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Help with simple Potentiometer controller (that outputs CV)
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mikeb



Joined: Nov 20, 2006
Posts: 59
Location: The Automotive Capital of Canada

PostPosted: Mon Nov 20, 2006 11:50 pm    Post subject: Help with simple Potentiometer controller (that outputs CV)
Subject description: A few questions about making a controller that involves a pot and not keys.
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Hey all,

I want to make a really simple controller for a synth that has a potentiometer and a single switch instead of keys. You know, as a quick way to make some theremin-like noises.

I'd be happy if it output say, 2 to 8V to a 1v/octave VCO.

Surely it can't be as easy as using the pot as a voltage divider and sending the output voltage straight to the VCO as the CV?

I took a look at http://musicfromouterspace.com/analogsynth/keybrdcontroller.html (The single bus controller circuit) and I know I can replace the keys and resistors with the potentiometer.. But the circuit seems overkill for that.. and I'd really like to know what to trim from this circuit. For example, I wouldn't need the portamento or whatever part of that circuit looks for voltage changes for the trigger, etc. I also wouldn't need a sample and hold on there..

Honestly, all I want is a potentiometer that sends out CV and a single switch that would output the trigger or gate (do I need both / are they the same thing? For example, Thomas Henry's XR2206 VCO doesn't seem to have inputs for either trigger or gate.. just CV.. although I'd like this simple controller to work with other VCO's).


Since I really like the simple route, I was thinking of this solution... I'd love to hear what problems there are with this.. since I'm new to a lot of this. I was thinking I could just say use +10V (off of a regulator to keep it constant) run it to the pot as a voltage divider.. and also run the +10V to a momentary switch for the gate/trigger (reducing this voltage depending on what voltage the gate/trigger input of the VCO would like to see). My understanding is that with that simple setup, I could sweep the CV from 0-10V and that it would be divided up evenly through the pot's rotation.. that is.. x degrees of rotation/octave.

I'm guessing if I did this, it would also need to be buffered? (so send it to an op-amp as a "voltage follower".. that's all I know).

Thanks in advance!
Mike
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ian-s



Joined: Apr 01, 2004
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PostPosted: Tue Nov 21, 2006 1:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Have you seen this?
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mikeb



Joined: Nov 20, 2006
Posts: 59
Location: The Automotive Capital of Canada

PostPosted: Tue Nov 21, 2006 2:44 am    Post subject: Reply with quote  Mark this post and the followings unread

g2ian wrote:
Have you seen this?


g2ian, I have not seen that.. Infact I've never heard of it before. It's like a modular "Paul's Box" (http://www.electrotheremin.com/PTE-TPage.html - The precursor to the Electro-Theremin), where he just marked the notes off on a Function Generator Smile

Thanks a lot for this, it seems to be the basics of what I'm trying to do. I wonder why the range and scale knobs are needed though. With that printed "keyboard" on there, you'd think having set the range/scale once and then just tuning it at the VCO would be easiest.

I'm also wondering if I can replace that "temperature compensated current source" with just a standard Voltage Regulator IC.. like an LM7912.
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fonik



Joined: Jun 07, 2006
Posts: 3950
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PostPosted: Tue Nov 21, 2006 2:57 pm    Post subject: Re: Help with simple Potentiometer controller (that outputs CV)
Subject description: A few questions about making a controller that involves a pot and not keys.
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mikeb wrote:

I want to make a really simple controller for a synth that has a potentiometer and a single switch instead of keys. You know, as a quick way to make some theremin-like noises. I'd be happy if it output say, 2 to 8V to a 1v/octave VCO.
Surely it can't be as easy as using the pot as a voltage divider and sending the output voltage straight to the VCO as the CV?

why not. that's exactly what some utility modules of modular systems do. they provide adjustable control voltages.
Quote:
Honestly, all I want is a potentiometer that sends out CV and a single switch that would output the trigger or gate (do I need both / are they the same thing? For example, Thomas Henry's XR2206 VCO doesn't seem to have inputs for either trigger or gate.. just CV.. although I'd like this simple controller to work with other VCO's).

VCO with trigger input? CV will control the pitch of the VCO. trigger is not the same as gate: a trigger is a short pulse used to (yes!) trigger a module and evoke an event. a gate signal is a pulse of a certain length. the signal goes high as long as you push the switch (for your example). with these signals (gate or trigger) you can control an ADSR generator which could control a VCA which could control the volume of the VCO! take a look at ray wilsons ADSR article to get the picture: http://www.musicfromouterspace.com/analogsynth/ADSR001/ADSR001.html
Quote:
Since I really like the simple route, I was thinking of this solution... I'd love to hear what problems there are with this.. since I'm new to a lot of this. I was thinking I could just say use +10V (off of a regulator to keep it constant) run it to the pot as a voltage divider.. and also run the +10V to a momentary switch for the gate/trigger (reducing this voltage depending on what voltage the gate/trigger input of the VCO would like to see). My understanding is that with that simple setup, I could sweep the CV from 0-10V and that it would be divided up evenly through the pot's rotation.. that is.. x degrees of rotation/octave.

(never saw a VCO with trigger input, but) the way you described it you'd get a gate signal which should work for most ADSRs (with limited functionality though).
Quote:
I'm guessing if I did this, it would also need to be buffered? (so send it to an op-amp as a "voltage follower".. that's all I know).

would be good practice, i think.

hope i got you and this was helpful - or did i get you completely wrong?
cheers,
matthias
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mikeb



Joined: Nov 20, 2006
Posts: 59
Location: The Automotive Capital of Canada

PostPosted: Tue Nov 21, 2006 4:45 pm    Post subject: Re: Help with simple Potentiometer controller (that outputs
Subject description: A few questions about making a controller that involves a pot and not keys.
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Hey Fonik, thanks for the reply..

I messed up when I said that the trigger/gate would be going into the VCO.. I do understand that it would be going into an ADSR or VCA to control the volume. Forgive me for being new to the idea of modular Smile

As for the idea of just having the pot as the voltage divider, when you said: "why not. that's exactly what some utility modules of modular systems do. they provide adjustable control voltages.". I'm guessing the why not is that there should at least be a buffer circuit on the output and possibly a few other things..

The only reason I mention gate/trigger (which I thought were the same according to wikipedia, I guess not) is that I want to have a momentary switch to press to make the tone until I release it. I don't want to have to build a VCA and as of right now, I have no need for an ADSR. All I want the switch to do is to enable sound from the VCO that the pot is controlling. A big bonus feature to me would be if this switch also outputted a gate signal so later down the road it could be used with an ADSR, etc.

A simple solution to getting the momentary switch to work would be if it interrupted the CV going to the VCO, but since I'm not familiar with the operation of VCO's I don't know what they do when you send them 0V and then suddenly go to say 4V, and if they make an audible popping sound when this happens or something. Maybe I should also work on some sort of super-simple VCA that takes a gate signal from my controller's momentary switch (That gets held for as long as the note should be held for)?

Anyways thanks for the help!
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fonik



Joined: Jun 07, 2006
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PostPosted: Wed Nov 22, 2006 12:05 am    Post subject: Re: Help with simple Potentiometer controller (that outputs
Subject description: A few questions about making a controller that involves a pot and not keys.
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mikeb wrote:
I want to have a momentary switch to press to make the tone until I release it. I don't want to have to build a VCA and as of right now, I have no need for an ADSR.
A simple solution to getting the momentary switch to work would be if it interrupted the CV going to the VCO, but since I'm not familiar with the operation of VCO's I don't know what they do when you send them 0V and then suddenly go to say 4V, and if they make an audible popping sound when this happens or something. Maybe I should also work on some sort of super-simple VCA that takes a gate signal from my controller's momentary switch (That gets held for as long as the note should be held for)?

a vco outputs audio continuously (if it's calibrated according to the 1V/oct rule it would output a C1 at 0V). A very simple solution would be to gate the audio from the VCO with the switch itself (simple on/off operation).
then you don't have a gate signal. the VCA solution would be more elegant. the gate signal would be used instead of ADSR.
for a simple controller take a look at this:
http://www.musicsynthesizer.com/Joystick/1209.htm
for simple VCAs (somewhere there might be more simple designs that can work from a single power supply?):
http://www.uni-bonn.de/~uzs159/vca.html
http://www.musicfromouterspace.com/analogsynth/vca.html

cheers,
matthias
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mikeb



Joined: Nov 20, 2006
Posts: 59
Location: The Automotive Capital of Canada

PostPosted: Wed Nov 22, 2006 1:50 am    Post subject: Re: Help with simple Potentiometer controller (that outputs
Subject description: A few questions about making a controller that involves a pot and not keys.
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fonik wrote:
mikeb wrote:
I want to have a momentary switch to press to make the tone until I release it. I don't want to have to build a VCA and as of right now, I have no need for an ADSR.
A simple solution to getting the momentary switch to work would be if it interrupted the CV going to the VCO, but since I'm not familiar with the operation of VCO's I don't know what they do when you send them 0V and then suddenly go to say 4V, and if they make an audible popping sound when this happens or something. Maybe I should also work on some sort of super-simple VCA that takes a gate signal from my controller's momentary switch (That gets held for as long as the note should be held for)?

a vco outputs audio continuously (if it's calibrated according to the 1V/oct rule it would output a C1 at 0V). A very simple solution would be to gate the audio from the VCO with the switch itself (simple on/off operation).
then you don't have a gate signal. the VCA solution would be more elegant. the gate signal would be used instead of ADSR.
for a simple controller take a look at this:
http://www.musicsynthesizer.com/Joystick/1209.htm
for simple VCAs (somewhere there might be more simple designs that can work from a single power supply?):
http://www.uni-bonn.de/~uzs159/vca.html
http://www.musicfromouterspace.com/analogsynth/vca.html

cheers,
matthias


If I just gate the audio from the VCO with the switch itself, from experience.. I tend to get a rather loud "POP" every time the button is pressed. I've tried a few things to help solve this with some caps and resistors as a friend mentioned, but I always got the pop sound.

The VCA thing does seem like the way to go but unfortunately even those simple circuits are rather complicated for something that I wanted to keep so simple. When I have to build a VCO, this controller (although that CIRCON circuit is very simple and elegant) and a VCA.. I may as well just and some 1/4" jacks and be building a whole modular synth setup! Smile
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fonik



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PostPosted: Wed Nov 22, 2006 2:17 am    Post subject: Re: Help with simple Potentiometer controller (that outputs
Subject description: A few questions about making a controller that involves a pot and not keys.
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mikeb wrote:
If I just gate the audio from the VCO with the switch itself, from experience.. I tend to get a rather loud "POP" every time the button is pressed. I've tried a few things to help solve this with some caps and resistors as a friend mentioned, but I always got the pop sound.

did you try to put a resistor (i.e. 47k) between switch and ground? this way the signal goes through the resistor to ground if you push the button (which is actually kind of attenuation).
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mikeb



Joined: Nov 20, 2006
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PostPosted: Wed Nov 22, 2006 2:31 am    Post subject: Re: Help with simple Potentiometer controller (that outputs
Subject description: A few questions about making a controller that involves a pot and not keys.
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fonik wrote:
mikeb wrote:
If I just gate the audio from the VCO with the switch itself, from experience.. I tend to get a rather loud "POP" every time the button is pressed. I've tried a few things to help solve this with some caps and resistors as a friend mentioned, but I always got the pop sound.

did you try to put a resistor (i.e. 47k) between switch and ground? this way the signal goes through the resistor to ground if you push the button (which is actually kind of attenuation).


Fonik> I'm pretty sure I had a resistor in this place and still had audible "pop"s. However I can quickly give this a shot..
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ian-s



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PostPosted: Wed Nov 22, 2006 11:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Pops are to be expected with fast attack times (vca or not). One solution that does not involve slowing the attack time, is to connect the sync input on the vco to the gate. The idea is that each burst starts with the wave at zero.
Only works properly with triangle or sine waves and depends on the specific vco design.
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mikeb



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PostPosted: Sat Nov 25, 2006 3:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

g2ian wrote:
Pops are to be expected with fast attack times (vca or not). One solution that does not involve slowing the attack time, is to connect the sync input on the vco to the gate. The idea is that each burst starts with the wave at zero.
Only works properly with triangle or sine waves and depends on the specific vco design.


Hey, thanks once again for trying to help me with this. That sounds like a pretty neat idea. I was planning on trying Thomas Henry's XR2206 VCO because I'm familiar with the XR2206 from previous projects. The schematic does show it having a SYNC input.. not exactly sure how it works or anything though. I will most definitely have to read more..
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