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
 

paulb

Member
2001-06-01 4:53 pm
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.
 
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...
 
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
 

Jamie F

Member
2001-12-15 12:33 am
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.
 

paulb

Member
2001-06-01 4:53 pm
Calgary
AudioFreak said:
...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.
 
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
 
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?
 
JoeBob,
I am assuming that you live in the USA or Canada in which case you can get the transformers from www.plitron.ca

for the front power box you will need to choose one of the following options:-

3 x 087060201 ~ US$60.50 each + ~ US$5.25 p&h
3 x 087016201 ~ US$60.50 each + ~ US$5.25 p&h
2 x 107060201 ~ US$65.50 each + ~ US$5.25 p&h
2 x 107016201 ~ US$65.50 each + ~ US$5.25 p&h

for each of the rear amps you will need one of the following:-
1 x 077015201 ~ US$45.50 each + ~ US$5.25 p&h
OR
1 x 077060201 ~ US$45.50 each + ~ US$5.25 p&h
OR
1 x 077016201 ~ US$45.50 each + ~ US$5.25 p&h

I would go for 2 x 107060201 for the front power box and 2 x 077060201 one for each of the rear amps this will cost around US$243 and will give you plenty of headroom so you wont have to worry about it.
 
You're sudjestion on using the 107060201's and the 077060201's was also what I thought once you posted the list. I'm canadian so no conversion necissary, but almost $400 solely on transformers... Oh well, if that's what's needed I'd rather go with better quality ones then cheaper ones, and also have headroom instead of going with cheaper lower rated ones. When you think about it, it isn't that much compared to store bought amps, and I also get the joy and fun of building it :).

You said 2 x 077060201 and 2 x 107060201. But wouldn't I need 3 of the 300VA ones? Maybe you didn't hear that I also wanted a center channel, just making sure I don't buy one extra, but I'm pretty sure you just didn't notice when I said that.
I

So I guess I'll build the four power-amp modules for my front speakers first, wouldn't want to accidentally fry $400 of transformers from a stupid mistake when I could ruin only $200 worth. If that goes well, then rear and center channels will follow shortly.

Well, I think I've learned alot about transformers (I read the ESP link I was given), and I'll probably make it through building my front amp modules without frying/destroying/blowing anything up, or electrocuting myself.
 
Correct me if i am wrong but you are going to build a power box for the front and center channels and an amp box (5 Amplifiers) for the same and then 2 seperate amp/power boxes for the rear. Total of 3 boxes.

If this is the case then you will stick the 2 x 107060201 in the front power box and one of the 077060201 in each of the rear amps. Total of 4 transformers. The 107060201 are rated @ 750VA each which is 1500VA total which is as i had suggested 300VA per amp. The 077060201 are 300VA and will suit the rear channels.

If all is right build the rear amps 1st one at a time and make sure they work as they will be the smallest investment.
 
Oh... I see, I was mistaken, I thought you meant 2 x 107060201 just for the 4 front modules (two speakers, bi-amped), I see that it'll also power the center. Oops, my bad. Well, that does look better, 4 instead of my mistaken 5.

And yes you're correct for a power box for the front and center, then a seperate amp box for the same, followed by two seperate rear amp-boxes.

I guess I can start with building the front 5 channels first... Shipping seems pretty low for shipping to the states, and me being only one province away from them here in canada, it's probably less, pretty good deal. One question, how long do they take to ship? I'm going to start this after christmas when I've got more free time, just wondering if I've got to order now if they take a while.

Again, thanks for all you're help, don't know how I'd figure this stuff out on my own.
 
Just a quick note regarding using separate transformers for each amp as you enquired about earlier ... there are 2 trains of thought in this matter ... on the one hand a single large transformer offers better regulation and less copper/core losses and therefore is more efficient usually resulting in stiffer rails on the other hand the use of separate transformers removes the chance of intermodulation distortion between amps .... in this case the PSRR of the amp chip is good and the amps are not audiophile grade so you would be better using the one large transformer or in this case as previously determined two large transformers for the front and seperate transformers for each of the rear channels.
 
Ah, well if a large transformer in better i nthis case... Could I use a plitron 1 x 097060201 to power the two rear channels instead of two smaller transformers? Cost wise, it's $90 for one
097060201 instead of the $125 for two of the smaller ones, unless I'm missing this should be possible, right?

Using two 107060201 for powering all the front channels is how I'd like to go though.