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MC624 Monitor Controller


mc624 front view



The MC624 is similar to the monitor section of a mixing desk: it lets you select one out of six stereo audio sources, adjust the level and route it to one of four monitor outputs.

The level can be set in very accurate 1dB increments.

The MC624 gives you the possibility to listen to normal stereo, right only, left only, left + right (mono), phase reversed right, left – right (difference), dimmed and mute.

Inputs can be configured for balanced or unbalanced (input 1 to 4), -10dBV or +4dBu level and an offset can be added if added to correct input level differences.

Outputs can be configured for balanced or unbalanced and an offset can be added to correct output level differences. Output 4 can be set as a Sub output (unfiltered).

An infra-red detector allows controlling the MC624 from an ordinary TV remote, with a learning function.

Several units can be linked together for surround. One unit is the master and controls the other units.



  • 6 stereo sources: 2 XLR, 2 jacks, 1 RCA, 1 RCA/front mini-jack,
  • 4 stereo output or 3 output+sub: 2 XLR, 2 jacks,
  • Relay driven passive stepped level control, 64 x 1dB steps,
  • Buttons for Dim, Mute, Mute left/Mute right, Mono, left-right difference,
  • "Wire with gain" headphone amp with 2 gains,
  • Minimalistic audio path,
  • Fully differential throughout,
  • All switching done by relays,
  • Controllable by a TV remote, with a learning function,
  • Expandable to surround by stacking several units,
  • Available assembled or as a full kit (everything included).


mc624 back view


Design considerations

The design objective has been to put as little electronics as possible between audio inputs and outputs. The total passive approach was ruled out because of the stress it puts on the audio sources, increasing distortion, when you want to achieve a low enough output impedance, necessary for the noise immunity. So we opted for a unity gain buffer, optionally switch-able to provide the 12dB of gain, necessary for the semi-pro -10dBV compatibility. The buffer used is one the best op-amps available today (spec'ed distortion at 0.00008%). This buffer is the sole active component in the signal path. The circuit is fully differential throughout and there is no capacitor in the signal path. All the switching is done by relays. The buffer feeds a passive precision logarithmic attenuator that is also switched by relays. The headphone amplifier is also a high end design with extremely low distortion, high bandwidth and high drive capability.


Bloc diagram

bloc diagram



INPUT SELECT Pressing one of the six push switches selects the corresponding input and sends it to the selected output and to the Meter Output. Inputs are mutually exclusive.

INPUT 6 Is a 3.5mm jack on the front panel duplicate of the back panel input 6.

OUTPUT SELECT Pressing one of the four push switches sends the selected input to the corresponding output. Outputs are exclusive except output 4 which can be set as a SUB output and activated in addition to one of the first three.

MUTE Turns off the monitor output signal (but not the Meter Output neither the headphone output).

L/R Mutes the left and right channels alternatively. Unmuting is done by pressing the MUTE switch.

MONO Sums the left and right channels

DIFF Inverts the phase of the right channel. When DIFF and MONO are both activated, the output is the difference left-right.


DIM attenuates the output by an amount of dB defined in "Functions Setup" (default: -20dB).

Infrared sensor Lets you control all the function from an ordinary TV remote.

3 Digits display Displays the current level in dB. It is possible to set "0"dB at the level of your choice. The display is also used to set-up some parameters.

LEVEL Sets the monitoring output level. It is also used to set-up parameters.

PHONES Phone level setting, independent of the monitoring level. There are two phone jacks, one on the front panel and one on the back panel.

O1 switch Power switch





1 Input select relays

2 Quad buffer zone

3 Logarithmic ladder attenuator

4 Output select relays

5 Headphone amplifier

6 Microcontroller



What's in the kit?

Absolutely everything!

  • Passive components (resistors, capacitors, inductors),
  • Active components (diodes, IC's),
  • Front panel elements (switches, potentiometers, knobs),
  • Connectors, relays,
  • PCB's,
  • Case and all hardware


You bring the solder (use good quality solder, 1mm diameter max).


Difficulty level

The MC624 kit is a medium difficulty kit. Nothing very difficult but a rather large number of components.

Setup is very simple but require a good quality DC millivoltmeter.


Technical specifications

Measure Conditions Value
Input impedance Phase to ground 15 kΩ
Maximum input level on differential inputs +26.5 dBu
Maximum output level on differential outputs +26.5 dBu
Minimum gain   - 63 dB
Maximum gain   0 dB or +12dB
Frequency response Deviation < -1dB DC to 250 kHz
Total harmonic distortion (THD) + N f=1kHz too low for the HP8903A
Headphone amp frequency response Deviation < -1dB 1Hz to >250 kHz
Headphone amp output power R load = 33 ohms 1165 mW
Headphone amp output power R load = 66 ohms 950 mW
Headphone amp output power R load = 132 ohms 580 mW
Headphone amp THD + N f=1kHz too low for the HP8903A



Technical documents