Sunday, 22 March 2015

Serge VCS Modification

Modifying the Doepfer A-171-2 Serge VCS for more extreme non-linear curves and more manual control over rise and fall times.

The Serge Voltage Controlled Slope is a classic design and an integral part of many modular musicians’ systems. The Doepfer A-171-2 is a faithful recreation of the original circuit. It works just like the Bananalogue VCS that has been in my case since pre-Maths times. That’s good, and bad.

The VCS easily allows one to set different contours for its rise and fall phases. But its non-linear curves are not as extreme as those possible with Maths. Setting times on the VCS is harder, as the useful range is limited to about 20% of the potentiometer’s throw. It’s these differences between these two similar function generators that this modification tackles.

The feedback that generates non-linear shapes is pre-wired on both the VCS and Maths. Calibrating the VCS output voltage from 5V to 8V has only a marginal effect. But if one patches the VCS envelope output to its ‘Both CV’ input with VC Rise + Fall set to linear, the Serge yields the desired curves. This suggests the internal CV feedback loop is capped.

Indeed, if we look at the left of the schematic, at switches SWF & SWR we find 330K resistors limiting the amount of feedback to the CV mixer to about 30%. If we lower this resistance we’ll get more feedback. I desoldered and replaced the 330K resistors with 200K but you could also try soldering another value in parallel e.g. 150K (= ca. 100K, 100%) or 470K (=ca. 200K, 50%).

This part of the modification helped clear up an oddity about the VCS: namely, why making the curves more exponential actually increases the overall envelope time. Negative feedback should make it shorter, and vice versa.

If we study the same part of the schematic again, we can see -12V across a 1M resistor feeding the summing points. Given the gain ratio set by the 100K resistor, this offsets the rise and fall rates by +/-1.2 volts. I find this counter-intuitive, so I removed both 1M resistors.

Both my Doepfer and Bananalogue modules are fitted with logarithmic potentiometers to manually set the rise and fall rates. This means, when using the VCS as an envelope or slew limiter, changes in the first 50% of the pot’s throw are imperceptible. Typical envelope settings lie between about one and three o’clock. Tapering the A50K potentiometers on the A-171-2 with a 5.6K resistor between the CW/ ‘hot’ lug and the wiper solves this. The useful range now spans from nine to three o’clock.

To solder these in place, you’ll have to unscrew the jacks and remove the board from the faceplate. I tape Gaffa around the ends of my pliers to avoid scratches. While you’re there, you can measure the output between the A50K wipers and the subsequent 82K resistors to understand how the log pots choke the voltage. I did try an S-curve taper with two sets of resistors but the quasi-linearization suggested here by Daverj worked best.

So, what does it sound like? Here are two recordings:

feeback mod: exp-fall, unmodified, 0:08 modded, exp-rise, 0:16 unmodified, 0:23 modded.

pot taper mod: cycling, rise = zero, fall manually altered. Stock VCS then modded at 0:39

These simple changes have given my VCS more whip and made it easier to use. Thanks to Dieter Doepfer for helping me read his PCB layout, Ken Stone for publishing his schematic, Tim Stinchcombe and Dave Jones.

If you’d like to try this yourself, take the usual precautions to avoid damage to yourself or your module. I will not be held responsible. If in doubt, ask Doepfer or your technician to carry out the modifications for you.


sergio said...

nice one! the pot mod is super necessary on this one..
i have a question, is there a big difference between taking the output and feeding it back to the expo in and making the feedback mod?


Navs said...

Cheers, sergio.
The normal inputs (not the expo) go to the CV mixer via 100K, so the strength of feedback when patched externally is even greater. I've been doing this with my Bananalogue and just wanted to avoid the extra cable. 1:1 seemed like too much so I settled on 50% or 200K. I use the expo input for keyboard tracking: higher notes, shorter etc.
HTH Navs

sergio said...

ok, i see! thanks for replying!
if i may ask you: how do you calibrate the vcs to output 8v instead of 5v?

thanks again for the great info!

Navs said...

There is only one trimmer on the PCB, that's what you need to change. I set my VCS cycling and hook it up to an oscilloscope. If you don't have one, measure a slow envelope with a multi-meter.

Anonymous said...

Thanks for posting this, I just applied these mods to my A-171-2 and they transformed the module from being "meh" to awesome!
I used what I had around, 5.1k resistors across the pots and 470k resistor in parallel for the feedback mod (also removed the 2 1Meg resistors), and the envelope curves and pot response are much better for me-- closer to Maths, but a bit different, which is what I thought the module would be out of the box...