[sdiy] 1-quadrant multiplier with 2164

Richie Burnett rburnett at richieburnett.co.uk
Thu Jan 20 19:11:33 CET 2022


Pitch shifter! ;-)

-Richie,

Sent from my Xperia SP on O2

---- cheater cheater via Synth-diy wrote ----

>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
>> > >
>> > > _______________________________________________
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