[sdiy] LM13700 crossfade query

Tom Bugs admin at bugbrand.co.uk
Wed Jun 15 13:47:08 CEST 2011


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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