5
6
7
8
CM-BHA Belt Pack Headphone
Amplifier
Introduction
The CM-BHA is a portable, battery powered headphone amplifier for use in live
applications such as in-the-field news reports, providing audio feed monitoring
by headphones, earpiece, or built-in speaker.
Up to +30dB of gain can be applied to the incoming signal. The volume control
simultaneously modifies the output volume and the input gain, allowing input
signals of up to +18dBu to be reproduced without distortion.
An adjustable limiter protects the listener from excessive levels. The limiter is
only enabled when an earpiece or headphones are used, and when it is enabled
by the toggle switch. It is disabled when no jack is inserted, allowing a higher
volume to be passed to the speaker for “squawk mode” use.
Fig 1-1: CM-BHA Belt Pack Headphone Amplifier
A low battery warning LED flashes when the 9V PP3 battery has depleted to 6V,
notifying the user to swap the battery. The low power circuitry of the CM-BHA
gives up to 200 hours use from a single alkaline 9V PP3 battery.
Fig 1-2: CM-BHA Block Diagram
2 Top Panel Controls & Indicators
Fig 2-1: Top Panel View of CM-BHA
Headphone/Earpiece Socket
The headphone/earpiece connector is a ¼” jack socket that provides a
differential signal to a mono earpiece, or to stereo headphones. When using
stereo headphones, the right ear will be anti-phase, however speech will be
perfectly discernible.
Gain/Volume Control
The gain/volume control simultaneously modifies input gain and output
volume, allowing the user to connect high level signals (up-to +18dBu) without
distorting.
Low Battery Indicator
Should the battery voltage fall below 6V, the 3mm red low battery indicator will
flash, warning the user that the battery should be changed as soon as possible
to prevent failure.
Limit Level Control and Indicator
The output level limiter (when enabled) prevents large signals from damaging
the user’s ears. The limit level is adjusted between -6dBu and +6dBu using
the recessed potentiometer, and limiter activity is indicated by a blue LED.
Persistent illumination informs the user that the volume should be reduced.
3 Side Panel Controls
Fig 3-1: Side Panel View of CM-BHA
Limiter On/Off Switch
The output level limiter is enabled/disabled using the toggle switch on the side
panel. The limiter is disabled when the built-in speaker is active.
4 Bottom Panel Connections
Fig 4-1: Bottom Panel View of CM-BHA
The audio connections are found on the bottom panel, and both have the
following connections:
Pin 1: Ground
Pin 2: Phase
Pin 3: Non-phase
Line Input
The line input is an electronically balanced audio input with 200kΩ input
impedance, allowing multiple units to be daisy-chained together, without
causing noticeable loss of signal level. The line input can accept input levels of
upto +18dBu.
Loop Through
The loop through is a non-driven, parallel connection of the line input, allowing
multiple units to be daisy-chained together without specialist audio cables.
Low
battery
Limit
Off/On
Headphones
or earpiece
Speaker
Threshold
Limiter
activity
Volume
Gain
Off
On
Limiter
Loop through
Line
input
<6V
5 Technical Specification
Line Input Impedance:
200kΩ Electronically Balanced
Input Liiter Threshold:
Adjustable -6dBu to +6dBu
Maximum Input Level:
+18dBu
Input Gain/Volume:
Adjustable -60dB to +30dB
Frequency Response:
±0.5dB (ref 0dBu 1kHz)
Distortion:
<0.015% (ref +8dBu input, 0dB gain, 10kΩ load)
Maximum HP Output Level: +16dBu (ref 9V battery voltage, <1% THD,
2kΩ load)
+12dBu (ref 6V battery voltage, <1% THD,
2kΩ load)
Minimum HP impedance: 32Ω
Maximum Speaker Level: +12dBu (ref 9V battery voltage, <1% THD)
+6dBu (ref 6V battery voltage, <1% THD)
Dimensions (Raw):
12.5cm (W) x 6.7cm (D) x 4.8cm (H)
4.9” (W) x 2.6” (D) x 1.9” (H)
Dimensions (Boxed):
23cm (W) x 13cm (D) x 8cm (H)
9.1” (W) x 5.1” (D) x 3.1” (H)
Weight:
Nett: 0.3kg Gross: 0.4kg
Nett: 0.7lbs Gross: 0.9lbs
w w w . s o n i f e x . c o . u k
t:+44 (0)1933 650 700
f:+44 (0)1933 650 726
