Having some fun with the old R.A.Moog 904B High Pass Filter. I recently acquired the 904B and 904C to add to my Moog. Needed a bit of rewiring to the harness, so it wasn’t just a simple drop in installation.
The 904B is on the left with the 904C coupler next over.
The 904B does not look like much, and only has a high/low range switch and the large cutoff knob. Missing is any resonance dial like the more popular 904A has.
Reviews on this Moog filter vary from “nothing special”, to “adds sizzle”, to “a plain high pass without regeneration”. It seems odd, that the 904B does not receive that same “must have” status as the 904A lowpass filter does. Combined with the coupler, the tandem of both 904 filters can make some very interesting tones. But wondered what it could sound like on it’s own, without the famed 904A helping it out. So I set out to make a patch using only this Moog highpass filter to explore it’s sonic possibilities.
The first patch I tried involved the Moog feedback trick. The 904B does not invert the output like the 904A, so I used a positive feedback loop into the CP3 mixer. To my surprise, I unleashed a monster! The filter immediately turned into a towering 20 foot tall demon that absolutely screamed at me.
A small sample of the filter. At first playing some octaves high to low while demonstrating the cutoff. Sounds good, but nothing remarkable. At 10 seconds I begin to turn up the positive feedback path through the CP3 mixer. It gets much louder, then at around the 7 tick mark begins to break up and howl. Sounds incredibly nasty!
Very unstable and cool sounding. To get a percussive type envelope, I had to invert the envelope, and have an short attack to sustain of 10. There is also a Moog 1120 pedal I’m using to get a sort of wah effect. Kind of sounds like a distorted fuzz guitar with a wah pedal. But this is just straight Moog, 901 oscillators with the filter/mixer combo doing all the crazy distortions.
Here is an example with a Minimoog bass sequence playing in the background to get an idea how this sounds in a mix.
A unique sound I would say. Not what one would automatically associate with the classic Moog modular tone. This same trick can be done on the Voyager. The Voyager shares a bit more in common with the 904 filter combo than any other Moog when in HP/LP mode. Was able to take the highpass only output and loop it back into the external input for similar high pass resonance tones.
Having too much fun with this patch and thought I would share.
The 904B highpass filter is just a ladder filter configured for highpass. The patent describes both configurations. I think Moog never put a resonance knob on it for the same reason you uncovered - it is very unstable as positive feedback is increased, not as controlled as the lowpass configuration. If one wanted a highpass filter with resonance, the SEM multimode did a better job.
Very interesting CZ, not only hearing it, but also as it relates to something I am about to deal with. Every time something like that, I can’t help but to launch a battery of tests on my own stuff and see what it is all about.
What about the Korg MS-20 filters? It has both low pass and high pass, both are resonant and if I remember from looking at the service manual, they appear identical. I did not use the high pass all that much on the MS-20. Sometime ago, I had a problem with the MS-20. I first thought it was keyboard related, bad, unsteady contact.. whatever. Pushing the trouble-shooting further, i finally diagnosed the problem with the resonnance part of the low pass filter, which is acting funny as the frequency increases. While looking at repair options, the one that came to mind was to replace the LP circuit with the HP one, which appears to function normally, resonance included. Just last week, I was talking to my synth tech, and he mentioned that the filter is on a small assembly board with some sort of shrink wrap (identified as Kork 35). The shrink can be removed and the filter repaired or rebuilt.
I think my logic and mechanical skills are good, but my electronics skills are 101 or just a bit further. I have made maintenance and repaired some stuff on the Model D, Opus 3, Prodigy (mods) , MG-1’s (mods) and Satellite so far. I have an MG-1 spread over my work table as we speak for a complete refurbish job. The MG-1 is simple electronics and I have all the parts (and confidence) to accomplish the task with success. For later on I have a Polymoog (and NOS Polymoog Road kit for parts) as the ultimate projet for whenever and if ever my electronics skills get to that point.
But back to topic, I want to learn more about HP filters (and LP too) and make the link about how the Korg filter is built vs the Moog. Some guidance, tips would be appreciated.
If it’s a true Korg 35 chip, it’s not in shrink-wrap.
It’s a ceramic monolithic board covered in (what I think) is a baked-on enamel.
Not too easy to get into, though I think there are solvents available to melt the coating.
One fellow brought me his MS-20 for repair when the lowpass filter died.
I swapped his HP Korg 35 to the LP location and he had a low pass filter again.
But (at the time) Korg 35 duplicates weren’t available, so I offered that I could wire plain pre-filter dry signal in place.
The HP freq knob would turn into a dry mix control causing filter cancellations one normally couldn’t obtain.
The owner took his MS20 away for about a year or so until he located a guy manufacturing Korg 35 clones.
He then bought one and returned the synth to me to put it back to stock.
However, he then wanted me to mod it so he could still have that dry mix control.
It seems he became very fond of some of the new filter responses and wanted to retain the mod in addition to the replaced filter.
I put it back to stock w/o the dry control because I explained there was no good location to add a new pot.
Moral of the story: Korg 35s sometimes break and aren’t so easy to repair, but a good mod can go a long way to help not miss it so much.
Thank you very much Kevin, that’s the essential piece of information I was looking for, the fact than the filters can be swapped with little work. I will study the schematics and try to locate some Korg 35s. As much a I enjoy doing and learning synth electronics as a hobby, I have no problem referring to a qualified technician if I feel something is over my skills.
Thanks also for Bulletin 1312 and other docs. They were invaluable for modding my Prodigys.
