Drunk Beaver's 8th Monthly Pedal Drop is the Odessa Envelope Filter - based on MXR's MX-120 Envelope Filter with a few circuit enhancements

Mar 31, 202318:00 by

Stefan Karlsson

Drunk BeaverEnvelope Filter and Auto WahModulation

Key Points

Drunk Beaver’s 8th Monthly Pedal Drop is the Odessa Envelope Filter, an enhanced take on the MXR MX-120 envelope filter. The original MXR (first produced 1978) had only Threshold and Attack controls and no LED; Analog.Man’s mod added an Emphasis knob, Up/Down switch and LED.

Vitalii’s enhancements add 2-way toggles for Invert (reverses sweep), Decay (Shorter/Longer) and Range (Broader/Narrower). Controls: Attack, Threshold, Range, Decay, Invert. Default/stock switches are UP. The circuit uses 2×4069 and 1×4066 CMOS (best results with NOS Motorola MC14069 and Philips HEF4066) plus added input/output BJT buffers.

Limited to 15 units on the Drunk Beaver Reverb.com Store; no official demos (MXR-120 demo used as reference). As of 2026-07-13, the price was £150 (listed as $150 plus shipping). For electrical safety or modifying a pedal, consult a qualified tech.

The original MXR circuit had just Threshold and Attack Knobs, and no Status LED - and was produced first in 1978. Since then there have been a few reissues and modded versions of this pedal - including a well-liked Analog.Man modded variant which has a 3rd Emphasis knob for frequency tweaking, an Up / Down switch, and an added Status LED.

Of course Vitalii always goes one or two steps further - and his enhancements include additional 2-way toggle switches for Invert (equivalent to Analog.Man’s Up/Down), Decay and Range Switches

Controls - Attack, Threshold, Range (Broader / Narrower), Decay (Shorter / Longer), Invert (Up/Down).

As Vitalii puts it - this circuit is really quite unique as it relies on 3 CMOS IC’s for both the Filter and Envelope Follower. There are 2 x 4069, and 1 x 4066 CMOS chips, and the circuit really doesn’t work in the same manner if you use different IC’s - Vitalii tested a whole lot of them - and got by far and away the best results with NOS Motorola MC14069, and Philips HEF4066.

Above and beyond the original core components Vitalii added input and output BJT buffers to match impedances with CMOS ICs (the original MXR pedal might alter the tone depending on the signal chain).

The 3 key Mods which are rendered by those additional toggle switches are :

INVERT : Reverses the envelope follower sweep - so filter goes in the opposite direction also
DECAY : Shorter / Longer Decay Time
RANGE : Broader / Narrower Filter Range / Sweep - Standard Range is c. 300Hz - 3kHz.

Default / Stock settings are all switches UP.

So you’re getting the highest quality of vintage circuit - with additional enhancements to help you get more out of the pedal.

The MXR-120 is rightly lauded as one of the best sounding Envelope Filters ever - and this is a really cool enhanced version of that legendary circuit.

Available right now in limited numbers (15) on the Drunk Beaver Reverb.com Store for $150 plus shipping.

As this is a rapid prototyping project - there are no sound samples or official demos - so I’ve included the below MXR-120 demo as a benchmark reference for this circuit!