The Line 6 FM4 Filter Modeler is a multi-effect unit from Line 6′s popular Modeler range. In this pedal, Line 6 took many famous filter effects, vintage synthesizers and vocoders as inspiration to inspire guitarists to look outside-of-the-box. Most of the models in this stompbox are pure unobtanium – you’d be hard pressed to see one on eBay, let alone have one of your own. The FM4 neatly packages sixteen models, with some sounds instantly recalling musical heroes such as Zappa, Fripp and Frampton. Examples of some of the inspired models are the Mutron III Filter, Oberheim Voltage Controlled Filter, Z-Vex Seek Wah, Korg x-911 Guitar Synth and waveforms from vintage synths such as the Prophet 600 to the legendary Arp Explorer.

This purple beast has four footswitches that allow you to instantly recall four saved presets. The five control parameters allow you to select the Frequency, Q, Speed, Mode/Pitch and Mix of each filter/synth setting. Expression Pedal control allows you to morph between two different saved settings in the one preset. For example, heel down of the expression pedal can be saved for 50% Speed and 50% Mix and toe down can be saved for 100% Speed and 100% Mix. This lets you morph between these two settings and stop at any parameter along the way – useful for keeping in time with a fluctuating rhythm section.

The Line 6 Modeler range has gained a bad reputation as unreliable due to some design limitations. Firstly, the switches used are not on/off switches, but spring actuators that “click” a surface mount switch on and off. These switches feel unsturdy and are normally the first things to break. Secondly, many users noticed a volume drop when activating the pedal. The output level was affected by component values in the buffer circuit with varied levels of “drop” in different pedals.

The DIY community was pleased when pedal designer Jeorge Tripps (Way Huge, Line 6, Dunlop) released information in tackling these initial limitations, it led to a number of pedal makers and modders to offering their services to improve the Line 6 Modelers. Robert Keeley offers a hi-fi mod involving the replacement of IC chips for Burr Brown’s and some part-time modders (DRASP at Harmony Central Effects Forum) improving on the flexibility of the pedal. After intensive experimenting successes (and failures), cubisteffects now offers a number of modifications to the Line 6 Modelers.

As you can see in these pictures on this page, I have custom sprayed the once-purple pedal to a Barbie Pink. This is not the colour of the original :) !

cubisteffects offers not only a modification to rectify the initial design flaws but also has a number of modifications that can improve the flexibility of the unit, and some that push the pedal to become a new experimental tool.


The Standard Modification involves three changes;

  1. SWITCHES – The spring actuators are removed and momentary SPST “soft-touch” switches are installed. These switches are screwed to the pedal chassis and are wired to the place of the switch providing thousands of clicks without touching the PCB.
  2. VOLUME DROP – Selected capacitors and resistors are removed and, if required, replaced with metal film components.
  3. LEDs – The four red LEDs are replaced with superbright, waterclear LEDs of your choice.


A rugged 24mm Alpha pot is placed on either side of your preference. This allows you to use the Expression Pedal controls without having an expression pedal take up pedalboard real estate. The modification enables you to save and morph between two settings per preset by rolling your foot over the knob. Settings are saved as per the Line 6 instruction manual.


This modification involves re-routing the output of the Modeler back into the input, and the new output selected by a switch on the top of the chassis. This is a feedback loop, and causes spontaneous bursts of noise, squeals, oscillations, drones – all depending on the setting selected. This modification is definitely for the more adventurous musician, but with every effect, the feedback can be used subtly. Most times the feedback can be deafening so it must be used with caution. However, I have installed a small knob that sits aside the switch allowing you to control the amount, pitch, speed of the feedback. A superbright, waterclear LED indicates whether the feedback loop is on or off.

The cool thing with this modification is that the FM4 becomes a stand-alone noisemaker or “synth” as a loose term. I found it best run through a mixer to control its levels more accurately and left as a table-top unit. Alternatively, if on your pedal board, running your guitar through it will produce a background oscillation (especially in the Synth models) similar to the Z-Vex Fuzz Factory.

Other modifications offered are:


This preset lets you save two different settings per preset as per the Expression Mod. However unlike the Expression Mod, the two settings are selected via a stomp switch. This allows you to instantly switch between two settings without the need for an expression pedal. The selected preset is indicated by a superbright, waterclear LED.


WARNING: Volatile and extreme modification. Not for the faint-hearted.
This mod involves two circuit bends that work around the same principle as the Feedback Mod but are much more pronounced and distorted. Crackling, white noise, fuzzed insanity can all be activated via two independent toggle switches with a knob controlling the output volume for both. Superbright, waterclear LEDs indicate whether bend is on or off (but you can clearly hear them if they are on!)

Below are two video demos of the cubisteffects modded Line 6 FM4 Filter Modeler - the first showing the Expression Mod and the second showing the Feedback Mod.
Clip Details: Epiphone Sheraton> cubisteffects modded Line 6 FM4> Marshall JCM600
Recording: SM57 > Mbox2 > Reaper


Related Posts with Thumbnails


  1. Shawn says:

    Greetings. What a relied I found this site.

    I have a hassle with the PITCH settings on my unmodded FM4.

    For example: On the heel down position I save the pitch at the root note, then I set the Heel UP position I set the pitch at on octave. I save the setting. Now when I use the expression pedal, the heel up position only takes the pitch to a minor7th and NOT the full octave. Is this just my FM4?

    Another issue, is there not a mod that can change the latency? It’ll be such a great design if the synth’s attack happened as you strike the string, not this 10ms delay.

    Hope you guys can help,

    GREAT site keep it up.

  2. domesticatedrobot says:

    Dig the feedback mod on the filter demo… but… what does it sound like when you actually play an instrument through it?

  3. cubisteffects says:

    The models I showed using the Feedback mod become Tone Generators so your original signal becomes redundant. However, on most of the synth settings you can have a feedback affected signal.

  4. cubisteffects says:

    Hey Shawn,
    Unfortunately your dilemmas are constraints of the programming of the pedal. These are not parameters that can be changed via analog circuitry. The only way would be to re-program the FM4 which is something only Line 6 can do.
    Check out the new M9 or M13 to see if they have updated those models from the FM4.

  5. Shawn says:


    I will have a look at the m9/13

  6. Imer says:

    Love the pink.

    Question the first- Does the TWIN PRESET MODIFICATION work like a bank? I’m not too clear on this; do i have my four presets then by pressing it will give me four more presets?

    Question the second! Does adding the FEEDBACK MOD still retain it’s true bypass?

  7. cubisteffects says:

    The TWIN PRESET mod works like the expression pedal. It will provide two different saved settings PER DELAY TYPE.
    The FEEDBACK mod is switchable, allowing you to change between the stock setting and feedback madness.

Leave a Reply