[sdiy] VCA crossfade/pan idea

[Tom Wiltshire]

Thanks Michael, that's exactly what I needed to know.

Yes, the Thomas Henry circuit looks pretty close to what I've got on the breadboard, although he's put a few more mixer op-amps on it than I have at present.

I must have something wrong or a bad component somewhere.

Tom

On 27 Jun 2016, at 05:03, Michael Barton <Michael at Bartonmusicalcircuits.com> wrote:

> Hi Tom,
> 
> This looks very close to the topography of Thomas Henry's crossfading VCA design: http://electro-music.com/forum/topic-54633.html
> 
> So it would appear you have a mistake or need to tweak more, but the idea is proven to work as I've built TH's design for myself.
> 
> -Michael
> 
> 
> 
> On 6/26/2016 1:21 PM, Tom Wiltshire wrote:
>> Hi all,
>> 
>> In the SSM2045 datasheet is the following snippet:
>> 
>> http://electricdruid.net/images/Crossfade.png
>> 
>> This uses a differential pair to provide currents to two VCAs on the chip. Since the total current  depends on the VL input (at the top) this can be used for envelope control of the amplitude. The VOF input (on the right) sets the differential voltage between the two bases, and thereby changes the balance of currents between the two VCAs. This gives a crossfade or panning function.
>> 
>> How difficult would it be to modify this to work with a LM13700?
>> 
>> It seems pretty straightforward to me, but I'm messing with things I don't understand. The overall current needs altering (from <500uA for SSM2045 to <1mA for the 13700) and VOF would need tweaking for whatever PNP transistors were used. But apart from that, should it work as drawn?
>> 
>> I'm asking because I breadboarded it and it didn't. I'll like to know if that's because I've got a mistake, or whether the whole idea is bad.
>> 
>> Thanks,
>> Tom
>> 
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