Chord EGG 2015

[toneman]

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?

[stefanv]

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.

[toneman]

stefan, very interesting approach! hear forward to seeing your mp3.

[stefanv]

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:



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:



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:

[toneman]

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.

[stefanv]

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.

[stefanv]

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.

[Kyhotay]

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!

[stefanv]

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.

[stefanv]

Here's a photo of the completed board (click to see full size):