CQ2 FM input questions

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The Grump
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CQ2 FM input questions

Post by The Grump » Fri Dec 01, 2017 7:03 pm

If I understand this correctly, the FM inputs on the Quantussy take an input voltage and either increase or decrease the frequency of the associated triangle oscillator. The internal voltage scheme is 0-+9v, so an input voltage of +4.5v should have no effect on the frequency, less than that lowers the frequency and more than that should raise it. Do I have this correct? That being the case, can relative "tuning" tables be used to drive the oscillators to bring forth some semblance of a melody?

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Doublecoolbossman
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Post by Doublecoolbossman » Fri Dec 01, 2017 9:03 pm

You will likely need a resistor between your voltage source and the fm input to have a good range.. But yes, you can bring forth some melody.

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Post by The Grump » Fri Dec 01, 2017 11:01 pm

Doublecoolbossman wrote:You will likely need a resistor between your voltage source and the fm input to have a good range.. But yes, you can bring forth some melody.


If the grounds are connected, why would you need a resistor? To reduce amperage or...?

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Post by Doublecoolbossman » Sat Dec 02, 2017 8:34 am

Something to do with CL looking for changes in current rather than voltage if I remember correctly (I may be wrong about that). The most valid thing that I can say is that it worked better with a certain value resistor for me.

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Post by The Grump » Sun Dec 03, 2017 5:19 am

Thanks for the heads up. Would you recall what that resistor value might be?

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Doublecoolbossman
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Post by Doublecoolbossman » Sun Dec 03, 2017 8:41 am

No problem! I believe I tried 10k, 100k, and 400k, and the 400 worked the best. I suppose it also depends on where your input is coming from. There also must be a way to use a potentiometer to figure it out, but again I don't really know what that is. I would just experiment. Im hoping someone with more electronics knowledge will chime in.

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Post by The Grump » Tue Dec 05, 2017 5:05 pm

Doublecoolbossman wrote:No problem! I believe I tried 10k, 100k, and 400k, and the 400 worked the best. I suppose it also depends on where your input is coming from. There also must be a way to use a potentiometer to figure it out, but again I don't really know what that is. I would just experiment. Im hoping someone with more electronics knowledge will chime in.
:nana:

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Post by pugix » Tue Dec 05, 2017 5:29 pm

The Grump wrote:
Doublecoolbossman wrote:You will likely need a resistor between your voltage source and the fm input to have a good range.. But yes, you can bring forth some melody.


If the grounds are connected, why would you need a resistor? To reduce amperage or...?
Yes, you need a series resistor to reduce the current. I think the point about Mido (4.5 volts) being the center point may be right. I'm going to try a test by putting a resistor between a low impedance voltage source (most modular CV sources) and a Quantussy CV input and see if varying the resistance when the input is 4.5 volts will produce a change.

You can easily prove that a zero volt source will act as a current sink by patching from the ground jack into one of those CV inputs. The frequency will be lowered. Peter even put this in a manual as a technique for getting the Coco delays to go really slow.
Richard
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"Everything in our world is actually always modulated by everything else." - Peter B

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Post by The Grump » Wed Dec 06, 2017 6:08 pm

Thank you for the insight, I will try using a pot inline to how that affects things. I asked Peter about this as well, and his response is as follows:

" 4.5 should be about no change. you
can put a capacitor in the line if you want to modulate that zero point
without dc (fm). you can put a resistor in the line too, it will attenuate
attenuverter affect. cheers! -Peter"

I'm trying to make sense of this, and considering what happens with both a cap and pot inline.

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Post by pugix » Thu Dec 07, 2017 9:35 am

The Grump wrote:Thank you for the insight, I will try using a pot inline to how that affects things. I asked Peter about this as well, and his response is as follows:

" 4.5 should be about no change. you
can put a capacitor in the line if you want to modulate that zero point
without dc (fm). you can put a resistor in the line too, it will attenuate
attenuverter affect. cheers! -Peter"

I'm trying to make sense of this, and considering what happens with both a cap and pot inline.
If you put a capacitor inline, the AC coupling makes it into a high pass filter. Meaning that below a certain frequency the level drops off. Cutoff point depends on the values of capacitor and (internal) resistor. For example, if you put a pot there, you'll only see an effect while turning it.

I used the capacitor/resistor approach to making a module out of a Tetrazzi board, to replace the bar inputs with CV input jacks. With a big enough capacitor, normal envelopes couple through nicely.
Richard
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"Everything in our world is actually always modulated by everything else." - Peter B

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Post by The Grump » Thu Dec 07, 2017 1:49 pm

So, if I take the output of the cap and invert its phase, then add it back to the dry signal, will it function as a low pass by phase cancelling the higher frequencies?

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Post by pugix » Fri Dec 08, 2017 9:59 am

The Grump wrote:So, if I take the output of the cap and invert its phase, then add it back to the dry signal, will it function as a low pass by phase cancelling the higher frequencies?
There's a very simple way to make a low pass filter with a resistor and capacitor. Example:

http://www.daenotes.com/electronics/dig ... er-circuit

But that wasn't what we were talking about.
Richard
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Post by The Grump » Sat Dec 09, 2017 8:04 pm

Image

This seems to be working in an interesting way.

Thanks all for the advice!

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