Resonant Filter v1.46


resonant_filter Technically speaking, a pickup is an audio voltage source followed by a second-order lowpass filter. The tone (colour) of the pickup is characterised by its cut-off frequency and its resonance contour — the so-called Quality factor or simply Q.

The Resonant Filter allows you to dial in the characteristics of virtually any pickup with two high quality dual-ganged Bourns® potentiometers. The Resonant Filter offers independent control of the cutoff-frequency and resonance (Q). The Resonant Filter is a single opamp VCVS second-order low-pass filter design with variable frequency, and variable resonance with constant gain compensation. The circuit utilises the audiophile grade OPA209, a modern precision operational amplifier with very low voltage noise density of 2.2nV/√Hz, and a rail-to-rail output swing with a supply voltage from 4.5 to 36 volts  maximising the dynamic range. The Resonant Filter module connects to the main switch using 3M gold-plated pin connectors. Cables included.

Listen to the Resonant Filter. Starting with maximum resonance (Q) and maximum frequency, I sweep through the frequency range until I find what I want. (It’s easier to find the right cutoff frequency with a high resonance for starters) Finally, I turn down the resonance to achieve the tone desired.

Frequency Control

The Frequency pot controls the cutoff-frequency from 2kHz all the way up to 20kHz. The sweet frequencies from 2kHz to 5kHz give you the classic tones of electric guitars. The human ear is most sensitive in this frequency range. As a rough guide at 2kHz, the tone is characterised as warm and mellow; at 3kHz, brilliant or present; at 4kHz, piercing; and at 5kHz, more brittle and thin. (source: The Secrets of Electric Guitar Pickups).



Here are some graphs of frequency responses of various pickups. The graphs are obtained from actual frequency response tests we have conducted.



Resonance Control

The Resonance pot controls the Q of the filter and determines the steepness of the curve. The higher the Q, the narrower and sharper the peak. A narrow peak gives a more pronounced filter effect at the cutoff-frequency. A gentler slope gives you a mellower, rounded tone. Q is continuously variable from less than 1 to greater than 4.


As you can see in the graph above, a higher Q produces higher gain. The circuit compensates for the gain with a corresponding gain reduction before the filter, so we have constant gain for all resonance settings. This prevents the filter from instability and clipping, and produces an overall balanced output, regardless of resonance setting.




The module has two parts, namely: Frequency and Resonance. They are separate to give you more flexibility to layout the controls in your guitar. The two parts are connected via a solder-free shielded cable as shown below. The Frequency control connects to the pins labeled “Freq.” Another set of pins labeled “Ins” (short for Insert) connects to the Insert Pins again via a solder-free shielded cable. The “Ins” pins provide power as well as connections to the filter’s input and output signals.

See XR [Flex] Installation Guide for complete installation instructions. For XR v1.46 systems [purchased before March 2016], please refer to the Cycfi XR Series Datasheet v1.46 for installation instructions.


Note small moulded triangle on connector for proper orientation.

Version 1.46, May 2015, Joel de Guzman, Cycfi Research, Inc.

Bourns® is a registered trademark of Bourns, Inc. The use of this trademark does not imply any affiliation or endorsement by the trademark holder.