Transformer help!

[NINfan]

I recently started working on one of Brians LM3886 chip amps. Anyway I picked up my transformer today. It runs on dual primaries, and dual secondaries, and has a VA rating of 120V. The voltage accross the dual secondaries in +22V and -22V@2.73A.

I'm very confused as to how I'm going to wire this. Should I put the primaries in parallel? that way the voltage across the primaries and secondaries is unaffected, but my current maybe diminished. I'm worried I may have to send the transformer back and wait it out for another one, just checked the .pdf sheet for the lm3886 amp on brains website and it mentioned that 160VA is what most people are using. Will a 120VA cut it?

[frdchang]

http://myweb.tiscali.co.uk/nuukspot/...ncloneFAQ.html

this is a website from a user named 'NUUK'.

i recommend reading it all, starting with the FAQ.

in the FAQ it describes transformers and how they are wired (depending if you live in a 220V or 120V country)

etc.etc.

i found the website very useful.

[aletheian]

Originally posted by NINfan
just checked the .pdf sheet for the lm3886 amp on brains website and it mentioned that 160VA is what most people are using. Will a 120VA cut it?

Depends on how you are running it. Peak output (Po) voltage is the square root of (2*Rl*Pw) Rl being your speaker load, and pw being the peak output power. Then to get peak currernt requirement for the power suppy, you need to do this: square root of [(2*Po)/Rl]. I can't remember the math for the supply rail voltage though... it's probably available on the manufacturer's website.

For example, for a 40W amp at 4 ohms, the current requirement is about 2.5A. if you use a 16 ohm load, the requirement drops to 2A.

[paulb]

Should work fine. If it starts sounding like crap, turn it down a bit.

[NINfan]

Thanks for the replies everyone, helped alot, thank you. I've come across another stump that I'm hoping some of you can address with me. Since I have a dual primaries how should I install the fuse? Do I need 2 fuses, 1 in series with each primary, or have the line part of the AC jack run right into a single fuse, and have the other end of it broken of into 2 lines, each one leading to a seperate primary. Also what value of fuse am I looking at, the lm3886 amp manual recommend 3amp for US, I'm in Canada, now.

[Nordic]

Remember to ask for T or slowblow fuses... the regular kind will just pop off straight away.

[wes-ninja250]

NINfan - electrical requirements in the US and Canada are, for all intents and purposes, identical.

Run your fuse in series with the hot lead from your power cord.

I did this the easiest way imaginable -- with an IEC connector which had the fuse integrated. Some here may disapprove of the built-in filtering, however.

The part I used is nice, as it allows you change the fuse without opening the box; prevents you from changing the fuse with the power on, and has a spot for spare fuse.

http://www.bgmicro.com/prodinfo.asp?...ri=IEC&stype=3

Wes