Hartman Envelope Filter



Inspired by one of the great envelope filter pedals of the 1970's and developed with guitarist Mark Karan (Delaney Bramlett, Bob Weir & Ratdog, Jemimah Puddleduck) the Hartman Envelope Filter is a recreation of a classic effect optimized for tone, flexibility, and ease-of-use.

"Mark wanted a pedal with a certain tone, range, and feel that could be switched fluidly among its settings via a simple user interface, and of course, analog.” says Hartman. “I was intrigued by the idea of tailoring a faithful recreation of the Mutron to his requirements, so we set to work. The result is a beautifully voiced musical effect that preserves the raw immediacy of first-generation analog.”

The Hartman Envelope Filter employs select, low-noise components for its signal path and an optically controlled state-variable filter for authentic timbre and response. A second foot switch permits change of filter sweep direction for on-the-fly access to a broad palette of classic filter sounds from funky chirps and saturated quacks to subtler flute-like attacks and horn-tones. An active Mix control blends filtered and unfiltered signal. Factory preset internal trim pots permit additional tuning of filter action.


  • Envelope controlled state variable filter
  • Gain, Peak, and Mix controls
  • Hi-Lo Filter Peak Frequency Switch
  • Foot-switchable Filter Sweep Direction
  • Plated-Through-Hole PCB (printed circuit board) construction.
  • Rugged powder-coated metal enclosure & heavy-duty true-bypass switching
  • 18VDC 2.5mm via Tip-Positive External power jack
  • 5-5/8”wide x 4-5/8”long x 2-1/2”high

Theo Hartman about the relation with the Mu-Tron III

  1. AUDIO signal path is identical Mutron up to but not including the unity-gain active mixer section (wet/dry) which occurs post-effect.

    However, though the audio signal path is the same, the Hartman does not have the Boost control on the outside of the pedal. Instead, this is an internally adjustable trimpot. Pedals ship with this factory-preset to unity gain, which keeps all but the strongest instrument signals from saturating the filter. However it can be adjusted upwards or downwards to suit taste and situation. Boost has been replaced on the outside by Sensitivity (see below). Audio output from the filter section is taken from what would be the "LO" tap on the original filter design.

  2. ENVELOPE signal path uses the Mutron topology for driving the (opto) LDRs. Since no one manufactures 2 LDRs driven by a single LED at the moment (and not being a huge fan of relying on the supposed darkness of the enclosure interior like some others (you have to turn out the lights to service them...) we match the pair.

    There are several tweaks to the Envelope signal path:

    1. We added adjustable gain to the front end, letting a user adjust the strength of the envelope signal upwards or downwards of the standard Mutron independently of the audio signal level. Handy for switching guitars/pickups with different output strengths. (Garcia never had to reconcile the strength of instrument signal to the Mutron because his fx loop received unattenuated signal 100% of the time, fx output was run back out to the instrument and the guitar volume knob acted post-fx, i.e., filter response never varied with guitar volume.) The SENS on the outside. Similar to Qtron, but implemented a little differently.

    2. There are further adjustments to the envelope section that take place when switching between UP/DN sweep directions. The upshot is there was no one set of component values for the envelope driver that gave us exactly the behavior and tone we wanted in ALL modes, so separate circuitry was created as necessary to permit us to set up and adjust the modes independently of one another. Interestingly, most of the variations still fall within the range of the theoretical tolerance of the original component values, but a few 100 ohm here or there turn out to make a big difference in certain settings, so we zero'd in on that stuff and attempted to the best of our ability to agree on what sounds best and provides the greatest utility.

  3. We were very concerned about creating a pedal that requires minimal fuss live and sounds good in all of its settings, rather than a forest of controls and choices. That drove every choice as to which controls to leave external vs. set/fix/adjust within. Now I guess we'll see if people agree, lol... ;).

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