[sdiy] 1-quadrant multiplier with 2164

cheater cheater cheater00social at gmail.com
Thu Jan 20 18:35:23 CET 2022


How would you do it?

On Thu, Jan 20, 2022 at 6:17 PM Mike Bryant <mbryant at futurehorizons.com> wrote:
>
> Yes :-)
>
> But infinitely easier if you go digital
>
> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of cheater cheater via Synth-diy
> Sent: 20 January 2022 15:25
> To: David G Dixon
> Cc: synth-diy
> Subject: Re: [sdiy] 1-quadrant multiplier with 2164
>
> I wonder if it's possible to build a frequency shifter that shifts higher harmonics more than lower harmonics.
>
> On Wed, Jan 19, 2022 at 6:48 PM David G Dixon <dixon at mail.ubc.ca> wrote:
> >
> > I must confess that I've lost the thread of this argument just a little bit.
> > However, what I like about my approach (which I have used many times
> > in many different contexts) is that, in order to build a nice linear
> > VCA from 2164, you really need to have a clean 5V source anyway.  I
> > keep a pile of LM336Z5 for just this purpose, and use two opamps to
> > buffer and invert this to get low-output-impedance +5V and -5V
> > references on all my multipliers.  If one uses precisely matched
> > resistors on the inverter, then one can get those references within a
> > mV of each other -- the actual voltage doesn't matter (and it is
> > usually around 4.90V), but as long as they are equal and opposite,
> > then they can be used for precise multiplication.  This is one of the keys to the precision of my Freak Shift frequency shifter circuit.
> >
> > I don't really understand how adding a stable DC value to a signal
> > increases the noise of that signal.  I must confess that I also don't
> > care at all about it.  My method is the simplest.  You don't have to
> > pre-condition the incoming signals at all.  The CV signal is
> > unchanged, and the DC reference levels are simply summed to the incoming signal.
> >
> > If you want to change the actual levels, you can simply change the
> > resistor values.  I do this all the time.  One of the keys to my
> > one-VCA four-quadrant-multiplier circuit (of which there are two in
> > the Freak Shift, made from a single 2164 chip) is to lift and diminish
> > the CV such that the zero point of the multiplier is at +5V and full
> > +/- unity-gain multiplication occurs between +2.5V and +7.5V.  This
> > gives lots of headroom
> > -- it essentially makes it impossible for the CV in the multiplier to
> > hit zero at the 2164 control pin (because the incoming CV signal will
> > never be anywhere near 20Vpp), which would give a dead zone on the
> > multiplication.  I achieve this simply by bringing the CV in through
> > 200k while using 100k on the reference voltages.  Of course, the
> > signal is now cut in half as well, so I simply double the feedback
> > resistor on the I-V converter.  As long as all of these 100k and 200k
> > resistors are within 0.1% of each other (and the 100k and 200k
> > resistors don't need to be in a precise ratio -- they only need to be
> > precise within their own values), and all incoming signals are AC
> > coupled through big back-to-back electrolytics, then the four-quadrant multiplication is very tight, which is important for frequency shifting.
> >
> >
> >
> > -----Original Message-----
> > From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
> > cheater cheater via Synth-diy
> > Sent: Wednesday, January 19, 2022 4:23 AM
> > To: Neil Johnson
> > Cc: SDIY List
> > Subject: Re: [sdiy] 1-quadrant multiplier with 2164
> >
> > [CAUTION: Non-UBC Email]
> >
> > I wonder if it matters that Dave's version will create theoretically
> > more distortion on the positive swing of whatever vs the negative
> > swing, whereas my version will apply distortion (non-linearity) more
> > or less symmetrically... do the numbers show that it matters at all? I
> > bet it would matter with some, let's say, crappy devices.
> >
> > On Tue, Jan 18, 2022 at 1:57 PM Neil Johnson via Synth-diy
> > <synth-diy at synth-diy.org> wrote:
> > >
> > > > This is certainly true but note also the importance of zero when
> > multiplying. The zero signal stays zero no matter what you multiply
> > by. In Rutger's case that zero is in fact -5V, so the origin of Neil's
> > graph should be at -5V signal and zero control voltage. That is why
> > the level-shifting solution is so effective and it is also why I
> > believe Rutger is correct to call this a one quadrant multiplier.
> > >
> > > Yes, this is just a bit of algebraic juggling.
> > >
> > > If we take Dave's approach:
> > > - convert the bipolar +/- 5V input to a unipolar 0 to -10V input
> > > - add a -5V offset to the output _after_ the VCA (so no bearing on
> > > the quadrantiness of the VCA itself)
> > >
> > > With a unipolar CV and a unipolar signal ... a 1-quadrant VCA.
> > > And don't forget that as-drawn the linearised VCA is inverting.
> > >
> > > Cheers,
> > > Neil
> > >
> > > _______________________________________________
> > > Synth-diy mailing list
> > > Synth-diy at synth-diy.org
> > > http://synth-diy.org/mailman/listinfo/synth-diy
> > > Selling or trading? Use marketplace at synth-diy.org
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
> > Selling or trading? Use marketplace at synth-diy.org
> >
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
>



More information about the Synth-diy mailing list