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

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Wed Dec 12 12:56:26 CET 2018

Agree with Tom and Roman.

It's not a case of "...if the switch generates a click".  If it's 
switching instantaneously between two arbitrary waveforms it's almost 
guaranteed to click, even if it switches cleanly without any rogue 
intermediate states!


On 2018-12-12 11:41, Tom Wiltshire wrote:
> 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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