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CQ2 FM input questions
MUFF WIGGLER Forum Index -> Ciat-Lonbarde  
Author CQ2 FM input questions
The Grump
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?
Doublecoolbossman
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.
The Grump
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...?
Doublecoolbossman
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.
The Grump
Thanks for the heads up. Would you recall what that resistor value might be?
Doublecoolbossman
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.
The Grump
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.


nanners
pugix
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.
The Grump
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.
pugix
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.
The Grump
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?
pugix
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/digital-electronics/rc-low-pass-fi lter-circuit

But that wasn't what we were talking about.
The Grump


This seems to be working in an interesting way.

Thanks all for the advice!
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