Splitter/unBlenderer.... sort of

[djp8djp]

Hi all. I'm hoping there's some bright soul out there who has something up their sleeve for this one. My headbone is weary, and I'm not seeing a simple solution.

I have an input signal, going thru an opamp-based active filter. And now I'd like to route that output to one of two destinations. What's so tricky about that, right? Well, there are two wrinkles.

First wrinkle, I'd like to see if this can be done with one pot. 10k, 25k, 50k, whatever. And this pot should function so that at one end it sends all the signal to one output, and at the other end should send all the signal to the other output. So full CW is 100% "right", and full CCW is 100% "left". Or vice versa of course. As wrinkles go, really not so bad taken by itself.

The second wrinkle is the interesting one. I'd like the middle position of the pot to be "neutral". It'd sort of equate to a large "negative gain" for both "left" and "right" sides. Full attentuation. Nil signal out. Choke. A rose is a rose by any name, right?

I guess this is kind of like the inverse of what a passive blend pot would do. Or if you will, I want a pan control with a hole in the middle. So the pot at center position makes for zero-"left" and zero-"right". And as the pot is rotated either way, an increasing amount of the filter output gets routed to one side. The opposite side should still get nothing. I'd be happy with a unity gain at full CW or CCW rotation, but a little boost might be nice. I'm sure I can get those ends sorted myself... it's the hole in the middle that's got me confuddled.   

[Eddododo]

Blend pot wired backwards with the less resistive side tied to ground is a good start

[Eddododo]

a couple of thoughts regarding using a 'reversed' blend pot.

the signal  into a 250k MN (blend pot) at center would have about 45k to ground and 200 k to the hot side... adding series resistance of 100k between the signal and the pot (center lug) would leave about 1/3 of the signal at the output.. perhaps that's a starting thought for you


what you could do is SHORT the blend pots less resistive taper side. you could open the pot, and use conductive paint from the center to ground, such that instead of the center having about 45k resistance to ground, you would instead have a short to ground.

[djp8djp]

Hmmm. Yeah, I was thinking of a blend pot, as you probably saw above. The reversal would work. But I'm not crazy about the leftover resistance, the whole point being to choke the "off" side to zero. The conductive paint could work, but it's a PITA and can wear out. Maybe I can get some better-quality ones. Any suggestions as to sources for those?

Otherwise I was considering some form of a symmetrical gain stage, but couldn't see it working without 2 more opamps.... and I already have plenty in the MFB filter. Anybody have anything about that, or otherwise beyond the reversed blend pot?

[PRR]

At first glance:

1) place a Custom order for a dual special-taper pot. Expect to pay for 10,000 units even if you only need one.

2) Use two VCAs and some analog computation.

3) find a Center Tap pot. Fender used some in a bias network. It was a bad idea for bias and perhaps for audio. You could fake a center-tap with silver paint but it is not easy to make a good tap.

There may be some clevererer way to get the above graph with a single or dual stock pot. But I don't see it, and I've bent quite a few pots.

A dual-lin pot with Eddododo's silver paint on the wafers is probably the best way to get this done and get on with life.

[Eddododo]

I found some stuff at radio shack, some sort of circuit paint. I will play with it a little and see if I'd recommend it at first glance

[lvs]

Possibly (<- never tried or measured one myself) a pot like the following one works, connected as dual voltage divider and like Eddododo said, with the less resistive sides (in this case, even non-resistive) to ground. So at center detent, it should mute both wiper outputs.

http://www.wdmusic.com/media/bourns_app_notes/bourns_PDB182_note_guitar.pdf

I remember seeing this kind of pot working as a pickups blender in a Bartolini diagram.

[PRR]

Yup.... that's the odd-part you need, at a fine price. Just wire it backward to how they blend two pups to one output.

BOURNS BLEND POT MINI WITH CENTER DETENT METRIC KNURLED SHAFT 250K AUDIO TAPER
SKU: BOURNS 250BLEND
http://www.wdmusic.com/bourns_mini_blend_center_detent_metric_250k_bourns_250blend.html