R a d i o S y s t e m s , I n c .
M A - 4 M a n u a l
P a g e 2
MA-4 MANUAL
1.
OVERVIEW
The Radio Systems MA-4 functions to mix 4 stereo balanced audio input signals to one stereo
output. Features include variable input stage gain straps, continuously variable mix-bus attenuators,
individual channel front panel test points and an on-board headphone amplifier.
The unit is also designed to house an optional 2x4 output distribution amplifier on the left-hand side
of the cabinet (Radio Systems model # DA 2x4a).
2.
MOUNTING
The MA-4 occupies only 1 rack unit (1 ¾ inch) of height in a 19 inch EIA rack. To allow for adequate
ventilation, do not mount the unit directly above large heat producing equipment such as power
amps or power supplies.
When stacking units, it is recommended that one rack space, (1 ¾ inch), remain open between
every three units.
3.
WIRING
Wire each input channel and the left and right stereo output feed to an individual 3-pin removable
barrier strip connector (provided). Pin-outs are as follows for all cables:
Pin 1 – ground Pin 2 – positive (+) Pin 3 – negative (-)
For unbalanced inputs wire the hot lead to the positive terminal (pin 2) and the shield to the ground
terminal (pin 1). Short the negative terminal (pin 3) to ground (pin 1).
For mono inputs, wire the feed in parallel to both left and right inputs for that channel. Inputs are
bridging (20k ohm) impedance so the output loading of your source device will not be effected.
Unused inputs can be left unterminated (unconnected).
For unbalanced outputs, connect to the positive (pin 2) and ground (pin 1) terminals. Leave the
negative terminal (pin 3) unterminated. Do not short this pin to ground (see the “Using Active
Balanced Circuitry” page at the end of this manual for more information).
4.
OPERATION
Apply an input level of approximately 0dB to the input terminals. The gain of any stage may be
increased by 20dB in 10dB steps by changing the internal gain straps to compensate for low-level
source equipment.
To locate and reset these input jumpers, refer to the parts layout for the location of JU1 through
JU8. The lid of the unit must be removed to access these jumpers. Locate the pins corresponding
to the appropriate input and set both the left and right channels. Place a shorting strap across the
center and rear pin to increase input gain by 10dB. Place a shorting strap across the center and
forward pin to
increase input gain by 20dB.
