Transformer/Power supply layout in chassis

[swinginguitar]

Not *exactly* stompbox, but close:

I'm planning a gainclone (LM3886) type power amp build, and contemplating  a hifi type preamp circuit - possibly in the same enclosure, but possibly external.

I have a transformer I scrapped from a Line6 Flextone (stereo) which uses the LM3886 chips in a somewhat similar circuit. It's an EI type xfo as opposed to the customary toroid.

My question is, what are some guidelines for laying out the xfo/power supply in a chassis with other component for lowest noise? what's the best way to orient the xfo? can it be layed on its side?

would it be better to create an external power supply box?

not sure what the VA rating is on the xfo (and not sure how to find out), but is it likely that it could supply enough current to run 2 stereo pairs of the LM3886 circuits? assuming a separate PS/filter circuit for each....

[teemuk]

Keep the transformer and mains wiring away from stages and wires carrying low level signals, especially if said stages and wires are of high impedance -type.

Contribute to creating good noding scheme for the whole power supply wiring and ground returns. Failing this is likely the greatest noise/hum contributor. (This is a well-covered topic and I'm not going to rephrase the content again and again).

With those two covered there shouldn't be any significant noise/hum issues.

No, you don't need an external power supply chassis; it actually introduces more drawbacks than benefits because it forces to use long supply rail and ground return wiring, which you then must compensate with more and more capacitance within the "amp chassis" and an even more tedious noding scheme for ground return. If you worry about magnetic fields just locate the PT outside the chassis. I have never had an issue with such scheme, in fact a toroidal inside the chassis but well away from input stages was also dead quiet.

Orientation shouldn't really matter unless the chassis also holds other transformers, chokes or such. ...especially those of audio-carrying type. Orientation is usually an issue only in tube amps, which have audio transformer and possibly a reverb and interstage transformers as well.

Anyway, with the power supply inside the same chassis as the amp just make sure the noding is done well, that the supply wires to power amp stage are not excessively long and that the current drawing stages have -some- local filtering capacitance right near the current drawing IC pins.

[swinginguitar]

Great info! Thanks..

do you have any links to noding scheme? Google returned a bunch of garbage...

[Kesh]

i just did this with an lm3875 and a tube pre

i put my trafos outside the chassis; rectifier, caps etc inside and right beside the gland the trafo secondary enters in; used tightly twisted pairs; located power opposite from lowest level signals (inputs and preamp); didn't run cables close or parallel; kept power and signal apart; kept power ground and signal ground apart; star ground; raised cables where necessary to keep them apart; used screened cable on input (only earth one end of screen).

[swinginguitar]

would it be of any benefit to surround the xformer in aluminum/etc for shielding?

(for space reasons i need it to be inside the chassis)

[defaced]

Great info! Thanks..

do you have any links to noding scheme? Google returned a bunch of garbage...

You'll find alot of this discussion in the tube amp realm of things.  
http://www.freewebs.com/valvewizard2/grounding.html
http://www.aikenamps.com/StarGround.html

Edit:
A really good book that covers the real effects of grounding (and other things) is Tim Williams' Circuit Designer's Companion.  The Amazon preview has alot of the first chapter which deals with grounding, and offers examples of how to handle grounding different segments of the amp.  It's well worth the read. 

http://www.amazon.com/Circuit-Designers-Companion-Second-Engineers/dp/0750663707/ref=sr_1_1?ie=UTF8&qid=1316691599&sr=8-1

[swinginguitar]

How could I determine if this xformer has enough VA to run 2 stereo pairs of LM3886 amps?

I'd like to have a 4 channel power amp. Obviously I'd need to beef up the filter caps.

[davent]

When i was looking at transformer recommendations for my gainclone build they were saying big, like 225VA for a stereo pair so you'd need to double that.  Check weights and dimensions, a 500VA Hammond (EI) measures 3.75x5.00x4.56 inches. a 500VA torroid weighs about 8-10 lbs. What's the physical size of the one you have?

I saw somewhere once that you can get a rough estimate of the unknown VA of a transformer by comparing the unknown one's size with the size of  transformers with known VA's.

dave

[defaced]

Assuming there isn't a label on the transformer indicating the secondary voltage and amperage ratings, you'll have to estimate it.  There are a few ways I know of.  I'd use all these methods, and probably take the lowest VA rating and go with it (i.e. take the conservative approach). 

-You can estimate it by weight/size of the transformer.  There's lots of discussion out there on the topic you can find by Googling "estimate va transformer" Some methods rely on weighing the transforemr, others advocate load testing it.  The method described in this link looks like it's pretty straightforward with few areas for error (and provides a citation to a book in the event you want to learn more): http://au.answers.yahoo.com/question/index?qid=20111102061157AAjsq4T

-You can look the fuse on the device it came out of.  If the mains fuse is 3 amps, and your mains wiring is nominally 120v, then the transformer cannot be larger than 3 amps * 120v, or 360VA.  (yes, I'm ignoring power factor).  I suspect this will result in an artificially high VA rating. 

-You can also look at the output chips.  Assume they are the devices using most of the power so the rest of the circuit can be ignored, and figure you can't ask for any more power out of the transformer than what can be delivered by the output chips.  If you want to double the number of output chips, you'll have to sacrifice max power to each individual chip so the entire power output is unchanged from the original design.  I suspect this will result in an artificially low VA rating. 

[swinginguitar]

The dimensions are roughly 4"x3"x3" (HxWxD)

it's an EI type xfo. Made by Suhil IIRC.

[PRR]

> is it likely that it could supply enough current to run 2

Line6 didn't get rich by over-sizing their big parts.

It will power what it was powering. What was that, two 75W channels?

Since it was guitar-amp, which are often run in full overload, it *may* power more channels in super-clean (never clipped) Hi-Fi. But I bet that's working awful close to the edge.