[sdiy] 1-quadrant multiplier with 2164
sines_list at scale.la
Thu Jan 20 19:32:05 CET 2022
I’m not very dialed in with the math, but I would say that Harald’s analogue design is the one to beat, and Don’s was more of a quasi-kludged modular building block that had mixed results. The math was right but the sound was a bit different, from what I can tell.
The 185 is basically a dual mixer that combines 2 phase shifters and a ring modulator that are hard wired as a hopped up ring modulator.
Daniel (LA67) himself mentions that the Bode / Haible design is an “easier, flexible, and more modern" route to go.
As you can see even a smaller clone can be quite costly.
The 285 rev 2, all analogue, does all of the above but puts all of the above components accessible on the panel to utilize the sections on their own.
The 285 rev 1 design, based on a Spin FV1 DSP IC is “reasonable” according to Dave Brown
"While not as good as the original all-analog frequency shifter, the performance is reasonable and the sound qualities are quite nice."
Haible’s design is still available, somehow, for sale at Random Source.
> On Jan 20, 2022, at 1:02 29PM, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote:
> I suppose that anything is possible, cheater. However, not really with a
> Bode frequency shifter, which is what the Freak Shift is.
> In case y'all didn't know, the Bode frequency shifter is really just a
> trigonometric engine. It realizes the so-called angle sum and difference
> identities, and this gives the frequency shifting. It will shift the
> frequencies accurately over whatever frequency range that the Dome filters
> give accurate 90-degree phase shift, and will give inaccurate shifting
> outside of that range.
> -----Original Message-----
> From: cheater cheater [mailto:cheater00social at gmail.com]
> Sent: Thursday, January 20, 2022 7:25 AM
> To: David G Dixon
> Cc: Neil Johnson; synth-diy
> Subject: Re: [sdiy] 1-quadrant multiplier with 2164
> [CAUTION: Non-UBC Email]
> 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
>> 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
>> 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.
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