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CS-80 style VCO
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elektrouwe



Joined: May 27, 2012
Posts: 45
Location: Germany

PostPosted: Thu Mar 21, 2013 12:15 pm    Post subject: CS-80 style VCO Reply with quote  Mark this post and the followings unread

here is a schematic for a CS-80 style VCO. It is only a LTSPICE simulation until now. A guy asked for help on a german forum ( http://www.sequencer.de/synthesizer/viewtopic.php?f=13&t=78218 ) and I found it interesting to help him, because Yamaha used a closed loop VCO in their Chip. Although closed loop VCOs typically have better linearity than our "bread & butter discharge cap. with switch"-VCOs they are not popular in music electronics, because the rising ramp has a slope with fixed rise time and the amplitude is not constant over CV-range.
Rise time at low freq. does not play a role, but for frequencies in the kHz range , waveform changes towards an asymmetrical triangle. I guess that is the reason why Yamaha added a waveform shaper that simply masks the rising slope with a constant amplitude "punch" from the closed loop one-shot.
the short (30us ?) punch does not add audible harmonic content and, I guess again, is "grinded" away in the (bandwidth limited) signal path.
Please comment


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steffensen



Joined: Jul 11, 2012
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PostPosted: Sun Mar 31, 2013 2:30 am    Post subject: Reply with quote  Mark this post and the followings unread

I have nothing to add here, only that im glad someone finally picked this up. Will follow with interest!
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keyman2



Joined: Jul 16, 2013
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PostPosted: Sat Jul 27, 2013 10:39 pm    Post subject: Re: CS-80 style VCO Reply with quote  Mark this post and the followings unread

Looks deceptively simple...What would be required to make this a 'practical circuit' in terms of compensation for temperature or other variations which might alter tuning/performance?

elektrouwe wrote:
here is a schematic for a CS-80 style VCO. It is only a LTSPICE simulation until now. A guy asked for help on a german forum ( http://www.sequencer.de/synthesizer/viewtopic.php?f=13&t=78218 ) and I found it interesting to help him, because Yamaha used a closed loop VCO in their Chip. Although closed loop VCOs typically have better linearity than our "bread & butter discharge cap. with switch"-VCOs they are not popular in music electronics, because the rising ramp has a slope with fixed rise time and the amplitude is not constant over CV-range.
Rise time at low freq. does not play a role, but for frequencies in the kHz range , waveform changes towards an asymmetrical triangle. I guess that is the reason why Yamaha added a waveform shaper that simply masks the rising slope with a constant amplitude "punch" from the closed loop one-shot.
the short (30us ?) punch does not add audible harmonic content and, I guess again, is "grinded" away in the (bandwidth limited) signal path.
Please comment
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itist



Joined: Dec 30, 2013
Posts: 16
Location: Germany

PostPosted: Mon Dec 30, 2013 12:46 pm    Post subject: Soundclip Reply with quote  Mark this post and the followings unread

Hi,

if you want to listen to the VCO from elektrouwe i was the guy who asked for.
Here is a link to a short youtube Sound clip.

Here you can here it without Filter with my SPI Based 12Bit MIDI to Voltage converter.It sucks in the low frequecies but this is an issue of the converter. The VCO is stable.

http://youtu.be/k6iqJHWC2OY



http://youtu.be/N1N52uKgiHo

Its the CS Style VCO thru a 24 db Transistor ladder Filter.


Here you can here the VCO with a discrete DAC and its fine over the whole keyboard range.

http://youtu.be/uGv4gvDUYMU

I am searching for an VCO schematic collection espacially linear VCO

if someone has ideas or want me to do some measurements feel free..



Greetz


Oliver


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keyman2



Joined: Jul 16, 2013
Posts: 4
Location: Canada

PostPosted: Mon Dec 30, 2013 4:53 pm    Post subject: Re: Soundclip Reply with quote  Mark this post and the followings unread

The ladder example sounds very saturated?

itist wrote:
Hi,

if you want to listen to the VCO from elektrouwe i was the guy who asked for.
Here is a link to a short youtube Sound clip.

Here you can here it without Filter with my SPI Based 12Bit MIDI to Voltage converter.It sucks in the low frequecies but this is an issue of the converter. The VCO is stable.

http://youtu.be/k6iqJHWC2OY



http://youtu.be/N1N52uKgiHo

Its the CS Style VCO thru a 24 db Transistor ladder Filter.


Here you can here the VCO with a discrete DAC and its fine over the whole keyboard range.

http://youtu.be/uGv4gvDUYMU

I am searching for an VCO schematic collection espacially linear VCO

if someone has ideas or want me to do some measurements feel free..



Greetz


Oliver


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http://www.ititst.de
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itist



Joined: Dec 30, 2013
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PostPosted: Mon Dec 30, 2013 11:25 pm    Post subject: Re: Soundclip Reply with quote  Mark this post and the followings unread

keyman2 wrote:
The ladder example sounds very saturated?


Hi,

Maybe there is a problem with the matching i will check.

I am not sure with the vibrato implementation. In the low frequencies the depth will be much greater then in the hicher frequencies. Same with the (linear) Detune.

Is it possible to {de) tune one vco of a CS Synth in thirds over the hole Keyboard?

Greetz

Oliver

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itist



Joined: Dec 30, 2013
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Location: Germany

PostPosted: Tue Dec 31, 2013 8:38 am    Post subject: made some new Measurements Reply with quote  Mark this post and the followings unread

Hi,

i think one of Videos is not the CS Style VCO.

Today i have made some new measurements above the range of
125mV to 8 V in octave steps.

The voltage source is not my DAC to eliminate errors in the Voltage steps.

Voltage Frequency red Frequency yellow
0.125V 56.5Hz 56Hz
0.250V 112.5Hz 112Hz
0.5V 223.2Hz 223Hz
1V 439.9Hz 443Hz
2V 874.1Hz 874Hz
4V 1696Hz 1701Hz
8V 3226Hz 3231Hz

Measured with the Oscilloscop!!

all above 3v sounds detuned.


See Update!!



The circuit was powered wirh an standard linear Power supply no Lab supply.

Greetz

Oliver


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125mV
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1V
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itist



Joined: Dec 30, 2013
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Location: Germany

PostPosted: Wed Jan 08, 2014 1:24 pm    Post subject: OpAmp changed!! Reply with quote  Mark this post and the followings unread

After Uwe, the developer of the VCO, wrote me an e-mail where he asked me about the used OpAmp, i wrote him that i used an OP07 (bipolar), he told me to use jfet type. So I changed it to a LF356.

Here the results:

CV F
0.125V 49.8Hz
0.250V 99.01Hz
0.5V 197.4Hz
1V 393.8Hz
2V 784.9Hz
4V 1520Hz
8V 3140Hz

I think this is very linear.

I did a simple temperature check (hair dryer)

15C° 1000Hz
20C° 999Hz
30C° 993Hz
40C° 984Hz
46C° 977Hz

The circuit was build with standard metalfilm resitors with +- 50 ppm

Thank you Uwe.

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itist



Joined: Dec 30, 2013
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PostPosted: Fri Jan 10, 2014 2:20 pm    Post subject: Soundemo Reply with quote  Mark this post and the followings unread

Hi,

Audio Demo CS Style VCO. Recorded direct to Soundcard no filters, no effects.



http://youtu.be/WoIxa03C_yE


The developer Uwe told me to check the used OpAmp. I used an bipolar instead of a Fet OpAmp
I changed the OpAmp from an OP07 to an LF 356 and the tracking Problems are gone...

Thanks Uwe.

Greetz
Oliver





http://itist.de/2013/10/cs-style-vco-core/

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Yipdeceiver



Joined: Apr 07, 2014
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Location: Athens GA

PostPosted: Wed Apr 23, 2014 7:54 am    Post subject: Reply with quote  Mark this post and the followings unread

I would love to make a +/- 12v version of this for my modular. What would I have to change to run this on +/-12v?
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elektrouwe



Joined: May 27, 2012
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PostPosted: Wed Apr 23, 2014 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

[quote="Yipdeceiver"What would I have to change to run this on +/-12v?[/quote]
It will run @+-12V without problems, but a V/Hz slope change (mostly) from Iref = -V/R9 will happen.
R9 can be changed to a 12/15 lower value to get (almost) the same V/Hz slope as with +-15V.
It might be a good idea anyway to add a trimpot in series to R9 to compensate for other component tolerances.
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itist



Joined: Dec 30, 2013
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PostPosted: Tue Jun 24, 2014 2:18 pm    Post subject: Built dual VCO Boards Reply with quote  Mark this post and the followings unread

Hi,

i built two dual VCO boards of the CS style VCO created by Uwe.

http://itist.de/2014/06/dual-vco-board/

And a first Impressions Sound Demo
http://youtu.be/QQPX25NYFqs


Greetz Oliver

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elektrouwe



Joined: May 27, 2012
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PostPosted: Wed Jun 25, 2014 1:00 am    Post subject: Re: Built dual VCO Boards Reply with quote  Mark this post and the followings unread

itist wrote:
Hi,
And a first Impressions Sound Demo

thanks for the proof of concept , Oliver !
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steffensen



Joined: Jul 11, 2012
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PostPosted: Thu Jun 26, 2014 2:56 am    Post subject: Reply with quote  Mark this post and the followings unread

itist,
Would you care to share more info/details about the sync-isse you had and solved by using a buffer?
Never seen a solution like that before, only seen it solved by using precision voltage reference's and such. This makes me curious!
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itist



Joined: Dec 30, 2013
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PostPosted: Thu Jun 26, 2014 7:05 am    Post subject: My explanation for the Sync issue. Reply with quote  Mark this post and the followings unread

In my first Impression Video I connected all outputs on the Diode D1 via a summing resistor Network
to an Opamp and then to the soundcard.

My assumption is that the outputs interfere with the trigger circuit (current return path??) and with the unity gain buffer this path is cut off.
Maybe it was only a kind of capacitive coupling. But with the buffer the sync issue has gone.

The circuit ist powered with a standard linear Voltage power supply but the control voltage is created with an LT1236.

For a linear VCO you need, especially in the lower key range, a precise Voltage. The lowest C is 125mV and the next C is 250mV
so each halftone is appr. 10 mV
I will do further testing but in my first tests the VCO seems to be linear over the Voltage Range from 125mV to 4mV.

You can follow the further development on my website:

www.itist.de

And please don't forget I am not an EE!
So my answers are maybe not correct! Smile

Greetz
Oliver

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itist



Joined: Dec 30, 2013
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PostPosted: Thu Jun 26, 2014 12:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

steffensen wrote:
itist,
Would you care to share more info/details about the sync-isse you had and solved by using a buffer?
Never seen a solution like that before, only seen it solved by using precision voltage reference's and such. This makes me curious!


i will try to reproduce this issue and will do Audio and Scope Recording.
Maybe its synced because of the Scope probes?
It's also necessary to check the second board. On this board one VCO sounds muffled. THe other three sounds crisp.

Greetz

Oliver

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steffensen



Joined: Jul 11, 2012
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PostPosted: Fri Jun 27, 2014 10:40 am    Post subject: Reply with quote  Mark this post and the followings unread

itist,
Thanks for the info, always interesting to hear other peoples experiences!

Your thoughts makes sense, could very well be like you describe.

I have similar results with some other VCO's i have, but only been planning on solving it by precision voltage reference instead. Gonna give this a go too as there is no buffer implemented atm. (sorry to go off topic, getting carried away with interest)

Let me/us now how it goes!
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itist



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PostPosted: Fri Jun 27, 2014 1:39 pm    Post subject: Quick Video showing the effect Reply with quote  Mark this post and the followings unread

steffensen,

made a quick Video showing the effect.
http://youtu.be/zEfeB6HNb0w

Greetz

Oliver

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steffensen



Joined: Jul 11, 2012
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PostPosted: Sat Jun 28, 2014 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Excellent video man, thanks for doing it!
You got a nice workbench-area there for sure. Very Happy
Hope i have some time on my hands to try this out soon, most curious.
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