The metering module is a small general purpose comparator based LED peak metering module. For the AX1250 preamp it is set to show -24dBu to +18dBu in 6dB steps. Both the range and increment can be altered by changing the ladder resistor values. A connector is provided to plug directly into the preamp module as well as a separate header for use in other applications.
The colours chosen for the LEDs are up to individual taste, for the AX1250 green was chosen up to and including 0dB, yellow for +6dB, red for +12dB and white for +18dB. The idea with the white led is that it is very bright and eye-catching when it illuminates so is easily seen even if not looking directly at the panel.
The module is mostly SMD to keep the size small. The pitches are 1.25mm or greater so should not present a challenge to constructors. The circuit consists of an active full wave rectifier based around a TL072 op amp which feeds a set of 8 comparators. The resistor ladder uses the 16V rail as its top reference so it will need adjusting is a different voltage is used. The LED string is fed with a current source (based around Q1) which is shunted using the comparators. When a comparator switches off the corresponding LED is lit. This method saves having resistors in each LED leg. The circuit is adapted from one described by Douglas Self in his book "Small Signal Audio Design".
|MPM-3C Circuit (pdf)||MPM-3C BOM (pdf)|
|MPM-3C PCB (pdf)|
The LED metering module