power supplies even for just the front channels. A Plitron (www.plitron.ca) 1000VA toroidal transformer is $110 CDN.
2 windings of 25 VAC should give you about 1.4*25 = +/-35 VDC, but this will droop under full load. I am of course neglecting the argument that power transformers should be lightly loaded for best sound, which is perhaps what AudioFreak intended.
  Reply With Quote
Old 17th December 2001, 10:56 PM   #8
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
Join Date: Oct 2001
Location: Queensland, Australia
JoeBob needs +/-28Vdc this should work in well with the voltage rating given the voltage droop @ full load and the use of the capacitance multiplier.

As for current 50W into 4ohms is :-
50/4=12.5
sqrt12.5=3.53Amps RMS
3.53x1.4142=5Amps peak
then add the 50mA for the chip

The chip is dissipating 48.5W rms.

So I said 7Amps per chip it is true you will get away with 4Amps per chip.

@4amps per chip the transformer is as follows 2 windings 22-25Vac @ 20Amps per winding giving 1000VA as PaulB suggested above.

Just so I am clear on a few things ...
*As you increase the VA rating of a transformer you also improve its regulation (if its a well designed transformer that is) and so you get less power loss in the windings and core. This also causes them to run cooler which decreases losses further.
*In my original calculations i used peak current draw x 1.4 for overhead..... this is *slightly* more generous then most people are
  Reply With Quote
Old 17th December 2001, 11:31 PM   #9
JoeBob is offline JoeBob  Canada
diyAudio Member
 
JoeBob's Avatar
 
Join Date: Nov 2001
Location: Montreal, Canada
I see, I'd rather be safe and have more, after all if I'm going to do something that costs money and time, I might as well do it right. You said at least a 154VA per amp in your last post. I'd rather not use the "at least" number, I'd rather use something that would give me enough headroom not to worry. Also, would I be better off using a seperate transformer per amp? Or would this increase costs by that much?
  Reply With Quote
Old 17th December 2001, 11:34 PM   #10
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
Join Date: Oct 2001
Location: Queensland, Australia
i just corrected my last post ... 200VA of transformer per Chip will be fine but i would use 300VA for peace of mind and the benefits listed above.
  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
fixed tovariable power suply salil_kamat2 Power Supplies 3 8th December 2006 01:41 PM
Power suply reduction leander Solid State 2 9th September 2005 07:54 AM
variable power suply csl113 Power Supplies 7 5th February 2005 02:19 PM
power suply help needed !!!!!!! mircea Chip Amps 0 28th June 2004 03:33 PM
Aleph5 caps for power suply cducati Pass Labs 2 20th February 2003 09:16 PM


New To Site? Need Help?

All times are GMT. The time now is 03:28 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