[sdiy] Center Detent dead zone on an "attenuverter"?

[Chris McDowell]

Thanks for the reply and input, Brian. A separate button to invert works great, and I do that with some 20mm sliders for the same reasons you listed. Thanks anyway! Just wondering if I'm overlooking something here that would make this simple, and tiny, design work a bit better.

And Tom, back to back diodes wouldn't be great as they'd distort anything at non-zero gains. If I were to use Schottky diodes, for instance, there would be a ~0.4V dead spot in the middle of whatever LFO or modulation source was fed to that channel, or a ~0.2V dead spot at the bottom of unipolar sources like a sequencer or envelope. yeah that would be pretty annoying for an envelope, I think, to cut off the tail like that. 

Now that you challenged it, I'm playing around a bit on LTSpice and it's suggesting that the distortion wouldn't be crazy significant, though of course it would vary a bit with real diodes. Worth trying in real life, I suppose.

Thinking on it more, it seems unrealistic to think I'd be able to kill tiny signals when the pot is (inconsistently) centered, but leave that range unaffected otherwise. I could do something silly like use a dual ganged pot, use one pot as it is now for one channel, use the other pot as a voltage divider fed into a window comparator narrowly around 0V, and use that signal to mute that channel. Bleck! Even typing it is lame. 

Carry on, then.
Chris

> On Apr 2, 2016, at 5:37 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
> Please explain a little more why back-to-bcak diodes don't solve this problem. I'm not seeing it currently.
> 
> Thanks,
> Tom
> 
> On 2 Apr 2016, at 21:50, Chris McDowell <declareupdate at gmail.com> wrote:
> 
>> Howdy list, 
>> 
>> I have a very simple 4 channel mixer that has "attenuverters" (gain from -1 to +1) on each input. As it stands, the "off" position is super narrow and not exactly in the vertical position of the pot. Both of those were expected, and it's fine for anything that doesn't need to be silenced. Using a pot with a center detent leaves tiny bit of gain one way or the other, since the pot and summer resistors aren't exact. So, I wonder, is there a reasonable method of adding a dead zone to the center of an attenuverter like this, without trimmers? 
>> 
>> I can link to a pic if needed, but the circuit is straightforward and many of you are likely familiar: Input goes through a 100k to the wiper of a pot, two inverting amplifiers are in series, with the one leg of the pot going to the first inverting amp, and the other leg going to the second inverting amp. 
>> 
>> Initially I thought to myself "yeah easy, back to back diodes like on a pitch bend wheel" but about 1 second later realized that it's not at all the same situation. Back to back diodes on the output would have the desired effect when the pot was centered, but would not with any gain applied. 
>> 
>> Any pointers appreciated! Thanks,
>> Chris
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