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

Roman Sowa modular at go2.pl
Wed Dec 12 10:04:36 CET 2018

I'm glad I could help.

Yes, it must work in dot mode. One way of making dot mode out of 
comparator chain is this:
with different wiring there's 1 of 8 dot mode possible or the one that 
does what's needed for crossfader based scanner - 2 of 8 dot mode.


W dniu 2018-12-12 o 03:53, David G Dixon pisze:
> Roman,
> You're right.  Your suggestion is superior.  I was simply unaware of 
> what a complete shit-show all this digital circuitry is. This is exactly 
> the reason why I avoid digital solutions whenever possible.  This module 
> should be a pretty straightforward application of an analog multiplexer, 
> but you are basically saying (and I concur based on my experience) that 
> I can't use such a thing for clean audio, and to me, that is a 
> sad indictment of these ICs.  I'm now switching these DG408s about as 
> cleanly as is humanly possible, and they're still misbehaving.  It just 
> shouldn't be this hard.
> So, I'm going to try to implement your idea, but I don't have the ICs I 
> need.  I'm also still trying to understand exactly how the switching 
> works.  I'm presuming that the 3914 must be in Dot mode, because only 
> one comparator can be on at a time for this to work, and therefore, this 
> circuit ONLY works with 3914, and cannot be implemented with a generic 
> flash comparator chain.
> So, I'm going to finish off the circuit as it is, because it works 
> pretty well, but I concede that I basically cannot use these fucked up 
> digital chips to switch active audio circuits, and will build the next 
> one your way.
> Cheers,
> Dave
> ------------------------------------------------------------------------
> *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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