Determining the value of volume potentiometers.

[mcasey1]

In a circuit that doesnt have a volume potentiometer, in which you would like to add a volume control at the end of the circuit, how do you go about determining the proper pot value?
Specifically, I would like to take the AMZ JFET Splitter-Buffer and add a volume pot for each output in order to split the signal from the preamp out of my PA and run to 4 headphones with independent volume control.

http://www.muzique.com/lab/splitter.htm

Thanks for any suggestions or suggested readings!

Matt

[Mark Hammer]

In that particular instance, just replace the 100k fixed resistor with a 100k log pot.  The two outside lugs go where the resistor currently goes, and you take the output from the wiper of the pot.

You can use this same approach in most cases.  IN some instances, you find a value whichdoesn't immediately present a solution.  For example, the output resistor on the Ross phaser (Tonepad Ropez) is 150k.  I decided I wanted to add a bit of gain on the last stage and have a volume pot to trim it back, or adjust for audible boost when the phaser was engaged.  What do you replace a 150k fixed resistor with?  I thought about it and realized that I had no use for any setting which was way below unity, so I replaced the 150k resistor with a 100k log pot and a 47k fixed resistor from the ground side of the pot to ground.  So, what I had was like a 147k pot that could never be turned down below 1/3 of the way, but could be turned up full.  Since the extra bit of gain was really only a 150% increase, I could get an increased volume output, or a decrease to a bit below unity, and points in between.

[mcasey1]

Oh cool!  I didnt realize it was so simple as replacing the output resistor. 

Thanks again Mark!

[john.hostetter]

Mark would this work for the Rocktave? I've been experimenting with adding a volume pot and I'm getting some strange results.

[Gus]

Keep in mind the series resistance it adds when the volume is not at max

Keep in mind the input resistance of what is after the volume pot

Keep in mind the output resistance of the circuit just before the volume pot

[Barcode80]

it helps to know that a potentiometer is really just adjusting two resistors. The resistance between lug 1 and 2, and the resistance between 2 and 3. These two resistances will equal the total resistance of the pot. as one side goes down, the other goes up.

[PRR]

> AMZ JFET Splitter-Buffer ... run to 4 headphones with independent volume control.

Headphones need "POWER".

Less than a speaker. More than an amp input.

Jack's great splitter is scaled for amp inputs, NOT headphones.

You have two choices:

Get a SPEAKER amp. 10W-20W transistor amp is fine. Maybe there's a 1982 Sansui hifi in your basement? Connect 100 ohm fixed and 100 ohm pot in series from red to black. Headphone goes from wiper to black. Do more resistor-pots for more headphones, up to 13 total.

Get four little headphone AMPS with volume controls. Y-cable all four from your PA board spare line output.

Rolls had a 2-in stereo headphone amp at a price you could hardly DIY.

RANE has/had nice 4-out/8-out headphone boxes very reasonably priced.