XR-Spectra: Tech Info

21st Century Pickups for the Modern Guitar Player.

The XR2 may well be the most versatile monophonic pickup ever.

Active or passive configuration.
Medium impedance (Mid-Z)
Low string-pull Neodymium magnets.
30kHz resonant frequency.
Uniform frequency response to 20kHz.
Humbucking dual rails.
Can be combined in pairs for PAF “humbucker” form-factor.
3.3″, 3.5″ and 4.0″ standard widths. 4.5″ and 5.0″ extended widths.
Optional Tone Block: a passive second-order (two-pole) resonant filter for sculpting the tone.

The Predecessor

The XR pickups are low-impedance pickups with a uniform frequency response from 20Hz to 20kHz. The XR pickup was engineered to capture the signal induced by the string with no added coloration. Traditional guitar pickups have a frequency response that starts to fall off around 2-6 kHz.

The XR is the monophonic sibling of the multichannel Nu pickup. It was the perfect partner for the Nu, and the pair eventually became the Nu-XR combo (image right). The Nu-XR combo was the perfect bridge pickup with its convenient PAF humbucker form-factor. The XR was perfectly suitable as a monophonic pickup, especially with the Resonant Filter for sculpting the tone, allowing you to dial in the characteristics of virtually any pickup.

Lo-Z

Both the XR, and the Nu are “Lo-Z” or low impedance pickups. The pickups operate under the same principles as a conventional magnetic pickup. The difference is that the XR and the Nu use far fewer turns of wire. Traditional pickups, especially passive ones, require significantly many turns of wire to deliver a signal output sufficient to drive guitar amplifiers. The higher the number of turns, the stronger the output. But this comes at the expense of coloration of the sound. As you increase the number of turns, you also decrease the resonant frequency, and the lower the resonant frequency, the darker the tone.

Traditional high impedance (Hi-Z) pickups have their own baked-in character. You will have to replace the pickup to get a different tone — defined by the pickup’s frequency response and characterized by its resonant frequency. The Lo-Z XR and Nu pickups, on the other hand, allow you to change the pickup’s tone anytime using post EQs.

The XR and the Nu pickups have an expansive frequency response well above 100kHz. Here is the measured frequency response of the Nu v2 pickup:

Mid-Z

Such a wide frequency range (>100kHz) is not really necessary since the limit of human perception is constrained to 20kHz anyway. Anything above 20kHz will do just fine. The original XR had 500 turns 42 AWG magnet wire per coil. The XR2, on the other hand, has 3000 turns 42 AWG per coil, making it a Mid-Z or medium impedance pickup. The main advantage is that the raw coils generate higher output, comparable to pickups such as the DiMarzio Injector™ (with 185mV output). That means that 1) the XR2 can be configured with or without the active preamplifier, and 2) the preamplifier gain required to bring it to a level comparable to other active pickups is considerably lower than that of the XR, which translates to lower noise.

With 3000 turns, the frequency response is still beyond 20kHz, with the -3dB point at around 30kHz. Here’s the measured frequency response of the XR2 pickup:

Tone Block

Available with the active XR2 configuration, the Tone Block is a small passive second-order (two-pole) resonant RLC filter with variable frequency and Q (resonance). It is specifically designed as post filter for the XR2 pickups, giving you the freedom to change each pickup’s character on the fly.

This second-order filter consists of an inductor, multiple capacitors and resistors in a parallel power of 2 configuration, switched using two miniature rotary hex switches, giving you 16 positions for frequency and another 16 positions for Q. The second-order filter provides a -12dB/octave (-40dB/decade) roll-off, which is exactly the response of a traditional high impedance passive pickup that the Tone Block is meant to emulate.

The XR plus the Tone Block allows you to dial in the characteristics of virtually any pickup.

The first rotary hex switch controls the cutoff-frequency from 1.5kHz up to 6kHz plus one all-pass setting. The sweet frequencies from 2kHz to 5kHz give you the classic tones of electric guitars. The human ear is most sensitive to frequencies in this range. As a rough guide, at 2kHz, the tone is characterized as warm and mellow; at 3kHz, brilliant or present; at 4kHz, piercing; and at 5kHz, more brittle and thin.

The second rotary hex switch controls the Q of the filter. Q determines the steepness of the curve and amplitude of the peak at resonance. The higher the Q, the narrower, sharper and higher the peak. A tall and narrow peak yields a more pronounced filter effect at the cutoff-frequency. A gentler slope yields a mellower, rounded tone. Q is continuously variable from 0.707 to 5.

Here’s a graph that shows the actual frequency response of the Tone Block (frequency = 3.5 kHz, Q = 5).

Modular Building Blocks

The XR2 and the Nu pickups are great modular building blocks that you can use to build complex pickup combinations. The form-factor is perfectly suitable for both straight and skewed orientations (e.g. multi-scale guitars). Here are some examples:

Quad Profile

Two XR2 pickups can be combined for a PAF “humbucker” form-factor.

For the passive XR2 configuration, adding a small capacitor in parallel with the coil output will lower the resonant frequency to taste, thereby setting the passive color from bright to dark (works well in conjunction with the passive tone control). This works for passive single XRs as well.