MOTM-480 3u Dual Filters

MOTM 480 dual filter modThe MOTM-480 CS-80 Resonant Filter consists of two identical state-variable Voltage Controlled Filters. Each filter circuit features:

  • 1V/Octave frequency control input
  • FM input with reversible attenuator
  • VC resonance input
  • High pass, band pass, and low pass outputs

In the original SynthTech panel the filters are connected in series, with the high pass output of the first hard-wired to the input of the second. This creates the CS-80 “variable band pass” response from the low pass output of the second VCF. The first VCF has a three-input mixer. Each filter has its own panel pots for initial frequency and initial resonance. The VC inputs are ganged: The 1V/Octave, FM, and Resonance VC jacks control both filters together. The Band Pass output of the first VCF is on a panel jack. The main output is the Variable Band Pass output, i.e. the LP out of the second filter. With a few changes and a bigger panel, this more flexible module can be obtained.In conjunction with Larry Hendry I designed a 3U-wide panel with symmetrical features for both filters, so they can be used completely independently. Each filter on the custom panel has:

  • 5 Pots: Inputs A, B, Frequency, Resonance, FM (reversible)
  • 8 Jacks: Input A, Input B, FM input, 1V/Oct input, Resonance VC input, High Pass, Band Pass, and Low Pass outputs.

The two filters are completely separate. I built the panel with no jack-normalization. But if desired, the builder could wire any of the RES, FM, and 1V/OCT inputs of the first filter normalized to the same inputs on the second filter, so they can be controlled in parallel.

Four output buffer circuits are needed for the four additional outputs. These buffers can be built with one MUUB-4 board from Tellun Corp. Each circuit is a replica of the buffer around U11B, consisting of a 3.3uf non-polar cap, 6.8K and 68K resistors and a 1K resistor for the output.

MOTM 480 mod kit

Parts list:

  • 1 Custom front panel
  • 1 Stooge 4-pot long bracket
  • 1 MUUB-4 pc board
  • 1 100K pot, Mouser 594-149-7104
  • 1 100K log taper pot
  • 2 Alcoknob 506-PKES90B1/4
  • 8 Switchcraft 112A jacks
  • 4 3M3 non polar electrolytic capacitors
  • 4 6K8 resistors
  • 4 68K resistors
  • 4 1K resistors
  • 2 TL072A op amp
  • 2 Ferrite bead, Mouser 623-2743001112
  • 2 10uf electrolytic cap

PHYSICAL MOUNTING

The MOTM-480 board mounts at the second set of mounting holes. This places the board far enough back to clear the pots and jacks on the panel. To prepare the mounting bracket:

• Drill two holes to align with the rear of the PC board, now farther back.
• Cut the bracket, removing a square of metal from the area of the lowest pot hole. There will be a jack in this position that needs to clear. Notice this cutout in the photo below on the lower right.

MOTM 480 mod boards

In the following description, the circuit referred to on the MOTM schematics as HP is Filter 1, and the one called LP is Filter 2.

CIRCUIT CHANGES

  • Omit all the board-mounted pots: VR1, VR2, VR3, VR4. These mount on the panel now.
  • Omit R25 and R68.
  • Cut four traces to disconnect VC inputs wired to both filters.
    1. To the right of R18, top of board
    2. To the right of R19, top of board
    3. To the right of R21, bottom of board
    4. Between R66 and J3, bottom of board
  • The right end of R18 will connect the 1V/OCT input of Filter 1.
  • The left end of R66 will connect the RES IN for Filter 2.
  • The right end of R19 will connect the +FM input of Filter 1.
  • The right end of R21 will connect the –FM input of Filter 1.
  • Wire the IN1A pot at the VR1 location (center and ground only).
  • Wire the IN1B pot where VR2 location (center and ground only).
  • The left hole where R68 was will connect the HP 1 output.
  • The lower end of R55 (33K) will connect the LP 1 output.
  • The left end of R40 (10K) will connect the HP 2 output.
  • The right end of R50 (4M7) will connect the BP 2 output.

WIRING OPTIONS

I used shielded coax, like that supplied in the kit, to connect the inputs to their attenuator pots on the panel and from there to the board. I did not use shielded cable on the outputs. Here is my wire list with lengths. All these wires except the MUUB outputs connect to the MOTM-480 PC board.

J1 – 1V/OCT 2 IN 4” Twisted pair
J3 – RES 1 IN 8” Twisted pair
J4 – BP 1 OUT 6” Twisted pair
J8 – LP 2 OUT 3” Twisted pair
R66 – RES 2 IN 7” Twisted pair
VR4 – FM 2 IN 3” Twisted pair
R19/R21 – FM 1 IN 7” Twisted pair
R18 – 1V/OCT 1 IN 6” Twisted pair
R68** – IN 2A 8” Shielded cable
R68** – IN 2B 8” Shielded cable
VR5 – FREQ 1 10” Twisted triple
VR6 – RES 1 7” Twisted triple
VR7 – FREQ 2 8” Twisted triple
VR8 – RES 2 6” Twisted triple
VR1 – IN 1A 6” Shielded cable
VR2 – IN 1B 6” Shielded cable
HP 1 OUT to MUUB A 6” Twisted pair
LP 1 OUT to MUUB B 6” Twisted pair
HP 2 OUT to MUUB C 6” Twisted pair
BP 2 OUT to MUUB D 6” Twisted pair
MUUB A to HP 1 Jack 6” Twisted pair
MUUB B to LP 1 Jack 6” Twisted pair
MUUB C to HP 2 Jack 6” Twisted pair
MUUB D to BP 2 Jack 6” Twisted pair
Power wires to MUUB 6” Twisted triple

In addition, you need two 7” shielded cables and two 3” shielded cables to bring the input signals from their panel jacks to their respective pots. I prepared the eight coaxial cables in advance, connecting them together in pairs where they would hook up to the pots.

MOTM 480 mod wiring step 1

WIRING STEPS

  1. First solder all the wires attaching to the MOTM-480 board.
    1. The four new VC input twisted pairs fasten to the end of some resistor on the board. Look around to find a ground point for the ground wire in the pair. The end of the Tempco resistor that goes to ground works for the one by R66. I used the ground of J2 (otherwise unused) for the 1V/OCT input ground. I did not twist ground wires with the FM input wires. Since those are reversible, each end of the pot goes to a resistor.
    2. The four new outputs also attach to the ends of resistors (with the exception of the HP1 OUT, which conveniently goes into the hole left by R68). For those outputs going to the MUUB, connect the ground of the twisted pair to the MUUB. At the other end the ground wire is left unconnected. Secure it with shrink wrap.
    3. For power to the MUUB I soldered to one end of each ferrite bead on the power connector side, and used the Tempco resistor lead for the ground. I soldered directly to the MUUB power connector holes.
    4. The FREQ and RES pot connections are wired just like in the MOTM instructions. Use three colors in the twisted triple and make sure the numbers on the board (1, 2, 3) match up to the numbers on the pots.
    5. ** The second signal inputs use the two 150K resistors that were omitted from R25 and R68. I connected them together at the right side hole where R68 used to be, each in series with the coax coming from its pot. These are the only resistors that don’t have both ends soldered into the board.
  2. Mount the MOTM PC board to the bracket, and the front panel to the bracket. The panel is held by three pots. Make sure each pot has a nut behind the panel as well as in front. Start wiring from the bottom up:
    1. Wire the two FM inputs. A 2” wire goes from each jack to the center of each pot. Add a jumper between the grounds of the 1V/OCT inputs and the FM input jacks, because the FM input wires carry no ground. Remember to connect the normal, middle, lug to the ground lug on all the input jacks.
    2. Wire up the four FREQ and RES pots.
    3. Wire up all the signal inputs. This is a bit tricky if you use shielded cable.
  3. MUUB wiring
    1. Solder the four MUUB output wires to JA9, JB9, JC9, and JD9.
    2. Solder the three-wire power cable from the MOTM board.
    3. Solder the four wires that carry the four new outputs from the MOTM board to JA1, JB1, JC1, and JD1.
    4. Mount the MUUB board using long screws and ¾” spacers to place it a good distance above the MOTM board.
    5. Connect the four MUUB output wires (orange/white in the photo) to their panel jacks.
  4. MOTM 480 mod wiring finished

YOU ARE DONE!

Share

4 Responses to MOTM-480 3u Dual Filters

  1. Tom Adam says:

    Richard,

    Just finished mine. Brilliant! Thanks for the instrustions;-)
    I built mine without cutting any traces, but I have some resistors standing up.
    Just for the records, don’t forget the 150K’s at the input, results in some strange oscillating sounds…

  2. Paul Newman says:

    So it’s possible to build this without cutting the traces? No issues or other workarounds to make it work that way?

  3. Richard says:

    I think Tom Adam left one end of each of the four resistors mentioned sticking up with a wire soldered to them, thus avoiding having to cut traces. This isn’t mechanically rigid and risks breaking resistor. I prefer to solder the resistor into the board at both ends, one end having the wire snugged under it, for mechanical strength. That means you have to cut the four traces. This is DIY! It’s up to you.

  4. Paul Newman says:

    Ah, ok, that makes sense. Thanks!

Leave a Reply

Please use your real name instead of you company name or keyword spam.