On Mar 15, 2013, at 12:41 PM, "
oliverfischhaupt@..." <
oliverfischhaupt@...> wrote:
> Hi Folks,
Hi Oliver! Cool work doing the Spice model and prototyping.
> I have build an circuit of the CS80 VCO chip like showed in the Service manual.
> First i build it with LTSpice after some troubleshooting i build the circuit on breadboard
>
> and it works. I do some measurements with the following results:
>
> CV Frequency
> 1 V 642 HZ
> 2 V 1253 Hz
> 4 V 2381 Hz
> 8 V 4360 Hz
>
> if I double the Control Voltage the Frequncy should also have the double value.
> My VCO don't do this. I think it is of course the tau Value(30µs) of the Monoflop.
> T = tau + I. The 3Oµs Peak is one "secret" of the VCO but how do the make this in their
I've seen info on this fixed time affecting the sound. Scott/Old-Crow did a lot of work on this and has a great webpage for it:
http://www.cs80.com/vco.htmlBasically, any sawtooth-based VCO (about 80% of those in analog synths) had this issue. It causes the VCO to go flat on higher notes compared to a perfect model because the fixed reset delay becomes a bigger and bigger fraction of the period. Most VCOs I've seen (e.g. Moogs) have a trimmer called something like high-frequency-adjust.
I just did a quick search and found this Rogue schematic that has "Hi-End Trim" for each VCO:
http://www.cykong.com/Synths/Moog%20Rogue%20(Oakley)/Images/Moog%20Rogue%20Schematics%20Coloured.gif
It also shows little pictures of the reset time affecting the waveform.
Here's another great DIY VCO page:
http://www.musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&VPW=1430&VPH=667I explain this in detail, along with other linear vs 1v/oct VCO stuff in my now famous toilet-as-VCO-analogy here:
http://m.matrixsynth.com/2007/11/david-rogoff-on-vcos.htmlTriangle-wave-based VCOs don't have this problem. They charge a cap at a rate set by the CV (actually a current) just like the sawtooth one. However, when they hit the max voltage threshold, they don't reset, the just reverse the current so the cap discharges at that rate. It will go negative and then hit another threshold at minus max voltage and reverse again. No issue with the fixed reset time and it's a nice +/- waveform.
It's more complicated
Here's the data sheet for the EXAR 2209. It's a great little linear VCO chip that's been around a long time and has a triangle core.
http://www.exar.com/communications/timing/voltage-controlled-oscillator-vco/xr2209# >
> What happens if the Tuning Knob on the panel has the max Value this means 6 Volts at the Note Voltage DAC and you make a high Pitchbend with the Ribbon control with the highest c depressed?
> What is the Range of the Ribbon Control.
More later.
>
> Sorry for this silly Questions but i don't owe a real CS80. I played one in the 80s.
I think they're great questions!
David
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