lm3886 transformer help

[doug deeper]

ok ive read through everything i can find using the search function (so no yelling! )
does anyone know of an easy to find tranny that will do the job?
(im planning on using the kit from chipamp)
something under $50 would be nice as i am planning on building 4 of these
thanks in advance!

[jrc4558]

I've been searching for a couple of days now, nothing yet...

[bancika]

I think that 9V 1A transformer will work fine

[R.G.]

http://cgi.ebay.com/23-23V-400VA-Toroid-Transformer-Stepper-Motor-Driver_W0QQitemZ270065634010QQihZ017QQcategoryZ4660QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/25-25V-100VA-Toroid-Transformer-Stepper-Motor-Driver_W0QQitemZ7608760730QQihZ017QQcategoryZ4660QQrdZ1QQssPageNameZWD1VQQcmdZViewItem
http://www.plitron.com/shopping/shopdisplayproducts.asp?id=26&cat=Power+Level+%2D+120VA
http://www.plitron.com/shopping/shopdisplayproducts.asp?id=25&cat=Power+Level+%2D+80VA
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=120-225
http://www.toroid.com/standard_transformers/rectifier_transformers/partnum2.htm#711.222

You want 44 to 48Vct or two 22 to 24Vac secondaries at 80 to 100VA.

They're everywhere.

You can even use two 24V/2A transformers.

[doug deeper]

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=120-225

i had my eye on this one but it looks to be center tapped (ive never built a power supply so im a little nervous 
about ordering the wrong stuff)

the board calls for a dual secondary transformer, how would i work it with this?

[R.G.]

Oh. You're using someone else's power supply kit.

Well, you can use the transformer by ignoring that silly dual-rectifier-bridge setup and using a single bridge. See "Power Supplies Basics" from 11/15/01 at GEO. Take a look at the third illustration, bipolar power supplies. The result of their power supply rectification is two big caps one on the positive side and one on the negative side, hooked together in the middle. The GEO illustration shows a way to get to that with a single centertapped transformer and a single diode bridge.

A centertapped winding is just two identical windings permanently hooked together in the middle.

And what the devil do they mean "snubberized power supply"? If your power supply needs snubbed, you did something wrong to start with. 

[doug deeper]

ha ha ha,
yeah i guess i should stay away from the audiofiles.
ill check em out!
thanks again.

[doug deeper]

ok, so if i did something like this with the formentioned transformer would it work?
also should this be regulated? or is this ok. (i kind of through it together from memory of what ive seen in supplys before.
so please let me know if i'd left somthing out! (im sure i have)

http://i10.photobucket.com/albums/a113/midfielectronics/supply.jpg

[doug deeper]

also!
on a side note, does anyone have an opinion on how a kit like this would work for guitar?
http://store.qkits.com/moreinfo.cfm/FK660

[MartyMart]

also!
on a side note, does anyone have an opinion on how a kit like this would work for guitar?
http://store.qkits.com/moreinfo.cfm/FK660

" 1V input sensitivity and 15k input resistance "

So it would handle up to a volt, ie: LP HB equipped gtr with some boost, but the 15k looks
like it would "load down" the input .... doesn't it ??
Is this kit expecting a CD/ipod line input then perhaps ...

MM

[d95err]

also!
on a side note, does anyone have an opinion on how a kit like this would work for guitar?
http://store.qkits.com/moreinfo.cfm/FK660

If you put some kind of preamp infront of it, I guess it would work fine. Still need the powersupply though...

[R.G.]

if i did something like this with the formentioned transformer would it work?

It would work, but it would have twice the ripple voltage because it's only half wave rectified.
You want something like the second circuit here:

Notice that it's essentially the same as the power supply in the kit, but it has only one diode bridge and uses a CT transformer.

While I was typing this, i figured out why they use two bridges. It makes it impossible to get the phasing on the two-secondaries transformer wrong. When you hook up two secondaries to a make a CT winding, if you just have to connect the end leads, it's possible to get them in phase or out of phase. If they're out of phase, the net voltage at the end of both windings is the same at any instant so the voltage across the outer ends not connected to the CT is zero - they are both swinging the same way all the time. If they're correctly phased, the voltage across the non-connected leads is twice the voltage of either secondary. Using two bridges makes it impossible to get this wrong.

But I still think it's a little silly.

also should this be regulated?

It does not need to be regulated, as LM3886's are quite tolerant of varying power supplies.

on a side note, does anyone have an opinion on how a kit like this would work for guitar?
http://store.qkits.com/moreinfo.cfm/FK660

It would work fine, with the problems that MartyMart refers to. It's a power amp. You need a preamp in front of it. And it has exactly the same problem as the LM3886, as d95err points out - it doesn't have a power supply.

Here's something you want to do - go to http://www.ssguitar.com and give a read. The whole forum is about Solid State Guitar amps.

[doug deeper]

sounds good,
i just like the looks of the descrete power amp more.
and the fact that it comes with a heat sink that wont require hacking up is a big plus. (i think it comes with it)
what rectifier should i look for?
as ive said before ive never built a power supply just want to be sure i dont order a bunch of stuff that wont work well.

[doug deeper]

also! (keeo thinking) are the 1000uf caps  safishant?

[doug deeper]

would this guy do it>?
http://www.smallbearelec.com/Detail.bok?no=321

[R.G.]

For a 50W amp, use 4700uF caps, and a beefier diode bridge. Like the article says, the capacitors only get a pulse of current every 8ms (at 60 Hz, 10ms at 50) and so they get pumped up once each half cycle. So they top up during the pulse and run down supplying the load until the next pulse. It's easy to calculate. If you're putting out 50W of sound, you need probably 75W of DC. It is being supplied at (in this case) around 60V, so the current is about 75/60 = 1.25A. A capacitor supplying 1.25A runs down by
dV = I*dt/C = (1.25)*(8mS)/C

So you select your cap by how much ripple you will allow. Classical push-pull A-B amps are pretty immune to ripple, but you do have to keep it in bounds or it gets everywhere. Maybe 5% ripple max is a good thing to shoot for. 5% of 30V is 1.5V, so the cap that would do that is
C = 1.25*0.008/1.5 = 0.0067F or 6700uF.
As I said, classical topology amps are relatively immune, so letting it go over 5% is probably OK. You could actually get decent results down to 1000uF, but buy all the capacitance you can afford and fit in the box up to about 5% ripple. Diminishing returns sets in in getting much lower than 5%, which is why I picked that out of the air.

A 3A or better bridge would be appropriate. A tiny 1A bridge might work... for a while, then it's toast supplying 1.25A on musical peaks. It only has to do this on peaks, but being overloaded constantly for little snippets of time will eventually kill it, and when that happens, the resulting meltdown kills the rest of your amp.
This would be a really handy, good one to use:
http://www.mouser.com/search/refine.aspx?Ntt=621-GBU404%09

[doug deeper]

ok, think im good to go now
ill report back once i start building!