[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

Rutger Vlek rutgervlek at gmail.com
Fri Dec 14 14:35:47 CET 2018


| It just shouldn't be this hard.

Ha, I totally agree! I once explored an approach to ring-modulation using a
single VCA and an inverting switch. It seemed like a nice, cost-effective,
approach (especially since I wanted to use a discrete VCA that would be too
bulky for another approach). After two prototypes of this circuit, I simply
gave up. In the analog domain it was simply too difficult to trigger the
switch exactly at the point where the VCA was fully closed, due to circuit
accuracy and noise. It was a hard lesson in the reality of analog
electronics. I went down the route of simply duplicating the discrete
circuit for the negative quadrants, and it works extremely well albeit not
super-linear, but that was a deliberate choice.

Regards,

Rutger

Op wo 12 dec. 2018 om 03:56 schreef David G Dixon <dixon at mail.ubc.ca>:

> It just shouldn't be this hard.
>
>
> ------------------------------
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *
> Roman
> *Sent:* Tuesday, December 11, 2018 1:20 PM
> *To:* synth-diy at synth-diy.org
> *Subject:* Re: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)
>
> I know it's too late, as the circuit at this stage has already gone
> slightly different path, and I already gave up, but couldn't help drawing
> this little schemo that explains my idea of using single switches with that
> obsoleted chip. Switches should be DG444 or similar (0=closed, 1=open), or
> replace diodes with NAND gates and use any regular dirt cheap switch like
> 4066.
> There's no switching happening while VCA is open, provided that VCA
> control triangles match the edges of LM3914 stages. The only switched
> channels are the ones routed to muted VCA, so in theory there should be no
> clicking audible.
> Basicaly this is just the circuit that makes this input switching sequence:
> VCA-A:12233445566..
> VCA-B:11223344556...
> by creating one step overlap between stages. Only 6 inputs in this
> example, but can be extended to any other number.
> http://www.synthdiy.eu/files/scanner.png
>
> IMHO switching active channel will always produce some clicking for many
> reasons: not matched channels, difference between transistion times H->L
> and L->H as Ingo said, break-before-make feature of DG407, slow response of
> 4532, and who knows if not also charge injection.
>
> Roman
>
> Dnia 11 grudnia 2018 18:24 Ingo Debus <igg.debus at gmail.com> napisał(a):
>
> Am 11.12.2018 um 08:10 schrieb David G Dixon <dixon at mail.ubc.ca>:
>
> There is now
> only one tiny problem: There is still an almost imperceptible click when
> the
> CV crosses 0V (and the logic control voltage crosses 2.5V) in the positive
> direction.  This is when the logic switches from 011 to 100 (i.e., all
> three
> bits change).  Interestingly, I don't hear the click at all when the CV
> goes
> in the other direction (100 to 011).
>
>
> Couldn’t this still be caused by a very brief „forbidden state“ during the
> transition from 011 to 100? In your case, the forbidden state would be 000
> (MSBit changes slower that the other two) or 111 (MSBit changes faster).
> Probably just a difference between rising and falling slope. Can you check
> with a scope?
>
> Ingo
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