Overall Volume reduced with a volume pot, why?

[coxy46]

I have been experimenting with adding volume pots to some of my old projects. Why does it appear that the maximum volume is less when you install a pot (1meg log pot) on the output side of the circuit (volume) than that of the volume with the output of the circuit straight into the jack output? Surely I would have thought 1meg would be enough to provide a full amount through. (the volume pot is set up as you would normally do- output of the circuit to 1, 2 goes to jack output and 3 goes to ground.)

Any help would be much appreciated, as on some of my circuits the overall volume drop detracts from the added flexibility.

Thanks in advance.

[The Tone God]

Well as long you don't mind the 1M to ground drop on the output, which you could simulate but connecting a 1M resistor to ground on the straight connected output but I doubt it will be noticable, I would say that your pot is not fully connection the signal lug side to the wiper. This means you don't get the full open output.

To check for this measure the pot's lug and wiper with a DMM and see how much resistance you get. If its not near zero then you found the problem.

Andrew

[R.G.]

To elaborate: it's all ohm's law.

There is a hidden resistor inside your effect - the output impedance. If you shorted your effect  to ground, infinite currents would not flow because the hidden inner resistance won't let them. Same situation if you hook an external resistance - the internal resistance limits things. So there's a resistor hidden in there where you can't see it. Pots are not perfect either. There is a resistance between the wiper lug and the resistor element that is usually small, but does show up. So that's there too.

Finally, there's the input impedance of whatever comes after your effect or pot.

If you take all of these resistances and measure them, you'll find that the voltages will match whatever the voltage dividers say.

Now - it is possible, as Andrew was saying, that the pot has a defect, something like an internal high resistance to the wiper or a high resistance from one lug to the resistive element that could give you problems as well. You didn't say that you were only using one single 1M pot, but if you are, check that one pot for defects.

The loading of a 1M pot is small, but if the output impedance of the effect is high or the input impedance following it is low, it can cause an audible loss.

It's that ohms law thing.

[coxy46]

Just measured the pot. (tested the three i have) and they all go from 1ohm up to opppps........100K. Sorry folks. I may have wasted your time. I was sure they were 1meg. Well that probably explains the drop in volume then. Apologies to all.
But thanks for your tips and theory, you never stop learning!