Newbie question about pots

  • Thread starter Thread starter Garry Sharp
  • Start date Start date
Garry Sharp

Garry Sharp

Lost Cause
This question will be ridiculously simple to many of you, but I’d rather be stupid than wrong, so…

I bought a 200W power amp module, finished and tested with little metal posts onto which you have to solder leads and connectors for input, output, power (well, that’s just two leads connected to the toroidal transformer) and three posts for a vU meter. All well and good; before building a box for it I tested it this evening, soldered leads and sockets together and I have to say it sounded great.

But I don’t like the fact that it won’t have a master volume control, (the manufacturer said just use the preamp volume control) so I think I need to put a potentiometer on the input. I had ignorantly thought that a pot was a variable resistor that would go in line between the live side of my input socket and the live input post, but that’s not right, is it? Why do pots have 3 tags?

Whilst I’m embarrassing myself, how do I wire up a fused IEC socket and an on/off switch? Don’t ask me how I did it to test it because I’m not saying, but nothing blew up and it worked…

Hoping I don't get flamed, any input much apreciated

Garry
 
Wiring up an EAC is fairly easy... you simply purchase such a unit (with a switch) or acquire one from an old computer power supply, route out the chassis for it, and mount it.

Then, you will notice on the EAC module that the switch is already wired in series with the "hot" and "neutral" connections to the AC cord, and you wire those to your amplifier accordingly. Ground, obviously goes to ground :)

A pot, or potentiometer, is a variable resistor, and it can be wired up in two ways.

Varying current. The first pin, or tab, connects to the current source, and the center pin, or tab, is the variable current output. As you change the position of the mechanical shaft, you change the output current.

Varying voltage. You connect the first pin, or tab, to the voltage source. You connect the third pin, or tab, to ground. Then finally, connect the center pin, or tab, to the input of the amplifier.

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From the above diagram, you see pin/tab 1 and pin/tab 3 are the ends of the static resistance, and pin/tab 2 is what can be varied based on the mechanical rotation of the shaft.

Never EVER place the center pin/tab, the wiper, facing the input source. Always face it to the amplifier inputs.

This is because inputs to amplifiers, if grounded, will simply not amplify anything. But if you short the output of the amplifier (preamp) before it, you will short circuit the outputs and damage the outputs, have puffs of smoke from the electronics, or worse, start a fire.

For audio amplifiers, typically an "audio taper" potentiometer rated in the 10K to 50K range is a dandy choice.

You want "audio taper" instead of "linear taper" because your ears hear exponential increases in volume as "double" the volume. This way your volume knob with markings 1-10 seem logical visually based on what you hear. If you make the mistake of using a linear potentiometer, it doesn't hurt anything at all, just that most of the volume increase will magically appear between the 8-10 setting on the knob, and simply irritate you until you replace it.

Good luck!
 
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