ARP 2600 Voltage Processors Part Four dot Two--DG401 Analog Switch Experimenter's Board

If you're interested in a DG401 experimenter's board--hear this useful analog switch IC for yourself--go to this blog's sponsor, PCBWAY....check out their Community Page....download the gerber for the PCB discussed in this post here.  Have fun!!!

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So whattzup.....this morning I am at the bench creating a work-alike for the Electronic Switch in the legendary ARP 2600 synthesizer. 

For a background for the project skim the previous post here. 

I wired up the toggle flip-flip and buffer breadboards from last time to an experimenter's PCB centered around the DG401 CMOS analog switch IC--the board courtesy PCBWAY.

With everything wired up I could hear the DG401 switch between >10V P/P waveforms.  Works!

DG401 test rig: works. Clockwise from left: DG401 experimenter's PCB, Siglent test gear dookie, the T-flip flop breadboard and the buffer breadboard. 

Here's the signal flow for the test:

A clock signal (from my Siglent signal generator or a bench Euro LFO--didn't matter) went into the buffer circuit described last post:

This feeds the Toggle flip-flop:

I found the circuit worked better with 6-8V to pin 16 of the CD4027....

The flip-flop's two outputs--basically a square wave and its inversion--go to data IN's--DG401--pins 10 and 15:

For S2 and S1 I buffered the incoming audio (or CV, up to you) with a unity gain op amp; for output pins 1 and 8 I mixed pins them together using a standard 2 op amp non-inverting summing mixer.

Basic idea: non-inverting summing mixer....to compute gain, get resistor values here.

The experimenter's board also had an external clock to logic subcircuit....get a simulation of that here. 

The circuit fragment below was not needed for the experimenter's board but I threw it in anyway. It allowed an external clock signal--say an LFO's square wave--to be split into a logic high (A) while (B) is low, then visa-versa on the next clock edge--the same idea as the output of the T flip-flop.  

However, the CD4027 and its buffer seemed a bit more tolerant of unusual (?) signals to activate the switch; for the final build I will probably use the flip flop, even though it requires more components.

Here are some useless build photos:

Happiness is new boards from PCBWAY!

  

So how did the DG401 sound? Well.....GREAT. The DG401 switched super fast, no audible clicks, pops, or oddness. Remarkable, really. And you can still find it for sale in P-DIP....and it's not too expensive....

In terms of making this switch switch--I had to do what its datasheet says--S and D are normally disconnected, to make them connect hit the logic pins ("In") with >= 2.8V.  

This meant I needed to power the CD4027 with  >6V or the CD4027's "Q" output was a bit too low.  

So what's next? I will make all the parts so far into a compact Eurorack synthesizer module. One of these days....Stay tuned for Part 4.3. Update: Done! Works! Post is here.