My adventures with the LM13700

[Tony Allgood]

Sorry if you've seen this already


> Further to all the previous correspondance on the LM13700 VCF:
> 
> Many thanks to all who have offered their help on this matter. I
> ought to have said what I wanted this filter for. It isn't going to
go
> into my modular set up, but to act on any audio input and then put
back
> into the final mix. Now CV breakthrough is a major problem here, as
is
> SNR. You see theres no VCA to gate the VCF's output at all, so the
> filter sits there outputing to the main mix all the time. Now the
> LM13700 I have used before, and it is good, with or without the
diodes,
> but it appears not to be as quiet as my Moog ladder filter. The 
> breakthrough can be minimised by using addtional circuitry or
tweeking
> offsets, but at the end of the day I believe that the LM13700 cannot
> come up with the goods for this particular application. Also, have
you
> listened to it without any audio input? Too much noise, and if the
VCF
> is being swept by the LFO then it swishes like some terrible sea-side
> sound effect. OK if you have a spare noise gate. The Moog does not do
> this. So I think I might have a play around with this topology again.
> 
> Does the patent still prevent commercial use of the ladder filter? I
> ask this only out of interest, because the Teisco S-100P synth used a
> direct copy and that was 15 years ago? 
> 
> Tony Allgood
> 
> Cumbria, UK
>  
> 'I may live in the middle of nowhere, but the views are nice'