Boredbrain Music Monitr Eurorack Stereo Output Module

Listen In & Cue Up

Monitr is a comprehensive audio output monitoring utility for eurorack systems. The module is equipped with a stereo VCA-controlled input mixer with master volume control and VU meters, plus a powerful headphone section for both live performance and detailed monitoring.

Stereo VCA Mixer

Monitr’s two stereo inputs, IN 1 and IN 2, are AC-coupled specifically for audio signals, with the R inputs normalled to the L inputs. These input signals are amplified and mixed via stereo VCA circuits to reduce noise and allow for external voltage control. The CHAN 1 and CHAN 2 parameters are for manual adjustments of audio levels, while the VCA1 and VCA2 jacks accept control voltages. The full range of each stereo VCA is 10 Vpp. The mixed stereo signal is buffered and passed on to the master volume circuit.

Master Control

The VOLUME parameter is a variable attenuator which controls the master stereo output level. It can also amplify the signal up to +6dB above unity gain.

The output volume is visually indicated by two 5-segment VU METERS, scaled appropriately for +4 dBu professional line-level output. Best practice is to keep the master signal volumes at or below +3 dBu (GREEN, YELLOW), with transient peaks occasionally reaching +6 dBu (RED). This will optimize the signal’s dynamic range when feeding into audio interfaces or amplifiers.

Monitr also features a MUTE OUTPUT switch that completely silences the main outputs. This is convenient once sound check levels are adjusted before a live performance begins, as the mute switch does not affect the VU meters or headphone cueing section.

Headphone Cue

The module’s headphone circuit is designed to drive a wide-range of headphones with enough detail and power for both studio monitoring and live performance. The LEVEL parameter adjusts the volume of stereo headphone plugged into the PHONES jack. Acceptable headphone impedance ranges from 32 Ω up to 600 Ω.

The headphone source is selected with a 3-way switch, which is useful for cueing during a live performance. The center position, MSTR (Master), monitors the primary master output signal post Volume, so the master VOLUME also affects the headphone volume. The other two positions, CH1 and CH2, exclusively monitor the two input signals respectively, and are not affected by their VCA levels (CHAN 1, CHAN 2, VCA1, or VCA2).

WARNING: Plugging a MONO cable into the PHONES jack and cranking the LEVEL can cause damage to the headphone output circuit, as this will short excess current directly to ground.

Balanced Output

OUT L and OUT R provide balanced line-level stereo output of the master audio signal via 1/4-inch TRS jacks, typically to an audio interface, mixer or amplifier. These balanced connections allow for low-noise signal transmission over longer cable runs, provided the destination input is also balanced. However, unbalanced TS cables can also be used and will simply carry the signal unbalanced.

Calibration Procedures

VU METERS

Feed a 1 kHz sine wave or similar into IN1 L and turn CHAN 1 all the way up. Use an oscilloscope to measure the peak-to-peak voltage from the OUT L jack (tip and sleeve), and set the VOLUME knob so the signal output is exactly 4.37 Vpp. Now adjust both trimpots on the bottom of the module until their respective RED LEDs just barely illuminate. In most cases it is better to match left and right as close as possible rather than be exact.

Patch Tips

SUBGROUP DUCKING – Patch a drum or drum mix into IN 1, while sending a mixed subgroup of other audio into IN 2. Patch an inverted envelope voltage triggered by the drum signal into VCA2 to “duck” channel 2 behind drum hits.

STEREO FX CONTROL – Patch just a reverb or other time-based effects return into IN 2, and a dry mix into IN 1. The effects level can now be controlled separately at the output, and modulated with CV at VCA1. Use abruptly changing voltages for more drastic results.