[sdiy] LM13700 crossfade query

Harry Bissell harrybissell at wowway.com
Wed Jun 15 14:17:54 CEST 2011

There was an EFM (electronics for music - Tom Gamble) board that did exactly that.
Itmight have been the VCA6, iirc. If you check the web there is a site (hosted by
someone on this list) that has all the old stuff. I have built this board and it
does work as a crossfader. Whether the 'law' is correct for smooth sounding panning
of audio signals, I don't know...

I've done similar crossfaders as well...

H^) harry

----- Original Message -----
From: Tom Bugs <admin at bugbrand.co.uk>
To: Synth-DIY DIY <Synth-diy at dropmix.xs4all.nl>
Sent: Wed, 15 Jun 2011 07:59:04 -0400 (EDT)
Subject: Re: [sdiy] LM13700 crossfade query

Haha -- it seems that whenever I post something to SDIY it somehow 
clarifies my mind..

I realised I could easily test the theory out with very minimal mods to 
my old UTL5 Fade/Pan module.

It seems to work very well.

It'd still be great to hear from people whether what I'm doing is good 
or bad though.

Cheers, Tom

On 15/06/2011 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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