old transformer lying around

[merlinx76]

I have a transformer that I picked up from a surplus store awhile back. There is no info on the unit itself only what sombody had written on a label... 59VCT 500VA.

I measured the output with no load 31.6-0-31.6 VAC. That gets me about +-45VDC w/ no load.

This is, of course, a little too high for any of the commonly used chipamps. It being a center-tapped transformer makes it unsuitable for any regulated supply I have seen used on GC's.

Which brought me to look up voltage regulators.

In all the examples in the datasheets for +/- voltage regulators, the way to hook them up is always different for the neg. version and the pos. version. The only example I could find for a +/- supply only worked up to +/- 20V.

I only wanted to build a GC for the fun of it and to see what kind of result I can get. I have to keep the cost low and transformer cost is damn near half of total cost.

Does anybody know where I can find some examples of a regulated +/- supply with a center-tapped transformer?

Any other alternatives? Diodes in series? Maybe a homemade regulator with a zener and BJT?

[shusha]

Your transformer must have two secundary windings. You should be able to separate two wires from center and get two pair of wires, each pair for one secundary winding.
If you manage to do that you can feed LM338 based regulators. Could you post some pictures?

[merlinx76]

I had already opened the case before to see if I could do what you said. There's a copper shielding that makes things more difficult.

there's also a 10V secondary

Here's a pic...

[merlinx76]

I have another transformer that has many secondaries a 7V, a 41V and 19-0-19. The latter of which were used to power the amplifier section of an old (70's) Kenwood 35Wx2 reciever.

Maybe this one would be a better choice(unregulated)? As long as it can handle the current this may be enough voltage for good output at 4 ohm at least. (my better speakers are 4 ohm anyway).

[leadbelly]

You can get decent but not great results from your big transformer, unregulated, if you go with half wave rectification instead of full wave rectification. You MUST use big caps if you do this to smooth the DC.

Your 38VCT if big enough is a good match for a GC.

[Hugh M]

I just finished up an lm3875 gc and am powering it with a surplus transformer that measured 60v ct. No load, rectified and filtered the voltage is +/-39.4. The amp works fine. no over heating or strange behavior. I realize this is close to the limit for the chips but it works.
A bleeder resister across the supply lowers the voltage slighty also.
Just my experience . The spec sheet says 84 v max between rails. I'll be fine as long as I don't get a line surge.
Hugh

[planet10]

There are a ct least a couple people running their GCs at or even above the ragged edge with no reported problems...

You might also try some CRC sections or even a couple small (but with fair power dissipation) on the legs of the trafo before the rectifier (this has the effect of reducing the diode spikes).

dave

[merlinx76]

I put a load on one of the 19V secondaries and even at 4.2 amps it reads 20.2VAC. I didn't have any more reasonably sized resistors to put a bigger load.

19-0-19 would give roughly +/- 26 VDC ~90W @4 ohm (180W peak)

Do you think this transformer can handle 2 X 90W channels? It is physically pretty big... A touch smaller than my 500VA. On the back of the amplifier itself in the power specs it said 350W.