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FarmVCO I


Modifyed VCO

 I modifyed Mr.kunii's VCO (top page is here but it's Japanese only ) to my own one.
I had actually assembled it using the universal board to use his VCO. And I found that depending on a setup of the knob of PWM, there is the point not to sound. And I found that he designed his circuit as that it define width of pulse first, next modulation depth will be apllyed. About me, from restriction of the size of a panel, I change it to be able to adjust pulse width from 50% to about 90% by manual. When inputting LFO as modulation source, it can be modulated up and down from 50%. and it changs this knob to the depth of modulation. It's the same as VCO's of Roland's 100M series.

From left,TL082,CA3086 and TL074(Here is my first version.)
Although restriction appeared in sound making, if I took out the chorus effect with this, I judged that there was no need shifting the center of modulation from 50%. For this modification, I can't make mosquito flying sound. But I want my synth to make fat, hot and strong sound, so I don't need mosquito flying sound.
But I avoid one of possibility to not to sound and I clear up my problem of size of panels.

The first version, I use one TL074 insted of two TL082 that to be assigned by orginal circuit for reducing PCB size. It has 4 opams in it, So I use them for summing stage, anti log converter and oscillator. But this version use it for summing stage, anti log converter, thermal loop control and square converter. And one TLO082 is used as the core of produce sound as the original circuit assigned.

And thermal loop control is changed as based on application notes coming from National Semiconductor in april 1982. it named Audio Applications of Linear Integrated Circuits I use LM329 and reduce one tune pot.
And I change scaling pot to summing stage for temperature stable. And I add octave switch. The people who gathering at local Japanese bbs about analog synth help me so much! (thanks!)

The circuit diagram that these modification is added is here. PCB pattern is here. red line is see through from component side. Purple line is jumper line. It have 2 Power connectors. One is for power source and the other is for proved power to panel. if you put in your box then you don't need 2 power connnectors.
If there is without notice, all PNP transistors are 2SA1015 and all NPN transistors are 2SC1815

Tune


From up, Hi tune, Init tune, scaling
There is three point to tune. Init tune perform as same as course tune on the panel. if you have big wide keyboards and don't need octave switch then you don't need panel's course knob. Arrange them as you like for your panels or something.

Basic way to tune is ...

  1. Use sequencer or something and play octave phrase.
  2. turn "scale" trim to match to a octave tune for up and down. Chromatic tuner for guitar makes this operation easy. turning this trim makes detune. So use Init tune trim to make your operation easy.
  3. if it narrow for a octave, (upper tune is downer than you want) then turn your scale pot to Spread ward. It makes total tune downer ward.
  4. if it wide turn it to Narrow ward.
  5. If it ok, then change the phrase to up to 2 or 3 octave up and turn "hi tune" trim.
Try this scheme from step 1 to end two or tree times in other tune.
At last set init tune trim as you like in combination of tune knob on the panel.

Output


saw

square

pulse
I capture it in same condition of
"Wave form of the old analog synth"(Japanese only))click picture for larger one.
This oscillator produce wave form of the form which clarified more nearly graphically than any synthesizer.I wonder that I can use an oscilloscope now well.

Stability


27.6 Degree C

34.2 Degree C I Tune up to C

Degree C
I had tuned at 34.2 degree C. Since my a thermometer has not proofread, it's a reference value. I put in to Chromatic tunner for guitar and I check the drift of tune.

When it hot tune will go down and its goes to cold then it go up. But as Anti log converter works so well, there is no drift for octarve scaling.
I don't care for Oscillator part for temperature drifting so there is it. But it's OK if you tune it before playing.


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"VCO I" Copyright 2002, Motohiko Takeda, Crow Hill Laboratory
Motohiko Takeda Mail:takeda@aleph.co.jp