The Stereo Headphone Amplifier Box takes balanced or unbalanced line level left and right stereo inputs and drives standard stereo heaphones over the wide impedance range of 30 Ohms to 1 kOhm. Input sensitivity is nominally 50 mVolts to 5 Volts RMS and a mono/stereo selection switch is provided.
The use of close tolerance (0.1%) components in the transformerless differential input design ensures high common mode rejection performance. A high efficiency headphone driver produces maximum output from the 9 volt supply while keeping quiescent power consumption to a minimum for extended battery life. An external DC input socket allows for mains operation from a standard 9 volt adaptor. The high input impedance provided is intended to bridge and not terminate the monitored line thus minimising any loading effects. Extensive filtering of the audio lines and comprehensive screening provides excellent protection from radio frequency interference.
The high performance of the unit offers a quality stereo headphone monitoring facility suitable for portable or permanent installation in educational, live music recording, multimedia, public address and industrial situations.
Typical modes of operation
To monitor balanced stereo lines connect the two (differential) signal lines of each channel along with the screens to each of the left and right input sockets using the pin numbers defined in the specification. Unbalanced (single ended) inputs can easily be accommodated by grounding one of the balanced input lines and using the other signal line and ground as an unbalanced input (see specifications).
The headphones are plugged into the 0.25 inch jack socket although a wide range of adaptors are readily available for use with alternative connectors. The socket is wired for use with standard stereo 3 pole plugs and a 2 pole plug should never be used. The volume is adjusted by way of the rotary control mounted on the top of the box.
The front panel mono/stereo slide switch can be used to combine the left and right inputs resulting in an identical mono output signal to each of the stereo headphone ear pieces.
Variations in use
For monitoring mono line sources the signal can be connected to either the left or right input, and with the mono/stereo switch set to mono, both headphone ear pieces will be driven by the single input.
The versatility of the Stereo Headphone Amplifier Box means that it can be used as a general purpose test instrument for tracing line faults or for quick operational checks. The high input signal level capability means that even speaker lines up to about 50 watts nominal power level can be monitored without danger of hearing damage by overdriving the headphones.
Stereo Headphone Amplifier
Precision balanced or unbalanced stereo line level input to stereo headphone output