Fun With Filters (Pt.1)

 Not So Brilliance

As you saw in the Concert Mates post I'm very much thinking about filters at the moment. The SS-30 Brilliance control isn't only not a VCF it's also not even an active filter and is in fact just a single capacitor. The SS-30 is, lets not forget, a strings synth and not a proper synth.

The SS-30M though is a not only M for MIDI it's also M for Modular (where possible) and one of the enhancements I've recently added is a send-return (or insert) for each voice section. That means I can insert a filter and play that paraphonically just like in those video in the last post, but as part of the SS-30M signal path and straight into the Orchestra effect.


I have a bunch of different filters at my disposal too. Not to brag, but, as well as the Moog MG-1 filter I can call on the filters of the Roland MC-202, System 500 & System 1M and the Mutable Instruments Braids, and also the 303 clones from Freebass (MAM) and Cyclonic, which are all very much in the style of the Roland sound, or thereabouts. I've also got a modified Korg Monotron which can serve as a filter too. Nothing from Yamaha though.

I do have a CS1x and a CS Reface. Both are digital, of course and neither has a filter input.

I Have No (YAMAHA) Filter

Eurorack are modues exist for just about any style of filter you could want. Roland style, Korg and Moog, Oberheim, EDP WASP, Polivoks, EMS, Steiner-Parker, ARP, and so on.
Look for a Yamaha style filter and only three options exist. Studio Electronics SE88, Old Crow's RR480 and the Gotharman CS. The SE88 is a full-featured, several hundred dollar module that takes the CS-80 filter design and runs with it. It's based on the SE Omega CS-80 filter board, which is clearly a standard set of OTA and Op-amp ICs. The Gotharman used actual Yamaha chips, which you had to find yourself, and was made in very limited numbers. Old Crow's RR480 is also a recreation using standard components, but it's DIY only.

The Yamaha synths almost exclusively used a single VCF chip - the IG00156. This was the core of the Gotharman module. The CS range of synths all shared a common chip, but the overall sound changed over the years. By adjusting the external components it was possibe to change the character of the filter. It's not clear which one the Gotharman went with, but I guess the CS-50/60/80. The SE88 and RR480 recreates the IG00156 from first principles somehow, but no-one would really know exactly what's inside that chip now, so it's just a good guess on their part.

I had nothing to hand though.

Organ Transplant

So, I went to my garage to look at the organ donor.

Wah Brass Filter from B75 organ

 I found that my B75 Electone organ also uses a couple of custom Yamaha VCF ICs. These are the IG02611. Some Electones did use the IG00156, but by 1981 only the
wonderous CS-70M was using that part. The IG02611 has a counterpart, the IG02610. The 2610 and 2611 are apparently the same, but there is very little info on them so who knows! The 2610 is used in the dinky little nephew of all the CS synths, the CS-01. The filter gets respect but no-one is doing much to recreate it in Euroland.

Anyway, this is a vintage Yamaha VCF and I have a couple on hand, so what shall I do?

Removing the boards

The organ has a curious 3-sided construction of boards.

Brass Wah section

On this board the Wah Brass filter section located at the end. So, I decided to saw it off!

Dirty off-cut Wah Brass filter section.

As with many old instruments, the boards are not just grimey with dust, it's probably had a cup of tea at some point!

Clean board

The board brushed up nicely with a bit of isopropanol. And now I had a board I could apply power and signal to and see what I got out of it.

Patio Electronics

It was a nice day, so I set up on the patio and started attaching wires to the off-cut Wah Brass board.

Wired up

Module-wah Synthesis

Back inside

Now I had the board wired up it was time to power up and see what I could get out of it!

This video shows what I managed to get working. With an inverting level shifter built around an 741 op amp I had to hand, I was able to get a reasonably good low-pass filter module on a bread-board.It starts with a cycling envelope from the System 500 fed with a square wave signal, followed by white noise and then a triangle wave and back to square wave.

I tried to get a resonant feedback going too, but without success, so I decided to keep it simple and make this into a eurorack module as it was.

The control voltage for cut-off frequency into the 2610 is negative going, so the inverting op amp is needed to mirror all positive goingvoltages to negative. Eurorack signals are either positive, like the envelope in the video above, or bi-directional with both positive and negative going voltages, used for LFOs or audio. The level shift is thus needed to move all negative input voltages into the positive. The inverting amplifier is actually a summing as well as inverting design, adding postive offset to the input shifts negative inputs to wholly positive before being inverted to wholly negative.

Simulation showing positive going input inverted to create the correct negative, mirrored, signal for the IG02611

I was a little concerned about postive voltage input to the IG02611 and what damage it might do, but it seems to cope okay with it. The shift is more important to avoid the signal being rectified and ensure the whole wave shape is fed through.
Here's the schematic (note green and red highlights match with the trace above).

Other features include an attenuator on the audio input and and LED to indicate negative input voltage. This has a slight impact on the current being fed to the IG02611 and adds a little proctection. A blocking diode would stop any postive going current to the IG02611, but I did not want to have any voltage drop across the input.

So! On with the construction.

Module construction 

I made the wiring such that when disassembled the boards layout flat and can be worked on without disconnecting anything.

I used Front Panel Designer from Schaeffer to lay out a panel design and printed it to paper to check the fit.

I have a 3U aluminium panel handy so I chopped off a 9HP section from the end. Then I taped the paper print-out over the panel and headed to the pillar drill. 

I was a bit worried about how well aligned the holes would be. It came out fine though! A bit of filing and the panel was perfect.

I decided to add the LED to warn when the input voltage was positive in a second revision.

This completes the first stage of construction. I need knobs and some print on that panel now though.

In the next part of fun with filters I will go deeper...