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FarmVCO II


More Modification to VCO I

I got temperature coefficient resistor (TCR). I planned modifying VCO I using TCR.

In VCOI, I used CA3086 as component for anti log converter. It has 5 NPN transistors in it, and 2 of them are combine with emitter internally. I use this as dual transistor and the others consist as thermal loop. One transistor feed back temperature drifting to the other that be used as heater .And it change the temperature of whole chip. I use it so that a drift may not be carried out.
But In this VCO, I use just a small Dual Transistor with TCR. And I omit whole thermal loop blocks to get it simple. this modification makes it easy to build it again.
NPN PNP
2SC3381 2SA1349
2SC5168 2SA1927
2SC1583 2SA798
Japanese Semiconductor makers are going to stop to create this kind of small signals transistors.It may become hard to get it. It's not easy to get even now, But I put the list of the transistor that we can get in Japan. There is the site of online shop dealing with it in October,2002. From now on, It may be getting easy to get from international online shopping or something.
I build VCOII with 2SC5168 that I found in "akihabara".

Home page of this transistors maker said that they still make this. But it's not easy to get in Japan.So somewhere some shop I don't know sales this chip. The maker that had been making 2SC1583 or 2SC3381 said they stoped to make them. Some oversea resaller still have some of them. But it should be going to be hard to get.
Someday We must switch to other chips.

Adding Octave SW


There is TCR and 2SC5168 pasted together in the middle of picture.
Removing thermal loop, I get one opamp on the board. So I use it to voltage follower for voltage of Octave switch.

For performing music we should tune VCO. First, you turn course knob to define with about tune. Then you use fine knob to fit your tune to other VCO or musical instruments. This way creates so wide range tune easily. But I think that it not easy to perform music.

If you use VCO for only sounds then it's enough. But when you want to use it for musical instruments, you want to switch tune per octave. and it makes to be enough for one knob to fit tune.
So I add VCO I octave switch that performs high, mid and Low. Basically I tune by course knob. if I want to change octave when I'm playing music then it performs so well.
I still have course knob. But I can't omit this knob. Because 3 octave switch is not enough to perform some music.
If I can switch 5 octave by it then it's enough and I can omit course knob.
This is my case.


1/2nn2n 4n 8n
2 para 1 only 2 seriase 4 seriase 8 seriase
In VCO I, I switched octave by changing voltage level to anti log circuit. It needed log curve. So I use old technique to build log ladder. I select register having same values and combine them together to make values that be needed.
But in this way, if you want 5 octave changing you need so many numbers of registers here. It's same as other part of this VCO. I think that this way is ok for 3 octave. If I want more changing, I should better to select another way. So in this case I made normal ladder and put it in voltage follower and put it to summing stage as same as other modulation sources.

the circuit diagram that added this modification is here.
Use 1k ohm metalized film register that has no difference each other as ladder.
PCB pattern is here

I use Japanese transistors. It has different pin structures from over sea one.I use 3 dual opeamps to make it easy to build.
If there is without notice, all PNP transistors are 2SA1015 and all NPN transistors are 2SC1815

Tune


there is 4 tune point. the basic tune is same as VCO I. see it

To tune octave switch, Set rotary switch to most high position and set your DVM to U2's 6pin and turn trim to 4.00V. Next, You can use this switch to scale tune.

Output


Saw

Squre
I modify anti log block only, so there is no difference with VCO I.


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