Ideas for Full Volume Balance/Blend Control

[sam21]

Hi guys,

I've got an idea I've been working on, but I've come to a bit of a stall with one section.

I want to utilise a blend control for two separate signals, however, I want it to work so that when the pot is set to half way, both signals are at 100%, and then either side of half way fades either side back down. So essentially 0-50% of the pot means that signal 1 is at 100% and signal 2 is faded from 0-100%, and vice versa when the pot is from 50-100%.

Anyone got any ideas on how to do this with analog circuitry?

[ashcat_lt]

There is probably some active circuitry way of making that happen with more standard pots, but the "easy" way is to make sure you get real Blend (Not Pan) Pots.  They have been notoriously difficult to find at reasonable prices, but as mentioned in that thread, I think found some at Mouser not too long ago:
500K Knurled Shaft
250K Plain Shaft
500K Plain Shaft

[sam21]

Hmm that's not such a silly idea actually. I really want to see if this is possible to do using active circuitry, it would allow for far greater flexibility.

[ashcat_lt]

Edit - NOPE!!!  I was completely wrong and hope nobody saw this post before the edit.  

[nsteen]

Even with a dual ganged potentiometer I don't really see how you'd bypass half the travel of the potentiometer (I don't know if bypass is the correct term so please forgive my noobishness)

Maybe with a logarithmic taper you could wire them in such a way where there was proportionally much less resistance than you'd see with a linear pot but it still wouldn't be straight through like what you're envisioning.

I'll try and come back to it tomorrow because it is an interesting idea though and I am tired at the moment.

[slacker]

You could hack a pot by adding a tap at the half way point on the track and shorting it out to one of the outside lugs. This will give a pot where for half the travel the resistance doesn't change and the other half acts as normal. Do this the opposite way round on each side of a dual pot and you've got what you want. Google will probably tell you how to add a tap.
Shouldn't be too hard to make a circuit that would generate the correct control voltages from a single pot to drive an active mixer/panner circuit. I'll have a think about it.

[Brisance]

wouldnt that make the volume like a valley and then a dropoff?

[slacker]

Ah yeah you're right, in my head adding a tap was the same as shorting out half the track but of course it's not. Should stop reading the forum before I've had my morning coffee.
What you neef is something conductive you can coat the track with, wonder if something suitable exists?

[Brisance]

Me too, my first thought was "hey, why didn't  I think of that" but then I was like "wait a minute"

[ashcat_lt]

Assuming you don't just buy the pots I linked, why wouldn't the tap work?(nm...got me too!   )

You certainly probably could work out an active solution.  Like a VCA that goes from -infinity > unity with control voltage from 0 > 5V, put 10V through the pot, and then clip it off at 5V via diode or whatever.

Not sure why we wouldn't just order the pots that do the thing we want to do, thouigh...

Edit - BTW I've tried that silver conductive paint on the pot track and it kinda almost works.  I don't think you actually have to cover the track, just run a line up along touching the edge.  I made a mess and it was kind of suck, but it might work for somebody with a steadier hand and more patience.

[slacker]

Thinking about it some more I think a tap will work.

See the scribblings below, number 1 shows a pot with a tap at the midpoint shorted to one of the outer lugs. This can be redrawn to look like 2, which can be simplified to look like 3. So when the wiper is above the tap the bits of the pot between the wiper and the top of the pot and the tap look like a series resistor whose resistance changes depending on where the wiper is. If you drive this into a high impedance then the difference can be made small enough not to matter. Eg; a 10k pot would give a series resistance between 0 and 2.5k which against a 1Meg impedance is bugger all difference.
When the wiper goes below the tap the pot just acts like a pot of half the value of the total pot. So if you drive it from a low impedance source so there's no loading from the pot and drive it into a high impedance it should work. Unless I'm missing something else?  



The lower circuit gives control voltages that have the right response. Out 1 follows the input voltage up to VCC/2 and is then clamped at VCC/2. Out 2 is clamped at VCC/2 until the input voltage goes above that and then drops down to 0 as the input voltage goes up to the supply. This could be used to control a panner/mixer circuit something from R.G.s OTA article would probably do the job http://www.geofex.com/Article_Folders/VCA%20Applications.pdf.