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Control voltage for XR2206?
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Phil93



Joined: Jan 15, 2012
Posts: 8
Location: Italy

PostPosted: Sun Jan 15, 2012 6:25 am    Post subject: Control voltage for XR2206? Reply with quote  Mark this post and the followings unread

Hi there. I'm new to the forum and also a newbie in audio synthesis. I'm trying to make a VCO for an analog music synthesiser with the XR2206, but I cannot understand how to control it! The datasheet says I can control the frequency with a capacitor between pin 5 and 6 and two resistors between pin 7 and 8, but it doesn't explains very well how to control it with an EXTERNAL voltage (instead of trimming some res), as it should be done... The datasheet also shows a configuration of R and Rc in pin 7 and gives two formulas, but I get two different results from them...
Can you please suggest me a schematic to control the frequency with an external CV (maybe from a DAC)? 1V/oct. would be great, but it isn't mandatory..
Thank you very much
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Dave Kendall



Joined: May 26, 2007
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PostPosted: Sun Jan 15, 2012 8:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Phil.

Welcome to electro-music!

Try this link;
http://www.birthofasynth.com/Thomas_Henry/Pages/XR-VCO.html

cheers,
Dave

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Phil93



Joined: Jan 15, 2012
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Location: Italy

PostPosted: Sun Jan 15, 2012 9:56 am    Post subject: Reply with quote  Mark this post and the followings unread

Dave Kendall wrote:
Hi Phil.

Welcome to electro-music!

Try this link;
http://www.birthofasynth.com/Thomas_Henry/Pages/XR-VCO.html

cheers,
Dave

Thank you Dave! I have already read the link by T.H., but I don't understand some pieces of the schematic:
1. What's the difference between exponential FM and linear FM?
2. Why does he use the LM394 and what's the gain of it?
3. What's the purpose of the op-amp part about linear FM? Seems like an integrator...
4. What's the meaning of the SKEW part before the inverting amp?
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LFLab



Joined: Dec 17, 2009
Posts: 206
Location: Rosmalen, Netherlands

PostPosted: Sun Jan 15, 2012 2:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

The XR2206 VCO's where the first VCO's I built, quite new in modulars myself as well. The easiest way to understand (soundwise) what linFM and expFM do is build a VCO and try it out, ditto for the skew.

I used bugbrands layout, it's a pretty quick build, it's about half way down this page:
Bugbrand old site
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Phil93



Joined: Jan 15, 2012
Posts: 8
Location: Italy

PostPosted: Sun Jan 15, 2012 2:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

JarnoBassplayer wrote:
The XR2206 VCO's where the first VCO's I built, quite new in modulars myself as well. The easiest way to understand (soundwise) what linFM and expFM do is build a VCO and try it out, ditto for the skew.

I used bugbrands layout, it's a pretty quick build, it's about half way down this page:
Bugbrand old site

Unfortunately, I dont' have time and money to build it if I'm not going to actually use it.. Maybe someone who already built it can explain.
I addition, I don't understand why he implements the AM in that way. Isn't it easier to implement tremolo effect by using a LFO sine as CV for a VCA?

Last edited by Phil93 on Mon Jan 16, 2012 5:52 am; edited 1 time in total
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Sebo



Joined: Apr 27, 2007
Posts: 541
Location: Argentina

PostPosted: Sun Jan 15, 2012 5:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Exponentinal Input:
You get an exponential frequency response from a linear voltage modulation, good for 1V/Oct and vibrato.

Linear Input:
You get a linear frequency response from a linear volatge modulation, good for FM sounds.

Skew:
Blends from a triangle or sawtooth wave to a sine wave two octaves below the triangle/saw.

I don't see amplitude modulation in the circuit.

The LM394 is the exponential converter, converts the linear voltage input into an exponential current, which controls the frequency of the VCO.

SDIY is about experimentation, making mistakes and learning from them, that's the fun of it, if you don't have time to spend in a circuit, may be is better to buy something that is already working.
With that being said, the functionality of this VCO is pretty standard except for the skew function. Most VCO circuits have linear and exponential inputs, and a couple of waveforms as outputs (sawtoth, pulse, sine, triangle). If you need a VCO you can't be wrong with it. The only 2 thing that could let you down are the V/Oct response, which seems pretty good judging from the table posted at the page, and the sound, and you can hear some audio at the page to judge for yourself (I think sounds pretty good).

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Phil93



Joined: Jan 15, 2012
Posts: 8
Location: Italy

PostPosted: Mon Jan 16, 2012 3:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Sebo wrote:
SDIY is about experimentation, making mistakes and learning from them, that's the fun of it

Sure it is, but it's a project for the school and I have limited time.. Sad
However, thank you for the answers. I will try breadboarding this circuit and see if it can be done, meanwhile.. is there a simpler method to implement pitch modulation with the 2206? The datasheet shows one but it's not so clear..
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LFLab



Joined: Dec 17, 2009
Posts: 206
Location: Rosmalen, Netherlands

PostPosted: Mon Jan 16, 2012 5:25 am    Post subject: Reply with quote  Mark this post and the followings unread

Phil93 wrote:
....the school....


Ah, but then it is important to figure stuff out yourself and learn something! Very Happy Go on, breadboard the sucker, you may want to use something else than LM394 since that one is hard to get, just match two common BC transistors.
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Phil93



Joined: Jan 15, 2012
Posts: 8
Location: Italy

PostPosted: Mon Jan 16, 2012 5:50 am    Post subject: Reply with quote  Mark this post and the followings unread

JarnoBassplayer wrote:
Phil93 wrote:
....the school....


Ah, but then it is important to figure stuff out yourself and learn something! Very Happy Go on, breadboard the sucker, you may want to use something else than LM394 since that one is hard to get, just match two common BC transistors.

It's what I'm already doing, but it's a bit hard to learn something without someone teaching you... Very Happy That's why I'm here. In addition, I don't have the ICs right now (i've ordered them online), so I'm trying to understand how the circuit works theorically. In the XR VCO project there isn't any formulas..
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MC1495



Joined: Mar 24, 2009
Posts: 19
Location: baltimore

PostPosted: Fri May 31, 2013 8:00 am    Post subject: Reply with quote  Mark this post and the followings unread

been working with the XR-2206 for awhile now and have been trying to come up with an exponential converter for an 2206 on a bipolar power supply. it seems like the henry design isn't working in this arrangement, but i'm imaging someone here has an idea on how to make it work.

thoughts/notes:

1. the potentiometer arrangement for Fig. 11 in the application notes works with the ground switched to V-. (http://www.jaycar.com.au/images_uploaded/XR2206V1.PDF)

2. the xr2206 does not seem to be respond to positive excursion voltage in the application notes or in my bipolar version.

3. i tried an extra inverting stage at the end of the Henry expo. converter (his design, i presume is outputting voltages between V+ and ground)...but that didn't seem to work either. i'm not sure attaching an op amp stage after a transistor stage for an expo. converter is effective in general.

4. my FSK input is not attached to anything, unlike the henry design - so i emitted to skew section. i'm wondering if the FSK affects expected polarity of the timing resistor inputs?

5. i don't understand transistor circuitry very well...but i'm wondering if there is something i can do with Q1 in the henry design to make it output a reversed exponential...
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MC1495



Joined: Mar 24, 2009
Posts: 19
Location: baltimore

PostPosted: Fri May 31, 2013 11:41 am    Post subject: Reply with quote  Mark this post and the followings unread

alright, a single voltage-to-current transistor gets me a lot closer to where i want to be.. but the 2206 is still only accepting negative voltage.. does anyone have any experience using these with dual supply?
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