FM-800By Atom Smasher Filter Mod for Korg EX-800 and Poly-800
Also see the HAWK-800 mod, and keep an eye out for the aTomaHawk-800.

atom@smasher.org

Samples, Reviews, Pictures, Tech Notes


This modification to the Poly-800 series of synthesizers adds the ability to control Filter Cutoff with DCO1. This is commonly known as FM control of a filter. Although it's technically not Frequency Modulation, the results are capable of sounding very much like an FM synthesizer.

A knob allows the effect to be turned from completely off, to far beyond the normal capabilities of a Poly-800.

This modification very nicely compliments the Moog Slayer modification for a total of 3 knobs full of fun.


In addition to the usual disclaimers (if you break it, it's your problem, not mine!), I'd like to also point out that I will not be responsible for blown speakers, amps & ear-drums.

Although this modification seems easier than the Moog Slayer, I suspect it will result in more casualties. Soldering directly to 1/8W resistors mounted tightly to a board is something that beginers should avoid (but it seems so easy!). I highly recommend replacing those resistors with vertically oriented 1/4W resistors before trying to connect wires to them. Unfortunately, replacing the resistors risks damage to the circuit board if you don't have a lot of experience replacing components on a circuit board.

If you have doubts about your ability to perform this modification, ask your local synth/guitar/amp technician if they can do it for you. The cost of having this modification performed by skilled hands is probably less than the cost of replacing a synthesizer.

If anyone wants to send me samples of their modded synth, maybe I'll post it here.

This modification is Copyright Atom Smasher and published under a Picture, Sample, Lunch-Ware License: If you perform this modification send me an email with pictures, samples and your location. If I visit where you live or you visit where I live (currently Wellington NZ) you owe me lunch.


  • Parts list
    • 5K (or 4.7K) linear taper pot (1/8W or more will work fine)
    • Knob to make the pot look pretty and feel good
    • 1K resistor (1/8W or more will work fine. 1/4W will be easy to find)
  • Optional parts (recommmended)
    • 62K 1/4W resistor
    • 22K 1/4W resistor

  1. Solder the 1K resistor to the arm of the 5K pot. Solder a length or wire to the resistor. Cover the resistor and it's leads with heat-shrink tubing.

  2. Locate the ground pad next to connector CN9 and solder a ground wire to it. Connect this to the low side of the pot.

  3. Locate R74 (62K) near IC4. One end of R74 is connected to IC4 pin 7; connect that point to the high side of the pot. Optionally, remove R74 and replace it with a vertically oriented 1/4W 62K resistor. If you connect the wire to the leg of the vertically oriented resistor it will be a better connection and less likely to cause problems.

  4. Locate R22 near IC1. One end of R22 is conected to R23; connect that point to the wire that's connected to the 1K resistor (connected to the 5K pot). Optionally, remove R22 and replace it with a vertically oriented 1/4W 22K resistor.

    Note that on older models R22 is 33K. On newer models R22 is 22K. This doesn't seem to be critical, but I would recommend using a 22K resistor if you replace R22.

  5. Mount the pot on the front panel and amaze your friends!

Black traces show the original wiring. Red traces show the FM-800 modification.
FM-800 wiring diagram

Have fun! And don't forget to send me samples and pictures! And take me to lunch!