ARP 2600 Voltage Processor--Part Two--Mixer/Inverter Clone

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Let's keep going with what we started last time: cloning circuits from the legendary ARP 2600 synthesizer--specifically it's voltage processor. 

The subcircuit cloned this time is its Mixer-Inverter:

A clone of ARP2600's "Voltage Processor's" top 3 sliders--a mixer/inverter

Original ARP2600.  We are cloning circuity in the bottom center.

Specifically this time we are cloning the functionality of these three 2600 sliders and 8 jacks!

Looking at the original ARP Schematic (here, page 29) the top three "voltage processor" sliders are tied to an inverting op amp based mixer. Simple enough..... 

So what's the big deal? I think ARP's take on this familiar circuit yields a lot of possibilities in a deceptively small amount of front panel space. Brilliant engineering from ARP, I think.

ARP'S DESIGN

The fun begins with the +10/-10 normals. This means with the sliders all the way to the left you end up with an output bias offset of 0V; the inverted output does just that: it inverts the signals you insert to the immediate left of the outputs.  

However if  you increase the pot's value, and/or put a different control voltage into one or both CV inputs, then mix it with an unattenuated input, you can alter the output's bias offset in extremely useful ways.  

Furthermore if you attenuate the unattenuated inputs (using a passive attenuator, or the ARP 2600's VCA, for example) now you have a 6 channel inverting mixer. Or gang 2 of these together to get more inputs....or use 2 in series to re-invert the inverted signal....or...

EURORACK PORT

The original 2600 design has 3 faders, two with attenunated CV normals, another with a normal Keyboard CV so you can quickly realize a Weather Report inverted keyboard vibe.  But--how to best represent this within our beloved Eurorack footprint?  

In my rack I have to save space so I didn't use sliders. And there was no real way to duplicate the "keyboard" normal in my modular setup. 

Instead, I decided to make 2 variations--one that has the pots vary the +CV/-CV normaled inputs--ARP sliders 1 and 3; the other allows you to attenuate the input signals--a dual version of the 2nd slider from the top. 

To get the full 2600 vibe, you might want to build both?

The photos from this post are the latter variant.  The other has the CV and Mix inputs flipped on the front panel and main board.

On to build photos:

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Each variation requires 2 PCB's, main board and front panel.

The op amp is SOIC--not too tiny....

The pots I use for these Euro projects have a longer shaft than will the knob can, so I cut them to size with a Dremel cutting tool:

The trick to cutting the pots' shafts without shattering a bunch of cutting disks--it's time consuming to replace the cutting disks when they break--was to cut one side, then the other, than back to the first. I did this over and over until maybe 1mm existed between the cuts.  

Then, I used pliers to twist off the remaining metal that is not needed.  

After the knob fitted flush with the front panel.

Ready to test!!!

"Seems working"

Ah, an inverting mixer!  It works, but I have thought of improvements to the main boards' layouts to make fabrication easier.

I am going to redesign 2 of the PCB's then go to PCBWAY with hat in hands to see if they will get me some improved boards.  

Then I will create a "Part III" post and make the gerbers and associated files available to you, my faithful readers.  

So!!  Upcoming--improved ARP mixer/inverter clones, and ARP 2600 sample and hold/2600 audio switch subcircuit work-alikes.  Should be ARParrific!  Until then, don't breathe the fumes.