[sdiy] LM13700 crossfade query

Tom Wiltshire tom at electricdruid.net
Wed Jun 15 15:26:13 CEST 2011

There's another similar example in the SSM2164 datasheet - different VCA, same principle. There's a "voltage-controlled Mixer" circuit which sums the outputs from four VCAs into a single I-to-V convertor like this. So it definitely works.

I've done it myself using the SSM2164, and it's fine, just like they say.


On 15 Jun 2011, at 12:47, Tom Bugs wrote:

> Hi All,
> I've got a question on summing together two LM13700 stages -- I can't clarify if I'm being clever or dumb!!
> (I know, I should probably just test it out...)
> Here's a little sketch: http://bugbrand.co.uk/images/LM13700_query.jpg
> (just showing the skeleton..)
> The idea is to have two VCAs with inverse control signals so as one comes up, the other goes down. (I've done this part before and it works well) but I'm stumbling on whether I can seriously simplify the summing stages at the end.
> I'm using external TL07X buffer stages rather than the transistor pairs on the LM13700 -- typical simple VCA solution is achieved as drawn but leaving off the 2nd LM13700 stage (ie output connection from pin 12 does not join the net from LM13700 pin5 to TL07X pin2)
> What I'm wondering is whether the summing sketch with the outputs summed as drawn is a good way or a total idiotic no-no. In which case I'd have to do two TL07X buffer stages (one for each half of the lm13700) and then have another summing stage.
> I'm sure there was a project that did similar but I can't track anything down now. The nearest I've found is the Crumar BIT1 which sums the pins of multiple LM13700 outputs together and then buffers in a similar way to my drawing but with one slight difference - there's now an input resistor before the TL07X (ie. after summing the LM13700 outputs).
> Thanks for any pointers here!
> Tom
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