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LFO with delay and speed-up the Roland SH-3A way
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Ricko



Joined: Dec 25, 2007
Posts: 251
Location: Sydney, Australia
Audio files: 27

PostPosted: Mon Sep 14, 2015 8:25 am    Post subject: LFO with delay and speed-up the Roland SH-3A way Reply with quote  Mark this post and the followings unread

Roland's SH-3A has the most amazing feature set: 4 LFOs, noise, sample and hold, ADSR+ 2 preset envelopes, five mixable sub octaves with 3 waveshapes, PWM, a Moog-style ladder filter, and solid tuning over 9 or so 8ves (the 3.5 of the keyboard, up or down 1 with the transpose, plus the 4 sub 8ves.)

But it gets no respect.

I think one of the reasons it has not had much DIY attention is that the main schematics on the WWW are missing many sheets, and so you only get parts of each circuit.

Anyway, I have redrawn the LFO2 with modern components, run it through TI-TINA (SPICE) simulation, and here are the results.

You take a very vanilla triangle core LFO, and run it through a diode waveshaper for a nicer sine. Next comes the unusual part, which I have drawn at the bottom centre and right.

The input is a trigger (or gate if you want the effect only on finger off) and you get a delay which does two things: first it slows down the LFO to almost halting, second it pulls the value of the LFO to 0V. The effect is that the first few cycles of the delay, as it comes to full level, are slower. This gives a better "singing" tone to vibratoes, slight more of a Leslie effect, and adds more interest to FM/AM/FilterFM effects.


(The output levels are quite low: I have left off a final buffer/amplifier to bring the levels up to 10V P-P or whatever you want. VG1 on the diagram is supplying the positive gate/trigger signal. )


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