voltage controlled slew / portamento circuit

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ESSY
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voltage controlled slew / portamento circuit

Post by ESSY » Tue Mar 24, 2020 10:15 am

So I have been using this slew circuit found in this thread by Harry Bissel for the module I am designing and it works great - does the job - low part count - simple! However... I can't help but want to add CV control to this circuit. After researching a bunch it seems the best way to do this is to use a vactrol. :despair:

So I went ahead and hooled up a vactrol in series with the potentiometer and it gave me my VC Slew, however, I learned there are some caveats to this approach. One is that the raw vactrol will output a resistance between 0 ohms and 10 Mohms, which is not ideal for tuning in a nice max/min slew amount. Second, the vactor cannot be attenuated with the existing potentiometer, which would be a real nice thing to have.

So poking around a bit more I found a circuit on this thread which shows how to keep the output resistance of the vactrol within a range using a "limiting resistor". Works like a charm.

Final tested circuit came out like this:
Screen Shot 2020-03-24 at 11.05.19 AM.png

Now I really lack expertise when it comes to circuitry so I wanted to ask the forum this question:

Is there a more ideal solution out there that doesn't involve using a vactrol / H11F1?
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Last edited by ESSY on Tue Mar 24, 2020 11:45 am, edited 1 time in total.

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Re: voltage controlled slew / pornomento circuit

Post by guest » Tue Mar 24, 2020 11:00 am

you can also use an LM13700 or SSM2164.
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Re: voltage controlled slew / pornomento circuit

Post by ESSY » Tue Mar 24, 2020 11:29 am

theres also this article suggesting I can do it with a JFET transistor?

The LM13700 or SSM2164's are a bit overkill as I I only need 1 vs 4

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Re: voltage controlled slew / pornomento circuit

Post by electricanada » Tue Mar 24, 2020 11:31 am

Came for the pornomento; left disappointed.
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Re: voltage controlled slew / pornomento circuit

Post by elektrouwe » Tue Mar 24, 2020 11:32 am

ESSY wrote:
Tue Mar 24, 2020 10:15 am
... I learned there are some caveats to this approach. One is that the raw vactrol will output a resistance between 0 ohms and 10 Mohms, which is not ideal for tuning in a nice max/min slew amount. Second, the vactor cannot be attenuated with the existing potentiometer, which would be a real nice thing to have.
your transistor control acts more like a switch, not as a linear control. Better use an opamp + transistor as voltage contolled current source for the vactrol LED with 3 summing inputs: 1 for ext. CV, 1 for CV-potentiometer, and 1 for CV-trimmer to adjust operating point.
Potentiometer is no longer in signal path then

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Re: voltage controlled slew / pornomento circuit

Post by jorg » Tue Mar 24, 2020 11:33 am

electricanada wrote:
Tue Mar 24, 2020 11:31 am
Came for the pornomento; left disappointed.
I should devise a circuit to upgrade my portamento to pornomento. That would be so fun.

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Re: voltage controlled slew / pornomento circuit

Post by elektrouwe » Tue Mar 24, 2020 11:34 am

electricanada wrote:
Tue Mar 24, 2020 11:31 am
Came for the pornomento; left disappointed.
say "hello" to Mr. Freud !

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Re: voltage controlled slew / portamento circuit

Post by ESSY » Tue Mar 24, 2020 11:47 am

ahah pornomento :doh: edited out

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Re: voltage controlled slew / pornomento circuit

Post by ESSY » Tue Mar 24, 2020 11:51 am

elektrouwe wrote:
Tue Mar 24, 2020 11:32 am
your transistor control acts more like a switch, not as a linear control. Better use an opamp + transistor as voltage contolled current source for the vactrol LED with 3 summing inputs: 1 for ext. CV, 1 for CV-potentiometer, and 1 for CV-trimmer to adjust operating point.
Potentiometer is no longer in signal path then
The thing with this is I kinda like being able to attenuate the external CV signal with the potentiometer, and the way I have it now I kinda get the best of both worlds. When no CV is present, the potentiometer will still adjust the amount of slew

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Re: voltage controlled slew / portamento circuit

Post by guest » Tue Mar 24, 2020 12:16 pm

an LM13700 is pretty cheap and easy to work with. it will also give consistent results, and you can make a dual slew rate limiter! it also allows for various contours on the slew. or you can use the other half for a S/H, VCA, VCO, etc.
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Re: voltage controlled slew / portamento circuit

Post by EATyourGUITAR » Wed Mar 25, 2020 3:55 pm

if you put the vactrol in series to C3 and increase C3 to 100uF that should get you down to 0 slew. then you can put the pot on the driver circuit with a normal connection in the jack for when nothing is inserted. you can also set it up as a input attenuator normalled to a 5v DC offset. I expect a vactrol to get down to 10K. I never design a circuit where I expect the Ron to be < 10K ohms even if one vactrol can do it. vactrols vary a lot.

this entire circuit is inferior to the discrete four transistor VC slew used by buchla, serge. that will get you to 0 slew or 60 seconds.
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Re: voltage controlled slew / portamento circuit

Post by ESSY » Fri Mar 27, 2020 9:01 am

EATyourGUITAR wrote:
Wed Mar 25, 2020 3:55 pm
this entire circuit is inferior to the discrete four transistor VC slew used by buchla, serge. that will get you to 0 slew or 60 seconds.
Do you think you can link to a schematic here? Is it this one? I would like to take a look. One thing to note about the op circuit is I need to insert 4 of them into a single module. So at some point part count as well as PCB space needs to be considered

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Re: voltage controlled slew / portamento circuit

Post by guest » Fri Mar 27, 2020 9:20 am

either the vactrol or 13700 circuit would be the easiest, in particular if you want a lot of them. the 13700 version is 1/2 of a 13700, an opamp, and a transistor per channel.
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Re: voltage controlled slew / portamento circuit

Post by EATyourGUITAR » Fri Mar 27, 2020 9:39 am

ESSY wrote:
Fri Mar 27, 2020 9:01 am
Do you think you can link to a schematic here? Is it this one? I would like to take a look. One thing to note about the op circuit is I need to insert 4 of them into a single module. So at some point part count as well as PCB space needs to be considered
I was thinking this http://www.synthpanel.com/modules/cgs775_vcs.html

it requires an LM3900 with 5 transistors. however, if you understand how the circuit works, that 4 transistor core is basically the entire circuit. everything else can be chopped off and replaced with a very minimalist support circuitry. the serge VCS, the buchla 281, make noise maths, MOTM 820, befaco slew, befaco rampage all have that 4 transistor slew core with different supporting circuitry. where would we be without it?

here is a more minimalist example https://befaco.org/docs/Slew_Limiter/Sl ... ematic.pdf
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Re: voltage controlled slew / portamento circuit

Post by ESSY » Sat Mar 28, 2020 8:12 am

EATyourGUITAR wrote:
Fri Mar 27, 2020 9:39 am
here is a more minimalist example https://befaco.org/docs/Slew_Limiter/Sl ... ematic.pdf
ahh this was honestly the first schematic I looked at when I was first trying to figure this out! I have NO idea what is going on in this circuit, but I will try and study up on transistors because it would sure be nice to understand how that famous slew circuit works

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Re: voltage controlled slew / portamento circuit

Post by ESSY » Sat Mar 28, 2020 8:13 am

guest wrote:
Fri Mar 27, 2020 9:20 am
either the vactrol or 13700 circuit would be the easiest, in particular if you want a lot of them. the 13700 version is 1/2 of a 13700, an opamp, and a transistor per channel.
I am also going to wire this up today and see what I get. It has been mentioned/recommended so many times I gotta see what its all about

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Re: voltage controlled slew / portamento circuit

Post by mush » Sat Mar 28, 2020 9:47 am

ESSY wrote:
Sat Mar 28, 2020 8:12 am
ahh this was honestly the first schematic I looked at when I was first trying to figure this out! I have NO idea what is going on in this circuit, but I will try and study up on transistors because it would sure be nice to understand how that famous slew circuit works
This link might be of help to you as it explains the pair of differential transistor pairs in a quite simple way. (it's almost the same integrator in befaco's slew limiter as in the serge vcs, dusg, etc.)
http://www.timstinchcombe.co.uk/index.php?pge=vcs
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