[sdiy] Synthesizers...

Wed Feb 18 19:10:24 CET 2009

I hear ya, Nicolas!  However, in this case, I wanted the multiplier gain to
be exactly proportional to the pot setting (which was sourcing an LFO output
for the I_abc inputs of my OTAs), and the parallel Thevenin impedance I was
getting by feeding the attenuation pot wiper directly to an inverter was
messing me up big time, so I put a follower between the pot and the inverter
and voila!, perfect gain linearity (as expected).  (I had no other use for
that last 1/4 of a TL074 anyway!)

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada

Tel 1-604-822-3679
Fax 1-604-822-3619

"PERFECTA FINGAMUS SERVIAT NATURA"

The information in this email and in any attachments is confidential and
intended solely for the attention and use of the named addressee(s).  It
must not be disclosed to any person without the writer's authority.  If you
are not the intended recipient, or a person responsible for delivering it to
the intended recipient, you are not authorized to and must not disclose,
copy, distribute, or retain this message or any part of it.

> -----Original Message-----
> From: nicolas [mailto:nicolas3141 at yahoo.com.au]
> Sent: Wednesday, February 18, 2009 3:02 AM
> To: 'synth DIY'; David G. Dixon
> Subject: RE: [sdiy] Synthesizers...
> 
> Sounds great David.  Hope you'll share your designs and an mp3 or two once
> they are up and running.
> 
> One thing with the linearisation of your attenuators is that often the
> non-linearity is desirable.  If your signal source has an impedance of 10K
> or lower and it is feeding a 100K linear pot as your attenuator, which is
> driving into an amplifier with say a 22K input impedance - then the
> attenuation becomes pseudo-logarithmic so that you can calibrate it in dB
> or similar.  Can be very useful.  See
> http://www.geofex.com/article_folders/potsecrets/potscret.htm
> 
> Cheers,
> Nicolas
> 
> 
> --- On Wed, 18/2/09, David G. Dixon <dixon at interchange.ubc.ca> wrote:
> 
> From: David G. Dixon <dixon at interchange.ubc.ca>
> Subject: RE: [sdiy] Synthesizers...
> To: "'Tom Wiltshire'" <tom at electricdruid.net>, "'synth DIY'" <synth-
> diy at dropmix.xs4all.nl>
> Received: Wednesday, 18 February, 2009, 8:48 PM
> 
> Synthesizers, eh? ...
> 
> Well, I've just finished testing my very first circuit design, a
> triangle-core VCO with log and linear FM, pulse with PWM, saw and sine.
> I'm
> very happy with its performance, and I did it all with just three chips:
> one
> LM13700 and two TL074s.  The triangle is perfect (thanks, Thomas Henry!),
> the sine has as little as 1.4% THD (according to simulations; it sounds
> pretty nice, too!  Thanks, Ray Wilson!), there is only a barely
> perceptible
> glitch (on the scope) in the middle of the ramp (which may disappear
> entirely when I move the circuit off of the breadboard), and the pulse is
> pretty square!  Tomorrow I'll lay out the pc board for home etching.  I
> plan
> to build at least two of them.
> 
> I thought of one little innovation today which is very simple: I put in a
> switch to select between the triangle and the ramp for the input to the
> pulse wave comparator.  When the resulting pulses are mixed with the sines
> or triangles from the same VCO, they morph them in different ways.  I'm
> not
> sure if the results really sound very different, but it is a very simple
> thing to do, so I'll probably keep it.  Has anyone else tried this?
> 
> I've also completed the design and simulation of a dual-OTA-based
> four-quadrant multiplier which is actually pretty linear (unlike the
> useless
> single-OTA design in the LM13700 datasheet which is impossible to bias
> properly).  The carrier signal is fed to the opposite inputs of both
> halves
> of an LM13700, and one complete TL074 is used to buffer and invert the
> modulating signal and sum the currents from the OTAs.  With the twist of a
> pot knob, it can be an inverting or non-inverting two-quadrant multiplier
> (VCA) at the ends, or a four-quadrant multiplier (ring-mod) in the middle,
> all with nearly unity gain.  I've laid it out on a breadboard, but haven't
> tested it yet, because I don't have an LFO to use with it.  I'll throw
> something simple together tomorrow and give it a whirl with my VCO.  This
> was all inspired by trying to figure out how to put a Grenader gizmotron
> (i.e., -1 to +1 adjustable gain opamp) under voltage control.
> 
> One detail which has come out of this is the importance of putting buffers
> after attenuators in order to maintain a linear gain response with respect
> to the attenuating pot setting when feeding a modulating signal into an
> inverter or other device with finite input impedance.  It is probably
> worth
> a few extra opamps in any design to ensure this linear response.
> 
> Finally, I've also been educating myself on the finer points of filter
> design (transfer functions, Laplace transforms, zeros and poles, etc) in
> preparation for designing my own VCF later this month.
> 
> If it sounds as if I'm reinventing the wheel, well, I guess I am.  But,
> it's
> an awfully good way to learn electronics!  I highly recommend it!
> 
> David G. Dixon
> Professor
> Department of Materials Engineering
> University of British Columbia
> 309-6350 Stores Road
> Vancouver, B.C. V6T 1Z4
> Canada
> 
> Tel 1-604-822-3679
> Fax 1-604-822-3619
> 
> "PERFECTA FINGAMUS SERVIAT NATURA"
> 
> The information in this email and in any attachments is confidential and
> intended solely for the attention and use of the named addressee(s).  It
> must not be disclosed to any person without the writer's authority.  If
> you
> are not the intended recipient, or a person responsible for delivering it
> to
> the intended recipient, you are not authorized to and must not disclose,
> copy, distribute, or retain this message or any part of it.
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> > bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
> > Sent: Tuesday, February 17, 2009 10:19 AM
> > To: synth DIY
> > Subject: [sdiy] Synthesizers...
> >
> > ...ones we built ourselves. Anyone fancy a chat about that?
> >
> > Surely someone's done something interesting.
> >
> > Myself, I'm still playing with the DCO design that I posted a problem
> > with the other day. Now it's going properly, I've been sorting out
> > the amplitude calibration, and looking at the way that the Rhodes
> > Chroma uses a ramp plus a PWM pulse wave to mimic the effect of two
> > detuned sawteeth. Apparently these two are equivalent, but I'm yet to
> > prove it to myself to my satisfaction.
> >
> > T.
> >
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> 
> 
>       Make Yahoo!7 your homepage and win a trip to the Quiksilver Pro.
> Find out more