pseudo double-gang pots with transistors...

[CRE]

Hi everyone,

I was wondering if it is possible to use transistors connected to one pot to create a multiple channel control.... the use here would be for a 5.1 sound card which only has volume control via software. I thought it might be possible to acheive the same effect as a multi-gang pot through connecting one pot (audio taper) to the base of multiple transistors?

The thought is that as the voltage from the pot increases the transistors open up that much more and the volume increases to the same degree accross all 6 channels.

Would this work?

[Kevin Weddle]

I don't quite get what you are saying. A gang pot must be a simple device. If you take 100 and reduce it to 50, the effect would be to reduce the other resistors by x amount. Sometimes it is desireable to have to independant resistor values. I don't get how you want to replace this with just resistors. This is a two channel resistor network.


I have taken new light to your situation. Why not use a variable resistor on each channel? Well you could, this what a gang pot is. It is several variable resistor networks in one package. The fact that they could gang tune to any amount is just foolishness. It is a simple array of the variable resistors that are independent. The package probably has an input for every output.

[CRE]

I you look here on page 2 you'll see a pic os a single vs a dual-gang pot. I've only ever seen multiple-gang pots in similar packages where it's essentially 2 or 3 pots in one case with the wipers all connected to a common axle.

Yes this can cause minor variances between the two "networks" when one's 68/32 the other might be 60/40... yeah this can case a problem if the circuit it's being used in isn't calibrated to account for this. That's not my issue at present though... I'm looking to avoid ganged pots all together. Especially since I've never seen a 6-gang pot.

If you want to look at it in regards to a variable resistor network, that's fine... can I use a variable resistor network to control a series of transistors which are each operating on their own circuit?

EDIT: That is, can I use a potentiometer to control the signal supplied to the base of multiple transistors to allow me to use the transistors as variable resistors?

[audioguru]

Hi CRE,
If you use a transistor (or FET) as a variable resistor the output will be terribly distorted because the signal voltage will also change the transistor's resistance.

Why not use digital pots? They are ganged by simply tying their "up" and "down" control inputs together. They have extremely low distortion and some have a memory to remember the last setting after their power was shut off, or others can remember with a backup battery. Check for digital potentiometers at www.maxim-ic.com , they have some stereo ones with an audio taper.

[MP]

CRE,
The use of FETs and OTAs are common in audio when needing multiple control from a single pot. Another option is the use of opto isolators such as the CLM6000 which is a combination of an LED and LDR in a light tight container. The CLM6000 by clairex is obsolete, but there are direct replacements that can be found with a search. You can also make your own with a little shrink tubing, an LDR and an LED. You can tune the response by how close the LED is to the LDR. Experiment a little with this. Have fun!

MP

[Kevin Weddle]

That is an interesting article. I should have suspected they would go into detail concerning pots. I have not worried much about them, except for trying to obtain discrete values is sometimes frustrating. It easy is easy to see why you can turn 100 into 50 and at the same time turn the other 100 into 50. But what if you wanted something else. It seems there might be some sort of modulation if you connected the pot to a turning mechanism. You could then change the gain of two transistors as you turned the pot. As you are aware the purpose of modulation is to insert a message into a more useable medium. By changing the gain you have simple amplitude modulation. By gang tunning you could keep a bias and realize a large amplitude. Many possibilities.