This two-channel, battery-powered headphone amplifier is designed for use in the field to monitor cue and a local source. Typically used by broadcasters, it may also suit the needs of sound crews in film and entertainment environments. Inputs are balanced, with extra gain available and the limiter on each channel is adjustable, to prevent problems with the varying levels often found out on location.
The amplifier is built in a robust standard extruded box, fitted with protective ABS bezels and belt clip. The upper face has two volume controls, one for each earpiece and two switches to provide 20dB extra gain for each input. The headphone socket is a standard stereo 6.35mm A-gauge connecter, wired with the left earpiece fed from the ‘local input’ and the right earpiece fed from the ‘cue’ input. The PP3 battery is fitted in a slide-out drawer.
On the lower face are three XLR connectors, two female for the ‘local’ and ‘cue’ inputs and a male to provide a loop-though connection. This is wired in parallel with the ‘cue’ input, typically fed with programme or IFB (interrupted feedback or foldback). A single screwdriver-operated control adjusts limiter sensitivity. The limiters are independent, but with a common threshold, operating after the volume controls. The threshold can be adjusted to a point where there is no limiting, where this is required.
Inputs: 0dBu nominal, 14dBu maximum, 50kohms. Gain: 0dB or 20dB, as selected, from input to headphone output, no load. Distortion: 0.02%, +13dBu, 600 ohms. Noise: -75dBu, 20dB gain, maximum volume. Limiter threshold: Variable from no limiting to -7dBu output. Output: 13dBu maximum, to suit headphones 8 ohms to 2kohms. Power source: 9 volt alkaline, MN1604, V4122 etc. Battery life: Up to 50 hours with alkaline battery. Dimensions: 107 x 93 x 50mm, not including control knobs.
The amplifier has no switching or mixing; one input is fed to each earpiece. The cue feed is commonly fed with IFB, where the cue feed is interrupted with talkback at a point upstream of the amplifier.