Halogen lighting transformers for a Gainclone?

[Atilla]

A saw an indea in a gainclone post somewhere - to use readily available halogen lighting transformers. As far as I get it they're 12v rated switching power supplies. The good thing about them is that they can come on a very good price and their size/shape makes them very convenient for use in a minimalistic amplifier. They also come with extra protection circuitry, for shorts and the like, so there's the extra safety factor added there. The power range of those can be anything between 60 and 200VA.

Now - the issue there is that we'd like to have 20-24v instead of 12. Since their output is +12, -12V - is there a way to chain two of those modules to acheive +24, 0, -24 output that can be passed on to the the rest of the power supply circuitry? I get how that can be done with traditional transformers, but to be honest, i'm quite clueless on how halogen lighting SMPS work.

Of course, I realize that SMPS have their disadvantages and many people woudn't consider them in a sound amp, but if all that is feasible, it'd be worth the try, perhaps with a tweak or two to clean up the output of any power jitter.

[theAnonymous1]

To get the desired voltage you will need to rewind the secondary of the output transformer. You will also need to add rectifiers and capacitors as they output AC voltage.

I was going to give this a try at one point, but I could never find any of the the larger ones at a reasonable price.

[AndrewT]

check your lighting supply is dual polarity. Most, if not all, are single polarity.
You'll need two for +-17Vdc and four for +-34Vdc.

If the output is nearer square wave then the DC voltage will be a bit less than SQRT(2)*Vac.

[theAnonymous1]

I thought I had seen this done before on this forum and I finally found the post.....

http://www.diyaudio.com/forums/showt...044#post989044

[Atilla]

Hm, I'm not really motivated to actually open up the halogen transformer and fiddle with the secondary winding myself. The point was, if possible to use something easily available in terms of power supply, given that it's cheap and then after building up the amp see if it'll be worth improving.

By the way the original suggestion came from http://orlygoingthirty.blogspot.com/...amplifier.html . But he doesn't mention in what way you actually wire them in series, which is the part I don't get.

Also, if i'm wring up two supplies rated at +- 12 volt, where am I loosing the 5 volt for the drop to 17?

[pacificblue]

Atilla.
The confusion is that you seem to think these "electronic transformers" are SMPS that deliver rectified and stabilised DC at the output. AndrewT knows that it is not.

The common thing is that mains voltage is rectified and the resulting DC is hacked into AC by some fast-switching transistors. That AC is transformed to 12V and at that point the "electronic transformer" is complete, while the SMPS has a second rectifier stage and usually a voltage regulator. In short that means your transformer delivers AC at the output.

If the AC was a sine wave, like what you get when you use a transformer with mains voltage, you would "win" 5V with the smoothing or power supply by-pass capacitors that you have to add. 12V times sqrt(2) ~17V, you may recognize the equation. You will win less, because these transformers are not transforming sine waves but square waves that are probably not very "clean".

Your reluctance to fiddle around with the secondaries is very healthy. Don't start fiddling.

The orlygoingthirtythread uses conventional transformers as opposed to the "electronic transformers" in the thread theAnonymous1 specified. Easy to copy. Lay the two transformers side by side. Connect the two secondary terminals that are now in the middle together and use them as your center tap. The other two terminals are your two 12V outputs. But...

you will be better off with a good audio transformer.

[aardvarkash10]

if you are linking the two outputs in series as suggested, you need ot make sure the outputs are connected in phase. Good cheap high output solution though - works well for me in both tube and GC applications, and the traffos are cheap as chips - last I bought were $5.00 each s/hand. Not bad for 60VA @ 12V outputs!

[theAnonymous1]

The transformers mentioned in the blog post are in fact ordinary transformers. These are marketed as "electromagnetic" transformers and not "electronic" transformers.

[Atilla]

Thanks a lot for the clarifications.

Yes, I guess I got confused initially with a different type of power source, that does have DC output.

I'll see if I'm going to continue looking at the halogen transformers, the experiment will depend on whether I can find at least tow of them, with reasonable power rating for a cheap cheap, price.

Otherwise I've already got the part numbers and prices for two really good 2x24 12VA toroidal transformers. They will cost more than everything else I'll ever get for that amp, but at least a good power supply is always nice to have

[AndrewT]

Originally posted by Atilla
I've already got the part numbers and prices for two really good 2x24 12VA toroidal transformers. They will cost more than everything else I'll ever get for that amp, but at least a good power supply is always nice to have

12VA transformer is tiny.