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Working on a 3-voice 40106 oscillator. Need help...

Explorer

Mar 28, 2014
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I’ve got each oscillator feeding one into the next with a diode and resistor in between each. (The schematics below recommend 1Ks.) But, there doesn’t seem to be agreement out there on which way to hook up the diodes.

The last schematic here shows the cathode going towards the second oscillator:

The third schematic here shows the cathode going towards the first oscillator:

I’ve constructed a little keyboard with multiple trimpots in the circuit of the third oscillator and am trying to figure out why my harmonically-strange higher pitched notes are so much lower in volume than lower pitched notes.

Are those diodes (and/or the 1K resistors) really necessary between the oscillators? I’m wondering if the slight voltage drop across each one is causing my voltage issue.

Thanks for any and all help!
 

kellys_eye

Jun 25, 2010
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Since the signals are all logic level there should be no difference in actual amplitude between them - other than your own ears ability to hear lower frequencies better than you do the higher frequencies.
 

Explorer

Mar 28, 2014
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THANKS to both of you. But, I’m still wondering about the absolute necessity of the diodes between oscillators and which way they should be wired, if they are needed.
 
Last edited:

bertus

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Nov 8, 2019
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Hello,

The diode is probably there to avoid current going back into the left oscillator.
You might also be interested in the following page:

Bertus
 

Explorer

Mar 28, 2014
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I emailed bbob Drake at fluxmonkey.com and he kindly sent this reply:
———
oh gosh, i am so sorry... that page has been up there for years, and no one has noticed my error! you are correct, the cathode (banded end) of the diode should be connected to the second oscillator, or the output. I will get that fixed later today.

diodes are like one-way streets: they let current flow in one direction, but not the other. think of the schematic symbol as an arrow, indicating the direction of current flow... so in http://fluxmonkey.com/electronoize/40106Oscillator_files/image014.gif, my intention was to have the left-hand oscillator's output flow towards the right-hand oscillator, modulating it. that's what the hackaday version does. i think if you build it according to my schematic, it'll still make sound (which may be why nobody's caught it before), but probably at a different volume level or something.

the idea with the mixer circuit is, to try to keep the output of one oscillator from flowing backwards into the other oscillators and influencing them. with the diodes, 3 one-way streets all converge at one intersection and then go out; with resistors, the input of your amplifier probably has lower resistance than the mixing resistors, and the signals flow towards the path of least resistance. you can use either approach, they will sound different, diodes probably provide more isolation.

the oscillators put out a very strong signal, much more than the 1.2v that a line-level in is expecting, so you can afford some voltage drop with whatever mixer you use. not sure about different levels for different notes... maybe try them one at a time so see if the levels are uneven prior to mixing.

i think i did a better job of documenting all this on my later PCB board pages, http://fluxmonkey.com/pcbDocs/40106Oscillators.htm, and http://fluxmonkey.com/pcbDocs/40106_files/Flxmnky_40106HexOscillator.pdf, maybe take a browse there. at least, it looks like i didn't make the same mistake with that schematic!
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Perhaps that will help others working with 40106 circuits. A schematic error. It’s not the first time I’ve had problems sorting out, for instance, circuit board layouts or build documents and schematics that didn’t agree with each other. What a joy when you find reference material in easy-to-understand plain English, with well-drawn schematics, everything matching and nothing left to guess about. Or authors like Mr. Drake, above, who quickly reply to queries. Maybe I’ll ask the question here later that I never got answered at Hackaday.com, re: a chip featured in one of their Sweet Logic articles that I have NEVER figured out how to work with. Onward! —Explorer
 
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