Ensemble Circuit Configuration Questions

[Paul R. Higgins]

Is it possible for someone on the list to post this 120 degree LFO circuit (from
the Korg Polysix)?  I'd love to see the schematic.

> The 2 LFOs with 120 degrees phase shift could be based around a single CD4069
> IC, as in the Polysix ensemble effect (thanks to Mike I. for telling me about
> this). The LFO's would control three 4-stage phase shift sections in 
> parallel. 

Has anyone tried using quadrature LFOs with phasers or analog delays?  Thomas 
Henry (of Midwest Analog Products) designed a couple of these years ago.  One of
his quadrature circuits is featured in Electronotes, and the other was published
in his book "Build a Better Music Synthesizer".  These were triangle wave 
generators with two 90 degree phase shifted outputs (sine and cosine).  I 
imagine that using the two quadrature outputs to drive the clock circuits to two
BBDs would lead to some pretty wild sounds.  (This might be a little too 
"electronic" sounding for an ensemble effect, but I bet it would be great for 
flanging, etc.).

I had no idea that the Dimension D was such a simple design--Roland really makes
it sound like there's all kinds of weird stuff going on inside.  The description
in their manual reads something like "a unique chorus effect without the audible
periodic sweep of ordinary chorus units".

> 2 BBD IC's, modulated by a single triangle oscillator, with the output of the
> oscillator inverted for one BBD channel:
> - Roland Dimension D

Would there be any advantages to mixing in the dry signal with the delayed ones?
It seems to me that you would get even more of an ensemble effect that way, with
more complex phase cancellations.  I imagine that sound quality would also 
improve; I've been building analog delays for years, and none of the ICs I've 
worked with (mostly the EG&G/Reticon BBDs) had superb fidelity even with short 
delays.

> Although the Eminent patent describes mixing the original signal with the
> output, the string ensemble circuits I have seen only mix together the
> delayed signals.

I would also be interested in people's experiences with using random 
low-frequency ("red") noise or S/H noise as a modulation source for the BBD 
clock.  Someone also mentioned using LF noise to modulate a VCLFO as a 
"humanizing" feature, which also sounds like a neat idea.

Using time delay techniques would, I think, give a less electronic "swooshy" 
kind of effect, and a more convincing ensemble quality.  I'm sure that many list
members have heard this before, vis a vis the phaser vs. flanger debate.  (Uh 
oh, I feel some flames coming on...).  Of course, using VC allpass stages based 
around an OTA is no doubt much simpler and cheaper.

> Each of the phase shift sections could be based around 2 LM13600/13700 IC's
> (Tom G. has an example of this on his website), with exponential modulation
> toget a nice vibrato sound. Mix the outputs of the phasers together, with no
> straight signal, and you might get a nice ensemble sound! Certainly cheaper
> than using 3 BBD chips + the associated clock circuits.
 
- PRH

_____________________________________________
Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
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