9V Battery Ripple Filter for Active Pickups

A tiny RC filter board that mounts on top of a 9V battery and removes the high-pitched whine that USB-rechargeable lithium 9V batteries inject into active guitar pickups (EMG and similar).

The Problem

USB-rechargeable “9V” lithium batteries are a Li-ion cell plus a small boost converter. At the very light load of active pickups (~160 µA for two EMGs), the converter runs in burst mode, producing a ~360 mV p-p sawtooth at roughly 8 kHz on the supply rail, right in the audible range. Under heavier load it switches continuously at ~1.2 MHz instead.

Active pickup preambs are micropower op-amp circuits biased from a half-supply divider, so supply ripple goes straight into the signal path and comes out of the amp as a steady high-pitched whine.

The Fix

A simple RC low-pass filter between the battery and the pickup power buss:

• R1, 1 kΩ: series resistor
• C1, 100 µF electrolytic: filters the ~8 kHz burst ripple
• C2, 0.1 µF ceramic: handles the ~1.2 MHz switching spikes

Corner frequency is ~1.6 Hz, giving roughly 74 dB of attenuation at 8 kHz. The ~360 mV of ripple drops to tens of µV, below the pickups' own noise floor. Voltage drop at 160 µA is only ~0.16 V.

Installation

1. Snap the board onto the top of the 9V battery.
2. Connect the board’s +V OUT to the pickups' red wires / +V buss.
3. Connect GND to the guitar’s ground bus.
4. Battery negative still goes to the output jack’s ring as usual.

Note: the preamps take a fraction of a second to settle after plugging in (RC time constant ≈ 0.1 s), so a small thump on plug-in is normal.

BOM

Notes

• Plain alkaline 9V batteries have no converter and don’t need this board.
• Works with any active pickup system that runs from a 9V battery.