Direct from Canada 🍁...
-- https://www.broughtonaudio.com/product-page/resonant-filter-equalizer
So the cool thing this does is let you have a boost in your EQ, immediately followed (or preceded) by a steep cutoff. This can be also be achieved using digital effect like the HX Stomp by combining multiple filters, but to the best of my knowledge the RFE is the only analogue, single-box solution on the market.
I couldn't get a head-on photo without shadows or reflections, so here's a bit of the stock one so you can read the labels:
Aaaaaaaand here come the graphs...
First the high pass filter:
Yellow: minimum resonance (-3dB) 7:00 on the dial
Green: medium resonance (6dB ish) 12:00
Purple: maximum resonance (15dB) 5:00
Left trace: minimum frequency (25Hz): 11:00 on the dial
Middle trace: medium frequency (80Hz): 6:00
Right trace: maximum frequency (190Hz): 1:00
The mid filter:
Yellow: minimum resonance (-15dB) 7:00 on the dial
Purple: maximum resonance (15dB) 5:00
Left trace: minimum frequency (200Hz): 11:00 on the dial
Middle trace: medium frequency (750Hz): 6:00
Right trace: maximum frequency (2kHz): 1:00
And the low pass filter:
Yellow: minimum resonance (-3dB) 7:00 on the dial
Green: medium resonance (6dB ish) 12:00
Purple: maximum resonance (15dB) 5:00
Left trace: minimum frequency (330Hz): 11:00 on the dial
Middle trace: medium frequency (1.5kHz): 6:00
Right trace: maximum frequency (20kHz): 1:00
But what does it sound like?
Bypassed:
A1 bypassed.mp3
All controls at "default" (12:00 resonance, 6:00 frequency):
A2 default.mp3
So what you're hearing is a small bump around 80Hz, with lows beneath that rolled off. And small bump around 1.5kHz, with highs above that rolled off. The mid filter's not doing anything here.
And after some twiddling until it sounded nice:
A3 adjusted.mp3
High pass at 1:00 resonance (7dB), 7:30 frequency (60Hz) (all values approximate)
Mid at 10:30 (-5dB), 5:00 (800Hz)
Low pass at 1:00 (7dB), 5:00 (2.5kHz)
Bypassed with a plectrum:
B1 bypassed.mp3
I forgot to write down the settings here, but it's an example of the odd, characterful sounds you can create:
B2 adjusted.mp3
Bypassed with flats, a foam mute, and a Poundland Jamerson impression:
C1 bypassed.mp3
The RFE can be used to mimic the way old-school speaker cabinets often had a frequency peak right before they started rolling off the lows and highs:
C2 adjusted.mp3
High pass at 3:00 resonance, 7:30 frequency
Mid at 1:00, 9:30
Low pass at 12:00, 5:30
Which gives an EQ curve like this:
It's pretty cool! As I mentioned, this kind of thing is easy enough to achieve in the digital realm, but there's something very direct and satisfying about having all the physical controls there to be fiddled with. It also works great with guitars, and can be powered with higher voltages in order to take line-level signals - there's probably fun to be had combining this with synths.