The Coils

      Electronics, Open Source, Pickups, Six-pack

Balanced Active Pickups

By and large, the current state of the electric guitar pickup design has not really changed much since the 50s. We still use Fender Stratocasters and Gibson Les Pauls with unbalanced, mono outputs. The typical electric guitar is notoriously very noisy (yes, even the ones with humbuckers) yet we send the outputs through distortion/overdrive and amplifiers often with a lot of gain. Then, we use noise gates to somehow alleviate the annoying noise.

Compare the state of the electric guitar pickup to its cousin, the dynamic microphone. Dynamic microphones have been using balanced wiring from way back. As a result, dynamic microphones are very quiet. We are in the 21st century now and we guitar players are still stuck with noisy unbalanced instruments from the 50s. Even “modern” active pickups are still unbalanced, last time I checked (Edit: Dave Schwab pointed out that EMG pickups have the two coils connected to the op amp in a differential manner).

The Six-Pack Hexaphonic Pickup will have six individual coils —one for each string. Unlike heavy, bulky, high-impedance passive pickups, our coils will have fewer turns (lower impedance). Each coil will have an on-board differential amplifier, just like those found in microphone pre-amplifiers, to boost the output and cancel the noise. The signals at each end of the coil are 180° out-of-phase while noise propagates equally through the wires in-phase. The differential amplifier subtracts the signals and the noise. The result is that in-phase signals (the noise) get canceled out while the out-of-phase signal gets doubled. If you are unsure what that all means, this link is a good tutorial.

magnet

Neodymium Magnet

bobbin

Bobbin

Fitting all six pre-amplifiers plus the six coils in one small package is a challenge and can be done only by using SMT devices (more on this soon). The coils are wound around a small bobbin with a 10mm diameter and 5mm height  (image at the right). At the center is a 5mm diameter neodymium (image at the left). Currently, we find that 1000 turns using a guage 43 magnetic wire provides sufficient response using a differential amplifier with a gain of five. The DC impedance is around 150 ohms. The output can go as high as ±4 volts peak to peak.

Like all active low-impedance pickups, the frequency response is wide (full range). Unlike passive pickups, there are no big peaks and dips in the response, which to many give the pickup it’s “character”. That should not be a problem since we can always shape the frequency response later on in the signal chain using various tone-shaping methods.

To help us prototype our coils, we hacked together a pickup winder (below). This quick hack is even better than some of the commercial offerings. Basic controls include setting the target number of turns (by 100s, 10s, and 1s), start, pause, stop, speed control, forward, reverse winding. For the logic, we used the cuddly little Arduino.

The Pickup Winder v0.9

The Pickup Winder v0.9

Further Reading

  1. Balanced Guitar Wiring
  2. Balanced Vs. Unbalanced I/O – How It Works
David says:

EMG pickups have the two coils connected to the op amp in a differential manner.

Balanced lines on mics are really to get rid of the noise picked up by the cable. This is because the output of the mic is fairly low.

Some neo rod magnets might cause a lot of string pull. be aware of that.

Joel says:

Thanks, David. Yes. I'm well aware of the string pull. I'm currently using a very small magnet (5mm) with sufficient space from the strings. The resulting sustain is on par with if not better than the LP and Strat PUs (when swapped).