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Electronic Projects - Coxing Amplifier

This amplifier is designed for the coxswain of a rowing boat - not that they generally need it!

Coxing Amplifier


The aim of the coxing amplifier is to amplify the cox's voice as they speak to the crew. The boat is normally fitted with two or three speakers under the seats of the rowers, and the cox is able to speak through these using a headset microphone. A cox box also connects to sensors on the boat that tell the cox the rate at which the crew are rowing. The coxing amplifier does not provide this information, it is simply a battery powered amplifier.

The requirements of the coxing amplifier are given here:

  • Connect to and be compatible with the headset microphone (In this case a BNC connector is required).
  • Connect to the speakers in the boat (In this case a Amphenol Series 44 circular connector).
  • Power the speakers for at least 6 hours continuously - The unit may be used by several crews in one day.
  • Output power of at least 5 watts (rms).
  • Rechargeable using an existing charger (In this case a 12v Neilsen Kellerman charger).
  • Packaged in a tough, water resistant housing. If no other Cox boxes are currently in use then this can be relaxed. Our boat house use Neilsen Kellerman equipment and so the new Coxing amplifier had to be compatible with the existing connectors, speakers, microphones and chargers.

Circuit design

The circuit is split into three parts:

  1. Microphone Pre-Amplifier
  2. Power Amplifier output stage
  3. Battery charging control circuit

The Microphone Pre-Amp could be a single transistor amplifier, but I decided to make use of a dedicated IC. The SSM2165 is a microphone pre-amp with a built in compressor and noise gate. The compressor adjusts the gain to give a constant output. This prevents the output stage from distorting when the cox shouts into the microphone, and increases the gain if the cox speaks quietly. The noise gate silences the output if the input signal is very small. The effect is to stop the speakers from hissing when the cox isn't speaking.

I aimed for a power level of 15 watts, which in retrospect is a little high. We don't generally turn the volume above 30% (about 5 watts) as the output is so loud it is picked up by the cox's microphone causing feedback. On the other hand, it is useful if you want to blast out music as you row along (not that we'd do that of course.) The Pre-amp and Output amplifier are joined by a potentiometer that controls the volume. I used a potentiometer with integral push switch, so the unit can be turned on and off without adjusting the volume.

The battery charger is a basic constant current source. It requires the charger to provide at least 2v more than the battery voltage. The circuit diagram is accompanied with a more detailed explanation of how the circuit works.


The circuit was built on a single PCB, with the heat-sink doubling as a bracket to hold the battery. The photos show this more clearly. The circuit had a single control for On/Off and volume. Two LEDs were used to indicate power and recharging. A Bi-coloured LED could have been used, but would have been harder to label.

The two connectors were fitted on the end - The BNC for the microphone and the NK 5-pin for the speakers and re-charger. The original casing was waterproof, but this was replaced with a smaller ABS box. The seal is sufficient to keep water splashes out but the volume control is not waterproof and so the unit should not be submerged.