amp testing

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There's probably a thread or 2 about this sort of thing out there somewhere, but I couldn't find one.

Would it work to use a fan controller in place of a variac when testing amps? I don't have a variac, and the're expensive and I would have to order one which would take a week, but I could get a variable speed fan controller easily. They're about $15US and rated 600W. I've had a channel of my Leach amp done for a couple weeks, and the power supply too, but I'm afraid to test it until I get something to bring up the voltage slowly, along with some current limiting, of course. I figure if a fan controller can control an induction motor, then it could probably controll my power supply transformer, as both devices are of much the same nature.
They function by hacking out part of the sine wave of the incoming AC. Your problem will be that when the AC switches on, the diodes in the rectifier will get hit with a massive rush of current as the caps attempt to charge. Even if we assume that the diodes can take it (unlikely), your current coming off the bulk caps is going to look pretty lumpy and you're going to have a lot of racket in the circuit as a result.
I know it's tempting, but I'd advise against it. Either buy a Variac or just grit your teeth and turn it on. A load resistor might be a good idea, rather than using an actual least until you're satisfied that the amp's working properly.

i use the best cheap solution to this: just a standard light bulb in series to the amp power input; this prevents big switch-on currents, gives safety (if the amp has a problem...) and you get some optical contol, how much power the amp is consuming; just use 2 bulbs switchable, like 40W and 200W, to be used for lower or bigger amps....and a switch to pass them, if the amp works and you make some power tests .

I agree 100% with Alf. I too use a 40W bulb in series with the AC and Amp (DUT) after I make EVERY modification before plugging it directly into the AC outlet. And not just power amps, I use it for almost all plugable electrical things I wish to test. This device automatically limits the current to the wattage rating of the selected bulb. No expensive variac needed. I learned this technique from Julius Siksnius of Audire, Inc. He's routinely used it when ever he tests his own new designs or warranty repairs for his equipment.

The setup is very simple to make: take a short extension cord (plug at one end and female receptacle at the other) and cut open one wire close to the female recetacle. Wire in series with this one open wire a light bulb receptacle that can take screw in 40 to 100 W bulbs. I tend to use a 40W most of the time.

Admitedly, you cannot check full power output on a power amplifier with a 40W bulb, but you cannot, with a low input, see if the amp is oscillating and a reasonable waveform is present. If you don't have a test bench set up, the other thing you look at is the glow of the bulb. If you have a low input level and the bulb is brightly shining, you probably have a short in the power supply. If the bulb is initially bright then dims, all is probably working well and you can usually safely plug the amp directly into the AC.

Hope this helps, Robert
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