• envelope trigger point A
  • envelope trigger point B.
  • frequency to voltage output Tip +
  • frequency to voltage output Ground
  • add a switch to override (short out) this 1k resistor to make the filter self oscilate when resonance is on
  • LFO to Filter cut of point A
  • LFO to Filter cut off point B
  • 7350-voltage regulator
  • filter injection point

How to circuit bend a Gakken SX-150 synth

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1--LFO-to filter cut-off (lfo frequency modulates the filter)

2--Filter resonance oscillate (when resonance is on filter distorts and produces a pitch frequency -- simply solder two wires either side of the resister labled above and add a switch too turn the short on/off

3-- frequency to voltage output (wire a output jack to the points shown above to have a volatge output that rises with higher pitch great when used with the theremin as input!!

4-- envelope trigger wire a switch between envelope pionts A and B (shown above) so you can manually re-trigger the envelope

5-- Optical Light control (add a norps12 LDR (min resistance 50ohms max 2m) across the same two pionts as the resistive strip (and a switch to turn it off and on) brighter light increases the pitch etc produces a constant tone.
as always check short before soldering Enjoy!

6-- The filter injection point (the point were R30 and R29 meet) can be used to inject an audio signal through the filter BUT i haven't tested this too much as yet it will need some form of biasing I guess using a 1-10k pot wired as a potential divider would do the trick the signal needs to be at least 0db (2.2v Pk-Pk) i.e. line level NOT guitar.

aylk, matt7277, Peter Swimm, and 11 other people added this photo to their favorites.

  1. Mr_Union 62 months ago | reply

    any chance of photos of the other side?

  2. Matt The Modulator 62 months ago | reply

    hi sorry i would have to dismantal it and unscrew the switches ive added ect. i left the wires a bit short and it was a pain to get together

  3. Synthesizers 62 months ago | reply

    Hi, I'm an admin for a group called Synthtopia, and we'd love to have this added to the group!

  4. Funky Cody 62 months ago | reply

    nicely done!

  5. Matt The Modulator 62 months ago | reply

    yep it sure is analog and cheers funky cody

  6. Funky Cody 62 months ago | reply

    :) where are the bateries??

  7. Breakfast for Dinner 61 months ago | reply

    hi matt, great looking mod. I'm interested to know what else has been soldered onto this board. can you make more notes? what are the other yellow wires running to the speaker? also, this mod would greatly benefit from a step by step guide. you're awesome!

  8. T. Read 50 months ago | reply

    Thanks for sharing this. These are great mods for the SX-150 and I highly recommend them. Not that you seem to be paying much attention to this anymore, it would be helpful for others if you actually explained what these mods, in fact, do. Also, what are the other yellow wires (R121) hooked up to? You seem to have incompletely explained the frequency to voltage mod.

  9. Matt The Modulator 50 months ago | reply

    Hi T.Read ive forgoten what ive done to this little baby ?? the frequency to voltage mod is simply an output wired to a jack socket to control other synths like doepfer etc ile get it back from a friend and have a look at all the mods ive done and correct / add any missing stuff thanks for having a look .Matt

  10. Matt The Modulator 50 months ago | reply

    ive added some better notes hope it makes the bends easier to understand ! enjoy Matt the modulator

  11. alienmeatsack 49 months ago | reply

    FWIW, you can also connect a 1M or 2M potentiometer or slide potentiometer to the resistive strip for a pitch control as well. With the pot shaft facing you, wire the blue wire (from the top of the unit, it's the right side of the resistive strip) to the left leg on the pot, middle to the wand, and white wire to the right leg.

    I'm working up the resistor values for a small keyboard mod for mine as well. I am hoping to make the pot mod above be the master pitch control, and then each key will run off that, so if I change the pitch, the keys all shift.

  12. alienmeatsack 49 months ago | reply

    A few bits of info for bending the SX-150...

    The stock potentiometers are as follows: (The 50k's on mine registered as 45-50k when measured)
    Pitch Envelope - 50k
    Cutoff - 50k
    LFO - 50k
    Attack - 10k
    Decay - 10k

    So far, the LFO pot seems to be pretty flexible for changing to a different size. I've gone as big as a 1M pot here and it works great. You get a way slower oscillation with it on the low end.

    I've not tried the others yet, but I would guess they would respond to alternative sizes as well for more variation in the sound.

    Notes for the bends noted in the OP...
    - LFO to Filter cutoff works great with a 25k-25k pot. Anything bigger has a lot of unused range.

    - Resistor override - a 1k (or 2k) pot works nicely here giving you some feedback like sounds.

    Using some photo resistors and LEDs you could do some really cool stuff I think. I'm going to dig for some good logic/activity spots for an LED and see what I can make it do. Technically any of the pots could be effected by an LED driving a photo resistor.

  13. Matt The Modulator 49 months ago | reply

    Great stuff thanks for adding the info Alienmeatstack please
    feel free to share your piano resistor values if you get it sorted Thanks

  14. alienmeatsack 49 months ago | reply

    Any of you tried adding diodes to the 5 unused Diode jumps on the board? I wonder if some LEDs would do something there.

  15. Matt The Modulator 49 months ago | reply

    can't remember seeing any! right that's it I'm off to get it back !!

  16. alienmeatsack 49 months ago | reply

    Here's the board:

    D1, D2 and D3 seem to jump in series together, as do D4, D5 and D6. I have found no reference to anyone using these to add a function or bend. Hmm...

  17. alienmeatsack 49 months ago | reply

    I think the trick for the piano/keyboard keys is going to be finding good quality accurate 1% resistors.

    My goal is to be able to put something like a 1M pot on the first key, then use resistors off that for each key. I'm not sure how accurate the resistors will be, and more importantly if, ask you change the pitch, if they stay on key or if they drift. I know that the pots aren't exact in the sweep, so that's where I'd gues the drift would occur.

  18. SalvorHardin2010 43 months ago | reply

    Hey dude. Nice work.
    I'm in the process of modding up the Gakken.
    I'm attempting to these mods:
    Plus a couple of yours, namely:
    (1) LFO to Filter
    (2) Env trigger
    I am also attempting res fdbk, but with a variable resistor (as 2CV has done).
    I was wondering if you'd know how to add an LFO depth knob to the circuit, bearing in mind that it'd have to control LFO depth to the cutoff as well (as in your mod). I'm using a 10k VR as I saw this done on a Japanese mod, but not sure how to do it, or whether it'd work how I want it to. Check it out:
    It seems like I should put it between R16 and R17, but I'm new to electronics and especially hacking them, and not sure if I'm understanding the signal flow/direction.
    Thanks for your time and awesome post.

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