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

Tom Wiltshire tom at electricdruid.net
Wed Dec 12 12:41:22 CET 2018


Nine VCAs and DAC channels seems like too much hardware.

There’s nothing wrong with the switches, but I’d do it with two switches to select the inputs to two VCAs controlled by two DACs, and then a uP to control the switches and DACs. You can easily make sure that the VCA is shut off when the multiplexer switches, so there shouldn’t be any clicks even if the switch generates one.

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 12 Dec 2018, at 10:02, john slee <indigoid at oldcorollas.org> wrote:
> 
> I suspect a more-digital solution would be easier to get right, for people more inclined toward such things, just more expensive
> 
> 9 VCAs, 9 DACs to control the VCAs, and a microcontroller to control the DACs.
> 
> No need for switching at all, if I understand your intent correctly?
> 
> I readily acknowledge that digital isn't for everyone, though.
> 
> John
> 
> On Wed, 12 Dec 2018 at 13:57, David G Dixon <dixon at mail.ubc.ca> wrote:
> 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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