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Calculating Resistance for a passive 2-oct switcher?
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Bodega



Joined: Nov 02, 2008
Posts: 75
Location: Montreal

PostPosted: Thu Jun 11, 2009 4:02 pm    Post subject: Calculating Resistance for a passive 2-oct switcher?
Subject description: Anyone have any helpful leads?
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Hi all,

I'm thinking of making a much, much simpler version of this dc voltage switch:

http://www.wiseguysynth.com/larry/jlh-822/822.htm

It would just be a single-channel input, an on-on-on switch and an output, if I can swing it.

So, position 1 would be input=output, 2 would be an octave down and 3 an octave below that. 2 and 3 would have resistors at a given value. And that's it.

It wouldn't be buffered... unless someone can convince me why it should be.

Does this sound like a feasible plan?

thanks lots!
Matthew
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andrewF



Joined: Dec 29, 2006
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PostPosted: Thu Jun 11, 2009 5:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

i think it would work with an op amp summer circuit, something like the CGS DC mixer.
just have your switch wired up to put out 0V, -1V, -2V, -3V, assuming you are planning 1V/oct
and have both your DC voltage and switch output as inputs to the opamp.....
is this something like you mean or am i totally off track Confused
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Bodega



Joined: Nov 02, 2008
Posts: 75
Location: Montreal

PostPosted: Thu Jun 11, 2009 9:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

You pretty much got it. It's for a MOTM-style system, so yes, I'd like a 0/-1v/-2v switch, or something along those lines.

andrewF wrote:
just have your switch wired up to put out 0V, -1V, -2V, -3V, assuming you are planning 1V/oct?


That's what I was really wondering, is how to calculate the resistor values. A voltage divider is easy, but that's not exactly what I need. It seems I need a voltage subtractor. Does that make sense? I'm new to this, so maybe they're the same thing on some level.

I'm also unclear as to why a passive circuit wouldn't work, but maybe that's a whole other topic onto itself....
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andrewF



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PostPosted: Thu Jun 11, 2009 11:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

i think this thread will help you - here

Everything in the bottom left corner will do what you want - the two op-amps, Cv inputs, oct switch. The voltage reference lm329 could just be replaced with a voltage divider or 7805.
Also, as you want to lower the octave, connect the output of the octave switch to pin6 U2 and connect pins 5&7 together
Also change R10 to 1k and have this connected to your output.
maybe you don't need the tune pot and R2?

With passive mixers the inputs affect each other, having an opamp 'isolates' each input.

Shame we don't do 0.7V/octave, then you could just use diodes in series Wink
have fun


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Bodega



Joined: Nov 02, 2008
Posts: 75
Location: Montreal

PostPosted: Fri Jun 12, 2009 6:44 am    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
With passive mixers the inputs affect each other, having an opamp 'isolates' each input.


Interesting - that does actually clear things up a bit.

In that schem, all those resistors in the lower left are 1k? That would make things considerably easier.

I definitely want to keep it as simple as possible, so no tune pot. Why is the pot connected to +12v? Does that VCO require both +/-15 AND +12?
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andrewF



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PostPosted: Fri Jun 12, 2009 7:37 am    Post subject: Reply with quote  Mark this post and the followings unread

No I don't think it has to be 12V, probably just a typo.

I had a look around Motohiko's page a bit more,
there is a simpler version of the circuit here
http://www.aleph.co.jp/~takeda/radio/img/farmVCOII-scheme.gif
a bit easier to make.
you could run this circuit with +/-15V or +/-12V.
for the opamps, TL072 or TL082 will do the job.

You could probably leave out the tune pot, but the scale and oct tune pots are essential. these can just be trimpots on the PCB, hopefully once you set this module it won't need to be adjusted.

The 1k resistors should be metal and matched (buy 20 and test them with a multimeter to find 4 of the same value).

i think you can see on the circuit, the oct tune pot and 4k7 resistor form a voltage divider circuit with the 1 k resistors. The opamp is set up as a voltage follower (and inverter if you change the connections as i described), it protects your voltage divider outputs from being affected by other parts of the circuit.
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Bodega



Joined: Nov 02, 2008
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Location: Montreal

PostPosted: Sun Jun 14, 2009 3:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, Andrew!

Maybe I'll try and perfboard up one of those in the next few days and see how it goes. I'll leave out the tune pot and R2.

Am I right in thinking that I can take out R3/R4 if I only have one CV in?
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andrewF



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PostPosted: Sun Jun 14, 2009 4:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Bodega wrote:

Am I right in thinking that I can take out R3/R4 if I only have one CV in?

You can leave out R3 OR R4, the CV in will have to pass thru 100k before it gets to the opamp.

have fun!
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Bodega



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PostPosted: Sun Jun 14, 2009 8:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Duly noted. Thanks again!
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BananaPlug



Joined: Jul 04, 2007
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PostPosted: Fri Jun 19, 2009 7:37 am    Post subject: Reply with quote  Mark this post and the followings unread

For the matched resistors you might want to buy a resistor network. Digi-key 770-83-R1KP-ND for example.
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Bodega



Joined: Nov 02, 2008
Posts: 75
Location: Montreal

PostPosted: Fri Jun 19, 2009 10:58 am    Post subject: Reply with quote  Mark this post and the followings unread

That would make things easier. Nice one BananaPlug.
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Bodega



Joined: Nov 02, 2008
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Location: Montreal

PostPosted: Thu Jun 25, 2009 4:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

I just realised that I'll have to order a switch and more pots before I can start in on this one, but does this look right to you?

I put in the pinout diagram to remind myself to connect pins 4 and 8.


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Bodega



Joined: Nov 02, 2008
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PostPosted: Sat Jun 27, 2009 7:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Built it. It worked! Made up a little video. It's a bit dumbed-down for the general public.

http://www.youtube.com/watch?v=TkeUYXMFfrY

I ended up taking out the scale pot - I'd rather not mess with a master detune.

Nice little project for a Saturday afternoon.
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lilpaula87



Joined: May 11, 2020
Posts: 6
Location: Italy

PostPosted: Fri Nov 13, 2020 11:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Bodega wrote:
Built it. It worked! Made up a little video. It's a bit dumbed-down for the general public.

http://www.youtube.com/watch?v=TkeUYXMFfrY

I ended up taking out the scale pot - I'd rather not mess with a master detune.

Nice little project for a Saturday afternoon.


Great job! I would like to create a module with a couple of octave switch for my VCOs. As I’m a real noob could you be so kind to share your strip board layout? 😊
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