[sdiy] LM13700 crossfade query

Tom Bugs admin at bugbrand.co.uk
Wed Jun 15 14:52:38 CEST 2011

Thanks Harry and Dave - yes, I'd looked at the EFM VCA6B - interesting 
for the inverted control signals, but the VCA outputs do not sum in the 
same way as I'm trying..

Cheers, Tom

On 15/06/2011 13:17, Harry Bissell wrote:
> 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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