The PastFX PX-101 Lowpass Filter is a compact filter pedal that recreates the discontinued Moog Moogerfooger MF-101 Lowpass Filter in a smaller enclosure. Like the original, it is a lowpass filter with an envelope follower: the filter cutoff moves in response to how hard you play, and the resonance control can push the filter into self-oscillation at extreme settings1. Guitar Pedal X described it as "a replica of the Moogerfooger MF-101 Lowpass Filter, adding an Output Level control while retaining its" core behavior2.
The original MF-101 was itself a stomp-box version of the Minimoog VCF, the filter circuit from the Minimoog synthesizer, with mono input and output and a switchable 2-pole or 4-pole filter3. The PX-101 carries that design into a pedal measuring 112 x 60 x 31mm (4.4" x 2.36" x 1.22", enclosure only)1.
A Moogerfooger MF-101 lowpass filter in pedal form
PastFX describes the PX-101 as "our take on the discontinued Moogerfooger MF-101 Lowpass filter"1. The MF-101 was a filter pedal built around the Minimoog's filter circuit, with knobs for filter cutoff, resonance, envelope amount and more, and a filter switchable between 2-pole and 4-pole operation3. Moog also built companion pedals in the same line, the MF-102 Ring Modulator and MF-103 12-Stage Phaser3.
The PX-101 keeps that circuit in a smaller box and adds practical touches the original lacked21:
- An Output Level control, added versus the original2
- A standard Boss style power input, replacing the original's special positive ground plug1
- An internal true bypass switch, letting the pedal run as an always-on preamp like the original or as a true bypass pedal1
- An ENV LED showing envelope strength, plus a dual color LED level indicator that shows how strong the input signal is as you set the Drive control1
Turning the Mix control fully counter-clockwise gives the "Preamp" sound without the filter, so the pedal can serve as a straight preamp when the filter is not wanted1.
The controls
The front panel follows the original's layout, with the same core controls plus the added Output Level2.
| Control | What it does |
|---|---|
| Drive | Adjusts the signal input gain; the dual color LED gives visual feedback on input strength1 |
| Filter Cutoff | Sets the frequency at which the filter opens and closes1 |
| Resonance | Changes the shape of the frequency response; oscillation can occur at extreme settings1 |
| Envelope Amount | Controls how much the envelope signal modulates the filter cutoff1 |
| Output Level | Sets output level; added versus the original MF-1012 |
| Mix | Blends filtered and unfiltered signal; fully counter-clockwise gives the "Preamp" sound without the filter1 |
| 2 / 4 Pole switch | Routes the signal through half or all of the filter. 2 Pole is a brighter sound, 4 Pole deeper and mellower1 |
| Smooth / Fast switch | Selects the envelope follower response1 |
PastFX warns that care must be taken to avoid damage to hearing and equipment when the Resonance control is used in high settings, since the filter can oscillate1.
The 2/4 pole switch is the same choice the original offered: Vintage Synth Explorer notes the MF-101's filter is switchable between 2-pole and 4-pole, which is what gives the two slopes their different character3.
Power and bypass
The PX-101 runs on a +9VDC supply with a Boss style 2.1mm negative tip plug1. PastFX points out that it "no longer need[s] a special positive ground powering plug like the original," which means it works with the standard 9V supplies most pedalboards already use1. Current draw is 120mA1, higher than many compact analog pedals, so the supply output needs to cover it.
Bypass is set by an internal slide switch: true bypass, or an always-on preamp mode matching how the original MF-101 behaved in a chain1.
Expression pedal control
The EXP jack accepts an EP-3 expression pedal or similar, using a 50K to 100K linear pot. With an expression pedal connected, four parameters can be controlled: Mix, Amount, Resonance and Cutoff1. Guitar Pedal X also notes that the Audiostrom MXP-1 expression pedal is supported2. For a player who wants to sweep the cutoff by foot, wah-style, the Cutoff assignment is the obvious one; see expression pedals for more on how these work.
What reviewers said
Guitar Pedal X covered the PX-101 at its July 2024 introductory pre-order, describing it as a compact, extended-range take on the MF-101 and noting the added Output Level control and Audiostrom MXP-1 support2. In the site's roundup of the best new modulation, pitch and utility pedals of 2024, reviewer Stefan Karlsson's verdict on it was blunt:
"An absolute essential for Bass / Funk Players!"
Guitar Pedal X, December 20244
The same roundup listed the PX-101 at $2094.
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Questions
Is the PastFX PX-101 true bypass?
It can be. An internal slide switch selects either true bypass or an always-on preamp mode like the original Moogerfooger MF-101. You choose the mode inside the pedal rather than from a front panel switch, so it can suit both a standard pedalboard and a filter that stays in the chain.
What power supply does the PX-101 need?
It runs on a +9VDC supply with a Boss style 2.1mm negative tip barrel plug. Unlike the original Moogerfooger MF-101, it no longer needs a special positive ground powering plug, so a standard 9V pedal power supply works.
Does the PX-101 need a positive ground plug?
No. PastFX states it uses a +9VDC supply with a Boss style 2.1mm negative tip and no longer needs the special positive ground powering plug the original Moogerfooger required. That means it can share a conventional daisy chain or isolated 9V output.
Can the PX-101 self-oscillate?
Yes. The Resonance control changes the shape of the frequency response, and PastFX notes that oscillation can occur at extreme settings. The maker also warns that care must be taken at high resonance settings to avoid damage to hearing and equipment.
Can I use an expression pedal with the PX-101?
Yes. The EXP jack works with an EP-3 expression pedal or similar, using a 50K to 100K linear pot. Four parameters can be controlled this way: Mix, Amount, Resonance and Cutoff. The Audiostrom MXP-1 expression pedal is also supported.
How much current does the PX-101 draw?
The current draw is 120mA at 9VDC. That is more than a typical analog drive pedal, so it is worth checking that a power supply output can deliver at least that much before building it into a board.
Sources (4)
- PX-101 LOWPASS FILTER, pastfx.com, 2026
- Guitar Pedal X, Guitar Pedal X, 2024-07-26
- Moog MF-101 Lowpass Filter, Vintage Synth Explorer, 2026
- Guitar Pedal X, Guitar Pedal X, 2024-12-19
