Variac as transformer for LM4780 GC?

[brietech]

Hello all. I'm new to the whole "DIY-Audio" world, but I do have basic electrical knowledge (and a BSEE, but I mostly do digital things!).

Anyway, I'm building an LM4780 GC kit (the dual, parallel version) from www.audiosector.com. There are 2 rectifier boards that each have 2 "22V AC" inputs connected to rectifier diodes. I happen to have a 7.5A, 0-140AC variac (rated for 1000VA) lying around unused, so my question is:

Can I just dial the variac to 22VAC, wire the output into all four rectifier inputs, and use it as my amp power supply? It would be nice not to have to buy a separate transformer.

I did a search and wasn't able to find a clear answer.


p.s. Not sure it matters, but this will be used to drive a pair of ~100W, 4-ohm speakers.

[DigitalJunkie]

NO.
For starters there won't be any isolation from the 120Vac line -this is VERY dangerous,possibly LETHAL.

There are a couple of other issues also,but that is the biggest one!

[AndrewT]

Hi,
you need a 100VA transformer for a single channel and a pair for two channels.
Alternatively you can try a single 200VA transformer for a two channel amplifier. I believe it is easier to reduce hum in a two channel amplifier if the transformer has a pair of secondary windings for each channel.
For 60W into 4ohm you need two 20Vac windings, not centre tapped. But your speakers deserve better than this. A parallel chipamp each putting out 50W into 8ohm will give you about 95W into 4ohms. This requires 150VA per channel and 25Vac windings.

Go look at the national datasheet for the design that you require.

[brietech]

So it is okay to wire the same transformer into both AC inputs (in parallel) on a single rectifier board?

I found a 24V 350VA transformer, with only 1 secondary, for a reasonable price. I can get 1 transformer per board no problem.

[AndrewT]

Originally posted by brietech
So it is okay to wire the same transformer into both AC inputs (in parallel) on a single rectifier board?

I found a 24V 350VA transformer, with only 1 secondary, for a reasonable price. I can get 1 transformer per board no problem.

the transformer has a pair of secondary windings for each channel.

you forgot to read what was said.
you NEED two secondary windings.

[brietech]

I must be slow today =)

Okay, because I am powering 2 separate LM4780's, each wired in parallel to drive 1 speaker, I should procure 1 transformer with dual-secondaries for each LM4780, correct?

[Peter Daniel]

You need transformer with dual secondaries ( 2 x 22V AC or so) because power to LM4780 is symmetrical: +/- 34V DC.

[Arx]

Originally posted by brietech
I must be slow today =)

Okay, because I am powering 2 separate LM4780's, each wired in parallel to drive 1 speaker, I should procure 1 transformer with dual-secondaries for each LM4780, correct?

you could run both chips off of a single tranformer with dual secondaries, also.

The main point, is that you need either a centretapped, or dual secondary. You can't use the same secondary for both sides of the rectifier.

Draw it out as a schematic on paper and see how it looks, and it's usually pretty obvious. (you'll see a short circuit)

As far as the variac goes, it won't work. It's not isolated, which can be quite dangerous, since even at low voltages, there's the potential to source very large currents into other devices.

Also, there's the (probably now obvious) fact that you've got only 1 secondary.

That's a pretty decent sized variac. It'll make a good piece of test equipment.

I use my variac, along with an electrical outlet box which i've rewired to put the two plugs in series. You just plug in an incandescent lightbulb rated a little higher than the wattage you expect to draw to one outlet, and your device under test to the other outlet.

Then gradually turn the variac up. If there's anything majorly wrong with your circuit, the lightbulb will come on, limiting the current to hopefully non-destructive levels.

-Nick

[Arx]

Originally posted by brietech
So it is okay to wire the same transformer into both AC inputs (in parallel) on a single rectifier board?

I found a 24V 350VA transformer, with only 1 secondary, for a reasonable price. I can get 1 transformer per board no problem.

Actually, you COULD make this work. You would not use one transformer per board though. You would use both transformers on the one board, and a parallel run to the second board.

I'm actually running my 6 channel amp on a pair of cheap single secondary non centre tapped transformers I had kicking around.

It works just fine.


Given the choice between that and a proper dual secondary, I'd grab the dual secondary, or better yet 2 duals, 1 per board. But, if you're getting a really good deal on those singles, it might be worth a go. I'm happy with mine.

-Nick

[planet10]

Each 4780 requires a power supply with +V/0/-V. (and probably at least 300 VA -- Peter?)

This can be done with a transformer with dual secondaries, or with a centre tap, or with 2 stacked transformers. This last is how you would use the 24V 350 VA trafos. You wire the primamries in parallel and then treating the 2 as a single transformer you treat the secondaries just as you would with a dual secondary trafo. You would have a "single" 700 VA supply to supply both channels/

dave