A Multimode Filter For The OB-8

1.) What's it all about?

Get two additional filter modes for your OB-8: 2-pole High Pass (HP) and 6dB/Octave Low Pass with Resonance (6dBLP). The new modes are programmable.
The 2-pole HP function is similar to the one found in the Oberheim SEM module. Besides the usual HP filtering this is useful for very fat sounds, when you set the corner frequency to the lower audio range and turn up the resonance.
The 6dB LP function is not a simple 1-pole filter. Its the signal taken from the first stage of a 4-pole filter, so it has a 6dB/Oct rolloff and a Resonance peak !
Nothing is for free: you sacrifice the NOISE for this modification. (Noise isn't of much use on the OB-8 anyway, because you can just switch it on and off; full level or no noise at all. Not very useful, IMO.) The new function of the two front panel switches, NOISE and 4-POLE is like that:
NOISE button 4-POLE button Result
off off 2-pole LP (as before)
off on 4-pole LP (as before)
on off 2-pole HP (new)
on on 6bB LP (new)
Note: There's one thing about the 2-pole High Pass mode I should mention. >From the structure of the OB-8's filter, this is clearly the HP output of a State Variable Filter. But to my ears, this does not sound comletely like an ideal HP. The resonant HP sound is dominant (and very pleasant), but there is also a little rolloff at higher frequencies. It is clearly not a 2-pole BP (this would be a different node of the SVF). Maybe this HF rolloff is because of circuit imperfections - the typical summing opamp of a SVF is made from one of the CEM3320's OTA stages, after all ! - but I haven't investigated further in that yet. If someone has done an in-depth analysis, please contact me !

2.) What does it cost?

You only need a few CMOS chips (CD4052 x8) and some thin wire. Should be approx. 10 Dollars. AND you need a lot of patience for soldering ... while this mod is rather simple from an EE point of view, it needs some skill in soldering.

3.) What's the basic idea?

The "new" filters are already there inside Your OB-8! You just need an electronic switch to choose them. And as you need a programmable CV for the electronic switch, the CV for the NOISE is used. (It's actually just an on/off CV.)

4.) What do I have to do?

4.1.) Don't trust the autor of this description!

Well, it works fine on my OB-8. I am happy with this modification. I would do it again without hesitation. But I cannot guarantee that it will work on your OB-8 too. Care has been taken to avoid errors in the following description, but I guarantee for nothing. I suggest that you pull out your service manual first, check my description one by one, before you heat up your soldering iron!

4.2. What do I risk?

You will solder chips on top of other chips which are mounted on top of a double sided pcb. If you kill the original chip, you are likely to have trouble desoldering it. But the chip itself is not expensive. If you make a wrong connection, shorting one of the filter chip's outputs, you will kill the filter chip. These *are* expensive, and hard to come by. (Anybody still here ???? - Ok, let's go!)

4.3.) Cut some existing connections

The following pcb traces have to be cut:
(a) from the CV MUX to the internal noise switch: Cut the trace from U16, Pin12 to U26, Pin5. This has to be done on each voice boards (upper and lower), i.e. 2 cuts in total.
(b) from the original filter output to the VCA's input capacitor: Cut the trace from UX07, Pin14 to CX21. This has to be done for every voice, i.e. 4 times on each voice board, 8 cuts in total.

4.4.) Supply the internal noise switch with a LOW control voltage

Solder a 10k resistor from Pin 5 to Pin7 of U26 on each board.

4.5.) Prepare the new chips for mounting them

Take the 8 new CD4052 chips (danger: electrostatic sensitive!) and carefully bend Pins 1-7 horizontally. Then bend Pins 9-15 upward (!). Only Pins 8 and 16 are left in their original position. Now take a short wire and solder Pins 1-9 together. (They are all at GND later.) Solder Pin 14 and 15 together. Solder Pin 11 and 12 together.

4.6.) Mount the new chip onto UX07 (4053)

Put the new 4052's on top of the existing 4053 in each voice (8x). Just solder Pin 16 to Pin 16 and Pin8 to Pin8.

4.7.) Connect the CV

With a rather long wire, the Pins 10 of all 4 new chips on a board are connected, and are also connected to the CV MUX at U16, Pin12. Do this for both boards.

4.8.) Connect the new signal paths

 The following has to be done on each voice (i.e. 8 times):
(a) Connect Pin14/15 of the new 4052 to the HP output. This is best accessed where RX46 and RX47 are connected together. (These are the two 100k resistors left to the filter chips, with a diode located in between them. the left side of the resistors is the right one!)
(b) Connect Pin 11/12 of the new 4052 to Pin 14 of UX07 (this was the original filter output)
(c) Connect Pin 13 of the new 4052 to CX21 (the side where you have cut the original pcb trace). This is the new filter output.
The connections should now look like this.

4.9.) Testing

Close the lid, switch the power on, press the NOISE button on a program with a somewhat medium cutoff frequency. If your sensual experience is of sonic nature (delicate HP filter sound in the like of Obie SEMs), everything is just fine. If your sensual experience is more of *visual* nature (dense smoke emerging from inside your Obie), oh well ... don't blame me!


For more information, please contact
Juergen Haible
All drawings copyright J. Haible (C)1996

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