[sdiy] CV inputs with bipolar "amount" knobs

[Harry Bissell]

Advantages: the 'null' point depends on the quality of the pot alone. In theory you could
get an 'infinite' null (zero).  The differential solution can only be as good as the opamp...
probably 80db max. Could be worse depending on resistor matching (the 80db usually assumes
.1% resistors... can you set the pot that well ?)

The dual opamp allows the use of a center tap pot, which could be grounded... making the null
a LOT more precise by design.

Disadvantages:

Needs two opamps and a reasonably high impedance for the next stage (10X higher than the
pot value) ~or~ another buffer opamp

I usually use the differential solution...

H^) harry


----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: Harry Bissell <harrybissell at wowway.com>
Cc: bbob <rdrake at data2action.com>, Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 19 Jul 2010 12:03:41 -0400 (EDT)
Subject: Re: [sdiy] CV inputs with bipolar "amount" knobs

Thanks Harry, that's what I thought. It felt right to me that the two halves of the differential amp should have the same values.

Interesting point about the impedance.

Finally, the other way to do this is too connect the signal and the inverted signal
on the end terminals of a pot, and take the adjusted signal from the
wiper (as Simon suggested). Is there any advantage or disadvantage to this technique as opposed to the differential amp method?

Thanks,
Tom


On 19 Jul 2010, at 16:08, Harry Bissell wrote:

> You can use just about any values, the two resistors in the inverting path must be the same value, and the
> pot can be anything.
> 
> Higher values of POT will give higher input impedance, which is NOT constant btw. It changes with the pot setting.
> 
> I'd use two 50K in the inverting path, and a 100k pot so that at center rotation, the circuit
> looks exactly like a proper differential amp with the inputs tied to the same point. This would be best
> for input offset current etc... (best matching)
> 
> H^) harry
> 
> 
> 
> ----- Original Message -----
> From: Tom Wiltshire <tom at electricdruid.net>
> To: bbob <rdrake at data2action.com>
> Cc: Synth DIY <synth-diy at dropmix.xs4all.nl>
> Sent: Mon, 19 Jul 2010 10:46:38 -0400 (EDT)
> Subject: Re: [sdiy] CV inputs with bipolar "amount" knobs
> 
> Thanks everyone.
> 
> The Fonik design and the EFM VCA5B design are basically the same; a differential amp fed with one signal. One thing I don't understand though - they used 47K/47K for the inverting feedback, but a 50K pot for the non-inverting feedback (essentially a 25K/25K split). Just for the symmetry of it, I'd have wanted to use a 100K pot here. Can anyone think of a reason why I shouldn't?
> 
> Thanks again,
> Tom
> 
> On 19 Jul 2010, at 15:00, bbob wrote:
> 
>> i think, check out the first section of fonik's dual voltage processor:  http://modular.fonik.de/pdf/dual%20voltage%20processor.pdf
>> 
>> On Mon, Jul 19, 2010 at 9:24 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
>> Hi All,
>> 
>> I know various people have built various devices with CV inputs that have bipolar "amount" knobs so that you can easily invert a CV input - particularly useful for envelopes and such like.
>> 
>> How many different ways of doing this are there? Can anyone provide a reference to a schematic online?
>> 
>> I just had a big hunt around thinking I'd find loads, but mostly they must be parts of other circuits 'cos I couldn't find any.
>> 
>> Any pointers appreciated.
>> 
>> Thanks,
>> Tom
>> 
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> -- 
> Harry Bissell & Nora Abdullah 4eva


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Harry Bissell & Nora Abdullah 4eva