best way for stereo output control from a single knob

i wanna control the stereo output of my tube mixer from a single knob.
summing amp is a mic pre with 100K pot for each side, transformer out.
very similar to NYD's 6SN7 micpre.

if i use 2 gang pot, i have doubts that it wont track properly,

i checked 2 deck 48 position Elma switches, but it seems like they are very $ hungry!
also, i dont really like stepped control, its always a less or more! for 250$ i rather have something smooth!

is there any other way, or suggestions ?

An alps dual pot tracks pretty dern well! I have one here in my sum box : outputs are within 0.3dB of each other at worst.

They're fairly exe at around 15usd each, but they have a great feel and I think are likely to last longer than most.

If it's  not a  dual pot, or a dual switched attenuator your options for variable gain are then  vca or stepped transformer which is real dollars!

alexc said:

An alps dual pot tracks pretty dern well! I have one here in my sum box : outputs are within 0.3dB of each other at worst.

They're fairly exe at around 15usd each, but they have a great feel and I think are likely to last longer than most.

Click to expand...

sounds good, thanks! will order one dual gang...

Audio taper pots are difficult to make and very hard to ensure they track well which is why a lot of people use stepped pots with fixed resistors. However, linear pots are much easier to make and track very well. The problem is their taper is not very good for audio work. A common way round this used in many pro consoles is to use a slugged linear pot. If you slug a 10K linear pot with a 5K resistor  from the slider to 0V, then at the midpoint the  bottom of the divider is 5K//5K which is 2K5. The top of the divider is 5K. The attenuation is therefore 1/3 which is close to 10dB. Not as good as the 15 to 20dB you typically get with an audio taper pot but perfectly usable and much cheaper.

Cheers

Ian

thanks Ian, sounds like a good idea...
i remember reading from your earlier posts, you guys were using same idea on Neve consoles!

ruffrecords said:

If you slug a 10K linear pot with a 5K resistor  from the slider to 0V, then at the midpoint the  bottom of the divider is 5K//5K which is 2K5. The top of the divider is 5K. The attenuation is therefore 1/3 which is close to 10dB. Not as good as the 15 to 20dB you typically get with an audio taper pot but perfectly usable and much cheaper.

Click to expand...

Maybe just do it twice by multiplying, with a dual 10K lin pot. Or a quad linear pot for stereo. I have no idea what the resulting curve would be, but the center position should be similar to an audio taper. R-X would be based on the load, to present 5K to the wiper. Just musing, although cumbersome, would this work?

Gene

i found this link

http://www.potentiometers.com/select_precision2.cfm

Gene Pink said:

Maybe just do it twice by multiplying, with a dual 10K lin pot. Or a quad linear pot for stereo. I have no idea what the resulting curve would be, but the center position should be similar to an audio taper. R-X would be based on the load, to present 5K to the wiper. Just musing, although cumbersome, would this work?

Gene

Click to expand...

The downside of a slugged pot is that the load it represents varies with the pot position. In the example I gave, when  fully up it is 10K//5K which is about 3K3 and when fully down it is 10K. If you cascade two such circuits, as well as obtaining more attenuation at the mid point you also increase the range of the load it represents. If you have a nice low driving source impedance this is generally not a problem but you don't always have a low driving source impedance (e.g. in tube designs) .

Cheers

ian

kambo said:

if i use 2 gang pot, i have doubts that it wont track properly,

i checked 2 deck 48 position Elma switches, but it seems like they are very $ hungry!
also, i dont really like stepped control, its always a less or more! for 250$ i rather have something smooth!

is there any other way, or suggestions ?

Click to expand...

I know this won't be helpful for your tube-based design, but when I did my monitor controller I used the TI PGA2320 device because its match between left and right channels is better than anything I could do with a pot or switch.