Precision balanced or unbalanced microphone or line level input
to headphone output
The Headphone Amplifier Box 
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.
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, Line 1 for typical domestic
line levels and Line 2 for professional 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.
Conford Electronics Conford Liphook Hampshire GU30 7QW England
Telephone +44 (0)1428 751469 Fax 751223 E-Mail contact@confordelec.co.uk
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