50240 top octave synth build

theshaggyfreak · Joined: Oct 29, 2006 Posts: 48 Location: Centreville, VA

So, I decided to go ahead and keep a couple of 50240 chips that I have instead of selling them. I found my old copy of Mimm's Engineer Notebook which has a fairly simple design to use it (You can actually view it via google here). Since I have two of these chips, I've been trying to figure out the best way to use them. I've been possible thinking about setting them up in a sort of parallel configuration where I'll build two of the same circuit and both will trigger when I press a key.

What I've been trying to figure out is what would be the best physical mechanism to trigger each of the 16 notes. I haven't seen any key beds that small available for purchase and I would like something that's a bit easier to use performance wise. If no one sells a key bed that would work, are there some type of large push buttons out there? I'd really love to put these two 50240s to use instead of just letting them live in the blister packs.

urbanscallywag · Joined: Nov 30, 2007 Posts: 317 Location: sometimes

I'm trying to imagine how 2 top octave divider chips would sound in parallel. I don't think it will provide any particular advantage, or may even sound bad.

theshaggyfreak · Joined: Oct 29, 2006 Posts: 48 Location: Centreville, VA

urbanscallywag · Joined: Nov 30, 2007 Posts: 317 Location: sometimes

OK I see the tuning deal now.

slabman · Joined: Sep 01, 2005 Posts: 102 Location: UK

If you can find them, Don Lancaster published a series of circuits in the 70s for CMOS modules that could be combined to form a complete Top Octave chip-based synth. If I remember right, the Top Octave chips fed divider chains. These went into CMOS switch keyers that were fed from global Attack & decay pots to give polyphonic envelope shaping. There were also 4046 phase locked loop 'trackers' that tracked the digial signal but, by adjusting the loop timings, would do portamento & glide. Possible Lunetta crossover!

theshaggyfreak · Joined: Oct 29, 2006 Posts: 48 Location: Centreville, VA

Interesting. I just ordered a used copy of the TTL cookbook and the CMOS cookbook. I was able to find them for next to nothing at half.com Smile

Dan Lavin · Posted: Tue Nov 11, 2008 7:33 am Post subject:

Probably the best use of the TOG was Craig Anderton's Organtua and John Simonton's Stringz n Thingz. Both were sold by PAIA in the late 70's and early 80's. I used to gig in college during that time with little more than a PAIA Stringz N Thingz and an Oberheim SEM with an EML CV-gate keyboard.

I think the Stringz N Thingz would be difficult to make from scratch as the pcb's are absolutely huge. The organtua wouldn't be too bad, but you do need a double bussed keyboard: one for the tones and another for the gate/trigger signal. The organtua used 2 TOG's.

I do have a pdf of the Don Lancaster articles mentioned which I found via a google search. If you can't find it yourself, PM me and I can send it.

Tim Servo · Posted: Tue Nov 11, 2008 11:54 am Post subject: 50240 top octave synth build

I'd probably rig up some analog switch ICs to gate the notes on and off. That way, you could control them with a variety of things (mechanical switches, logic, etc.). You could also do the trick of running the square wave into the switch control input, and run a DC voltage into the switch input. This way, you get volume control via the DC voltage at the switch in (the square wave essentially gates the DC voltage on and off) - a free VCA for each note!

Other than that, you might want to look up some schematics for the old PAiA "Chord Egg." It was a gizmo that used a top-octave chip and some logic to slowly fade in and out through some pre-selected chords. Sort of "space music in a box." Someone on the list mentioned having one, and it looks interesting. Also, you should be able to find the schematics buy buying a $5 back issue of Polyphony from PAiA. I'd imagine that two Chord Eggs and a little reverb + chorus might sound pretty cool!

Tim (has "space music" in his head) Servo

Tronato · Posted: Tue Nov 11, 2008 12:20 pm Post subject:

Hello!

About a year ago I got a hold of a couple of the 50240 ICs with building a Paia Stereo Chord EGG or two in mind but have not been able to find any PCB... I had one a couple of hundred years ago and it was really cool.. it's a pity it got lost through time...
If someone comes up with an extra PCB or is willing to design one let me know!
I have all the info but suppose it cannot be posted here...

Thanks!

TRON

RF · Posted: Tue Nov 11, 2008 12:35 pm Post subject:

Here's a parts list and single side board layout...and a bunch of info...on the Chord EGG.

Dang - If I had one of those IC's I'd make one of these...

bruce

egg.pdf

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Tronato · Posted: Tue Nov 11, 2008 1:08 pm Post subject:

Hello!

The article published is one of the ones I have... I also have the Original Paia manual that came with the kit.
Can that be published here?
Let me know if so...

Thanks!

TRON

richardc64 · Posted: Tue Nov 11, 2008 1:10 pm Post subject:

richardc64 · Posted: Tue Nov 11, 2008 1:18 pm Post subject:

theshaggyfreak · Joined: Oct 29, 2006 Posts: 48 Location: Centreville, VA

loss1234 · Posted: Thu Nov 13, 2008 6:35 am Post subject:

shaggy...where did you get your chips?

theshaggyfreak · Joined: Oct 29, 2006 Posts: 48 Location: Centreville, VA

Dan Lavin · Posted: Thu Nov 13, 2008 7:09 am Post subject:

Shaggy,

The fatar keybeds can be used, but you'll need to do some cutting and shorting of traces.

Fortunately, Fatar uses 2 switches in parallel. Each switch closes to a common bus called BR or MK. The BR's could be wired together in series to create a gate buss and the MK's could be wired together in series to create the audio buss. On the MK's, the diodes would need to be replaced with mixing/summing resistors and the connections between every 8 keys would need to be cut. Also, on the BR's input, they would all need to be shorted together and tied to +5V for the gate signal (or maybe+12V or whatever Organtua used as a gate signal).

Be prepared to use a lot of wire. That's what keeps me from doing another TOG project.