The Headphone Amplifier Box enables a wide range of audio signals to be monitored on standard headphones with negligible interaction on the original signal lines. Input sensitivity can be selected to suit signal levels directly from microphones or line outputs in the range 1 mVolt to 10 Volts RMS. A loop through facility is provided at unity gain.
The use of a transformerless design with close tolerance (0.1%) components ensures good line balance and high common mode rejection performance. Bridged headphone drive maximises the available output level from the 9 volt PP3 battery, while quiescent power consumption is kept to a minimum for extended operational life. An external power socket allows for mains operation from a standard 9 volt DC adaptor. High input impedances for the microphone and line level inputs are designed to bridge and not terminate the line being monitored, thus avoiding any level changes. Careful attention to screening and filtering provides protection from radio frequency interference.
The low distortion and noise performance of the design offers a high quality headphone monitoring unit suitable for portable or permanent installation in educational, live music recording, news gathering, public address, entertainment, multimedia and industrial situations. Microphone boom operators can monitor their audio signal without interrupting the ongoing transmission feed.
Typical modes of operation
To monitor a balanced line connect the two signal lines and screen to the input connector using the pin numbers defined in the specification. Unbalanced signal sources can be accommodated by grounding one of the input signal lines and using the other signal line and ground as an unbalanced input. The line can be taken from the output connector to a final destination if required (Loop through). The input sensitivity switch is then used to select Mic input for typical microphones and Line for typical line levels. The headphones are plugged into the 0.25 inch A gauge jack socket although a range of adaptors are readily available for use with alternative connectors. The jack socket is wired for use with standard three pole stereo plugs and both ear pieces are driven together in parallel. A two pole mono jack plug should not be used. The Headphone Level control on the top of the box is a variable gain control which is used to adjust the volume as required.
Some unusual applications
The versatility of the Headphone Amplifier Box means that it can also be used as a general test instrument for tracing line faults, or giving a quick performance assessment of a suspect microphone. Connected directly to a microphone in this way produces a simple one way intercom system. The wide range of input signal levels catered for enables even speaker lines up to about 100 Watts nominal power level to be monitored without danger of hearing damage by overdriving the headphones.
A further application is as a practice amplifier for musical instruments fitted with inductive pickups such as electric guitar or violin. Although lacking any special effects the unit allows private playing of the instrument through headphones without any trailing leads.