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

David G Dixon dixon at mail.ubc.ca
Wed Dec 12 03:53:54 CET 2018


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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