Boredbrain Music Xchangr

Signal Interchange

The availability of quality stereo effects in pedal format still outnumbers those available in eurorack format, and thus processing eurorack-level signals with them is often desired. This requires proper attenuation and amplification, especially as the inputs of these digital effects can be unforgiving if the signals are too hot, have excess noise, or a slight DC offset.

Xchangr is meant to facilitate this signal interchange with its overall simplicity and purposely designed circuitry. Its three separate sections together exchange stereo audio with external processors at proper levels and with sonic transparency.

Level Scaling

The upper two SND sections are identical, and both apply attenuation to pairs of discrete inputs (3.5 mm) to send into stereo pedals and line-level processors. Each is equipped with a two-position switch to select the amount of attenuation, either -12 dB or -18 dB. The right side input is normalled to the left, so if no plug is present, the right SND output (1/4-inch) will copy the left.

The lower RCV section applies amplification to its pair of discrete inputs (1/4-inch) to bring external audio back up to eurorack levels. It is also equipped with a switch to select the amount of amplification, either +12 dB or +18 dB. The right side input is normalled to the left, so if no plug is present, the right RCV output (3.5 mm) will copy the left.

When interfacing with a stereo pedal, this leaves an extra SND pair on the module, which can be used for parallel processing or as an additional line output to a mixer, interface, or amplifier.

±18 dB is suggested for high-fidelity pedals, and ±12 dB for rackmount or line-level processors.

Audio Fidelity

Xchangr’s connectons are AC-coupled specifically for audio signals. This means control voltages are not meant to pass through the module, and any DC offsets will be removed.

The inputs of the two SND sections use first order low pass filters just above 25 kHz to attenuate any high-frequency noise in the signals like EMI (Electromagnetic Interference). The signals also run through 1.5 Hz high pass filters to remove any DC offset just prior to being output.

The inputs of the RCV section employ 5 Hz rumble filters to reduce any unwanted low frequency noise prior to being amplified back up to eurorack-level.

The combined filtering sections, along with low-noise buffering, allow only the desired parts of the signal to be processed and returned. The result is clean and undistorted external stereo effects processing.