ARP 2600 Voltage Processors--Part One--Eurorack Lag Processor

DiWHYers: If you want to build this eurorack module, you can get schematics, gerbers, a BOM, board/schematic layout, and more, from these guys; their community (here), and this project page. 

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Hello again, I went to a geeky synth meetup recently--we went through the ARP 2600 patch book. I wasn't expecting much--I already know all this right? Wrong! The patch book is a wealth of great ideas and covers a lot more than patching a vintage ARP 2600 or 2600 clone. For instance, I didn't know you can create a high pass filter by inverting a lowpass filter output signal and summing it with the original? That's in the book...

We ran out of time...I wanted to keep going through the book, but don't have room for a clone or a vintage ARP 2600 (even if I had USD $10K+ to spend....)

I have 2600-like VCOs, VCFs, VCAs, Reverbs and so on, already in my rack however. 

This time: an ARP2600 lag processor clone

But! I don't have the 2600's voltage processors....these subcircuits are an important part of the 2600's cool identity:

Original ARP 2600 Voltage Processor

With that long ado: in this and coming posts, I will create clones or work-alikes for the 2600's voltage processors. This brings some 2600 coolness to my rack without spending a box of money.

This time: the lag processor.

Dual ARP Lag Processor Clone--done, works! 

The original ARP schematic can be found here, go to page 29; subcircuit is in bottom left corner.  

It's a familiar design, "lag" here means "glide". See my portamento post here for more information about how the design works. 

However, two components ARP used in the 2600's lag processor were unexpected--a .1uF cap and an A1M pot. This means very short portamento times even when the A1M lag pot is "turned to 11". 

But on the clone PCB as well as on an original 2600, the "lag" processor sounds good to my ears, an "Oh Lucky Man" outro vibe.

Lag boards! Shout out to my sponsor, PCBWAY, for helping out.....

Time out--I need to do my usual shameless plug for PCBWAY, this blog's sponsor. They never have dictated what I post and are extemely friendly; I owe them my gratitude. Please visit their site and community page--doing this helps me keep creating these projects and posts. 

Back to it....

ARP 2600 LAG PROCESSOR DESIGN

The module clones the original ARP design, with a few minor changes discussed shortly.

If you build this module, you will need 2 PCB's: a front panel and "main board" in 4HP Euro skiff config. Consider Fabricating the front panel in black with white silkscreen; make sure to check the box "do not use an order number on the PCB".

I used a single TL072, the 2600 uses LM301's; I figure any dual DIP op amp will work. 

My clone also uses a 1K resistor in series with the output jack. This is not on the original design; I put it in there to keep a short to ground (a dangling patch cable that shorts against a metal enclousure) from drawing too much current and possibly damaging an op amp or other components.  

This means the voltage at output might diminish a bit as the lag value increases. This is caused by forming a voltage divider between this 1K resistor and the load resistance of whatever you plug the lag processor into--this may not be what you want. 

If not, put some jumper wire in instead of R3 and R10 and try to be neat when patching or (aguably) use 100ohms.  

Using this protection resistor (or not) is more complex a topic than I initially thought. A forum post about this resistor, to use or not to use, what value should it be, etc., is here.

BUILDING THE MODULE

No surprises here, one mistake is on the front panel, LEV2 is the top pot, not the bottom one (I kept changing this and ended up getting it wrong). This is fixed on the gerbers ready for download at PCBWAY (again, get gerbers here). 

Beyond that the dual lag processor can be built in maybe an hour? Something like that. This is an easy build.

Ready to test....

"seems working...."

MODIFICATIONS AND PATCHING IDEAS:

I have to remind myself to use this lag in unexpected places. The patch book has some ideas. 

For longer lag times increase the value of C3 and/or C5, and/or change the value of the "Lev2" and "Lev1" pots. I left mine at A1M pot and .1uF to match values found in the ARP service manual.

There are two lag processors in this 4HP design, the ARP2600 has only one in its voltage processor section. It would be easy to modify the circuit to normal out one to out 2, putting the two in series, giving you longer portamento times. Easier: use a short patch cable. Viola!

 

OUTTRO

Next time I will clone the Mixer/Inverter section of the voltage processor--an interesting single op amp design that is unique to the 2600 and its clones as far as I know. Until then, don't breathe the fumes....