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 Chord EGG 2015 
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Joined: Thu Sep 24, 2009 2:14 pm
Posts: 90
Post Re: Chord Egg 2015
Was wondering what frequencies you plan on using for each of the 8 oscillators? For a "top octave" wouldn't U want 12 or 13 oscillators?

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Sat Oct 24, 2015 4:24 pm
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Joined: Wed Dec 09, 2009 9:41 am
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Location: Moorefield, Ontario, Canada
Post Re: Chord Egg 2015
The original Chord EGG only alternates between four chords in one key (C-E-G, F-A-C, D-G-B, and E-G-C'), so only 8 notes are needed (C, D, E, F, G, A, B, and C').

Unlike the original Chord EGG, which used a top-octave synthesizer chip generating an equally tempered scale (of which only 8 of the 13 notes were used), mine has separately tunable oscillators, so I'm going to tune them using just intonation (specifically, Ptolemy's intense diatonic scale). This should give slightly nicer sounding chords.

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Last edited by stefanv on Sun Oct 25, 2015 8:21 am, edited 1 time in total.

Sat Oct 24, 2015 9:03 pm
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Joined: Thu Sep 24, 2009 2:14 pm
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Post Re: Chord Egg 2015
stefan, very interesting approach! hear forward to seeing your mp3.

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Sat Oct 24, 2015 10:38 pm
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Location: Moorefield, Ontario, Canada
Post Re: Chord Egg 2015
I just finished wiring and testing the part of the Chord EGG 2015 that I'm most proud of, the chord randomizer. First, here is the original PAiA chord randomizer circuit:

Image

Two low frequency oscillators at different rates generate a 2-bit random number, which is then decoded to one of four outputs using IC2C, IC2D, and IC3 (these are all NOR gates, not OR gates as they were drawn).

I decided to see if I could do it with op-amps instead, and this is what I came up with:

Image

The two oscillators are basically the same (with a slightly different topology since I'm using standard op-amps, not Norton amps). The 2-bit to 4-line decoder is decidedly different. :-)

R145 and R150-152 form a 2-bit digital to analog converter. IC3, R153-164, and D101-106 form a 4-level decoder. Depending on the input voltage, only one of the four outputs goes high. The voltage divider resistors (R153-157) are chosen to yield thresholds half way in between the possible output voltages of the D/A converter. The diodes and R162-164 provide negative feedback between the stages to ensure only one output is high.

Here's a photo of the above circuit (with a few other components visible as well). IC3 is the one on the left:

Image

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Sun Oct 25, 2015 6:19 pm
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Post Re: Chord Egg 2015
Do you think the oscillators will be stable and always return to the set frequency after power up? Guess U will be using a bi-polar supply? plus and minus 12? One of the nice things about the EGG is that it ran on a single voltage.

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Wed Oct 28, 2015 2:26 pm
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Location: Moorefield, Ontario, Canada
Post Re: Chord Egg 2015
I'm not too worried about absolute stability. They are bound to drift with temperature. However, they should all drift about the same amount, so the relative tuning between the notes should be quite stable.

My redesign uses a single 12V supply, just like the original.

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Wed Oct 28, 2015 2:40 pm
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Post Re: Chord Egg 2015
First recording!

http://www.stefanv.com/media/2015/10/ChordEgg2015.mp3

Photos of the rest of the construction to follow when I get a chance.

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Wed Oct 28, 2015 8:50 pm
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Joined: Mon Sep 14, 2009 7:43 pm
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Post Re: Chord EGG 2015
I am curious if you intend to allow control voltage inputs and outputs in order to be able to integrate the randomness into a modular setup. It would make a really cool Frac module!


Thu Oct 29, 2015 10:41 am
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Location: Moorefield, Ontario, Canada
Post Re: Chord EGG 2015
No, it's pretty much a direct remake of the original functionality, without using any hard-to-get parts.

However, it ought to be fairly easy to build a subset of it (for example, the note randomizer), for use as a pseudo-random control voltage source.

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Thu Oct 29, 2015 10:57 am
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Location: Moorefield, Ontario, Canada
Post Re: Chord EGG 2015
Here's a photo of the completed board (click to see full size):

Image

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Sun Nov 01, 2015 9:05 pm
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