Doepfer/ Serge A-171-2 VCS Mod Schematics

[Navs]

Finally got round to writing up my Burst, Hold and EOR modification for the Doepfer/ Serge A-171-2 VCS:

http://navsmodularlab.blogspot.de/2015/ ... matic.html

I've created a thread here because I'd welcome any corrections or improvements to the circuit.

Hope it's of use and look forward to your thoughts!

Cheers,

Navs

[Joonatin]

Lovely! I was going to make a small extra panel for my quadslope (dusg) with switches for trig out -> trig in and attenuated outs but this would definitelt be useful and easily added to the same panel too. Thanks for sharing!

[Navs]

No probs, it's nice to have finally documented it.

The hold function can be fun at both audio (waveshaping) and low ('sequences') rates.

[maltemark]

Wow, I haven't really begun DIY yet, but would love to do this to my VCS:s - they are mainstays of my system. Thanks!

[negate]

Navs, shouldn't the inverted EOC (IC1 pin 10) go to switch logic IN2 pin 16, not the double inverted EOC as in your schematic?
As you wrote: "As the EOC gate is ‘high’ at the wrong time, we want to flip its activity."

Cheers.

[oldenjon]

Navs, shouldn't the inverted EOC (IC1 pin 10) go to switch logic IN2 pin 16, not the double inverted EOC as in your schematic?
As you wrote: "As the EOC gate is ‘high’ at the wrong time, we want to flip its activity."

Cheers.

You're right, thanks for pointing that out.
The schematic should look like this I think:

*edit* schematic was wrong

[Navs]

Oop. Didn't get a notification for this, sorry

I'll check the module later to confirm.

[ReToxx]

I think the EOR function on the schematic shown above is wrong.
The DG201 logic says its normally open and closes with a high signal.
For EOR we want the "not attack" signal, which starts, when the envelope
enters its falling portion and stays there until the the envelope rises again i.e.
the envelope starts again. So we take that and use the EOC which happens to be high from the end of envelope until the next starts, to shorten the "not attack" signal closing the switch.
With the inverted EOC signal which would be always high when the "real" EOC is not, we would just get another EOC as EOR.
Did I get that right?

[oldenjon]

Threw this together real quick. It shows you the logic behind Nav's circuit.
The inverted EOC opens and closes the switch so that the NOT ATTACK signal is only high during the fall and low when the EOC goes high.

I got the same result by using a simple transistor NOR gate on the ATTACK and EOC signals.

[Navs]

Thanks for that, oldenjon!

It's been a while but I remember that figuring out the logic made my head ache a little at the time.

ReToxx, I can have another look with a scope at some stage if that graphic doesn't answer your question.

[ReToxx]

Ok, I made a graphic, too.
You can see, how the EOR would be like with EOC and –EOC doing the switching. The truth table of the DG201 says 0=ON, 1=OFF.
So for me this clearly looks like the regular EOC is the right signal to take.



switch ON / eoc LOW / not attack LOW --->
switch ON / eoc LOW / not attack HIGH --->
switch OFF / eoc HIGH / not attack HIGH --->
switch ON / eoc LOW / not attack LOW ---> etc.

this would be one cycle of the switch action, as I think.
But maybe I don't get something.

[oldenjon]

Ok, I made a graphic, too.
You can see, how the EOR would be like with EOC and –EOC doing the switching. The truth table of the DG201 says 0=ON, 1=OFF.
So for me this clearly looks like the regular EOC is the right signal to take.

You're right. Didn't realize DG201 was a normally open switch.
So the non-inverted EOC signal is the one to use

[Navs]

... The DG201 logic says its normally open and closes with a high signal.

It's entirely possible that I've got things the wrong way round and I can't remember so I'd have to check the circuit itself. But is this logic table from the datasheet not saying the reverse?

[cygmu]

Some confusion is arising here because "normally open" means "normally off". This switch is normally closed.

But I think ReToxx is right about the logic. If you have two logic signals -- call them A, B -- and you apply A to the input and B to the drain of the switch, what you get at the source (provided you pull it down properly) is

(not A) AND B

[Navs]

Thanks for clarifying that, cygmu. ReToxx, sorry, I now see you correctly described the logic states in your second post.

But now I also see that Oldenjon posted an amendment to the schematic in 2015. Is that what you are referring to as incorrect, ReToxx?

Here is the schematic I originally posted. I'll gladly amend it if it's wrong. Maybe make mark-ups in red so they're obvious?

If that still doesn't solve it, I'll get the module out and re-trace my tracks, so to speak. Please let me know which pins of which chips you want me to check.

[ReToxx]

I meant the "corrected" schematic version.
The one you posted now looks right to me.
I will etch a PCB on Monday propably, so will be able to tell
if it works or not in the next days.

[ReToxx]

took me a little longer, unfortunately, but i can confirm now that the last schematic posted by navs works.

[Navs]

Excellent

I'd be keen to see pictures of your mod if you have some.

[ReToxx]

my phone camera is super dusty so u would hardly see anything. will try to take some with my other camera.
basically what i did is not a mod of the doepfer module but a completely new module i put together. my own kind of super function.
thanks for your work, that really helped me a lot.

[appliancide]

I just posted pics in the 2018 build thread of a pair of DUSG+ that used your mod schematic. Works great!

[fivesteppath]

Hmm, 8k2? I am not seeing this on the Serge VCS schematic. Is the Doepfer module different or 8k2 is a typo for 82k?

https://www.elby-designs.com/webtek/cgs ... 5_vcs.html

To the right of the TL074 there is a 82k, I am assuming I make the cut between 7 and 82k?

[Navs]

Hmm, 8k2? I am not seeing this on the Serge VCS schematic. Is the Doepfer module different or 8k2 is a typo for 82k?

https://www.elby-designs.com/webtek/cgs ... 5_vcs.html

To the right of the TL074 there is a 82k, I am assuming I make the cut between 7 and 82k?

It's been a long time so I had to check which resistor you're looking for - I'm assuming it's for the HOLD mod.

If so, the 8k2 resistor is in the Serge VCS schematic that you linked to:

Screenshot 2021-09-19 at 10.37.00.png (21.95 KiB) Viewed 1068 times

[fivesteppath]

Hmm, 8k2? I am not seeing this on the Serge VCS schematic. Is the Doepfer module different or 8k2 is a typo for 82k?

https://www.elby-designs.com/webtek/cgs ... 5_vcs.html

To the right of the TL074 there is a 82k, I am assuming I make the cut between 7 and 82k?

It's been a long time so I had to check which resistor you're looking for - I'm assuming it's for the HOLD mod.

If so, the 8k2 resistor is in the Serge VCS schematic that you linked to:

Screenshot 2021-09-19 at 10.37.00.png

Thanks Nav! Well would you look at that, there it is.

What is awkward, even though I posted the link which you're referring to, I was actually looking at the schematic over here which does not have the edits.
https://sdiy.info/wiki/CGS_voltage_cont ... e_Eurorack

Just to be sure, so the trace cut goes there, and the pin 6 of the DG201 is to the TL072 pin 7 also?

I wish I had the A-171-2 schematic, this would make things more clear for me.

Also, thought to show off my custom panel for this mod.
It's not fully assembled, still need to dress the knobs and nuts after it's all tested and good.