MFJ-616 Speech Intelligibility Enhancer
TM
Instruction
Manual
7
INTERNAL SETTINGS
The
Speech Intelligibility Enhancer
TM
headphone circuit is normally attenuated to protect you
against hearing injury due to an accidental high volume setting. If the sensitivity of your phones
is unusually low, you may bypass the attenuators by removing the cabinet cover and installing
shorting plugs on JMP4 and JMP5. JMP4 and JMP5 are located on the main PC board directly
behind the headphone jack (see below).
Front Panel
JMP5
JMP4
Note that these jumpers are not pre-installed at the factory, and most headphones do not require
installation. Install them only as needed!
THEORY OF OPERATION
A
B
Input
Volume
Equalizer Section
300
600
1200
2400
U2 A-D
U1A
U1B
Spkr
Spkr
Phones
Balance
Bypass
U1C
U1D
U6
U7
U3
12 Vdc
8 V Reg
Power
AF
Speaker
A/B
Attn
Power
SW1
SW3
SW5
SW2
Left
Right
R21
R20
Transceiver sources are fed to inputs A and B at line level. SW1 (
Input A/B
) selects a source and
routes it through
Volume
control R21 to pre-amp U1B. Following U1B, active filters U2A-U2D
provide four octaves of adjustable audio conditioning at the input of summing amplifier U1A.
Alternatively, audio is routed around speech enhancer circuitry via
Bypass
switch SW3. A
bridge-type
Balance
control at R20 splits the AF signal and sets a drive ratio for pre-amplifiers
U1C and U1D. Monolithic AF-power amplifiers U6 and U7 then boost the divided signal to
speaker level.
Speaker On/Off
switch SW2 provides the option to disconnect speakers for
headphone listening.
Power On/Off
is a audio routing switch that bypasses all amplification
circuitry when off. DC power remains connected to the unit at all times.