The Presspower 2 has two balanced inputs, each switch selectable for mic or line
level signals. In mic mode, a Bauer RE115Kepc mumetal shielded transformer
provides low distortion, impedance gain of 18 dB and high common mode
rejection. Faraday shields in the transformer and RF beads used at the input XLR
offer broadband RF rejection. Capacitors are also used for AM band rejection. A
phantom power switch applies DC power to both mic inputs simultaneously. The
value of the phantom voltage is dependent upon the type of power source
energizing the Presspower 2. In line input mode, the mic preamplifier transformer is
bypassed and an active balanced input is employed. The line input is capable of
receiving signals up to +23 dBm.
From each input circuit the signal is applied to a potentiometer circuit that has an
audio like taper, allowing fine tuning of the level from the 9 o’clock through the 3
o’clock range. The 12 o’clock position yields unity gain in either mic or line mode
with approximately 6 dB of gain adjustment 90 degrees in either direction. Below
the 9 o’clock position, the potentiometer attenuates the signal in larger increments
and fully counter clockwise the attenuation is 65 dBm which will mute the input.
Above the 3 o’clock position, gain increases rapidly up to + 20 dBm with the control
turned fully clockwise. After the gain circuits, the signal passes through a
switchable low cut filter. The filter has a 24 dBm per octave slope, down 3 dBm at
120 Hertz. This filter is recommended for mic use as it eliminates low frequency
handling and wind noise.
The inputs signals are then summed and applied to two separate output busses,
one for mic level outputs and one with an additional 20 dB of gain for line level
outputs. The line level feed also is applied to the headphone monitor section and
the LED level display. The mic and line level busses are routed to the active jack
boards via a balanced cable harness with plug terminations. The Expander
receptacle is also connected at this point in the circuit.
At each of the 16 outputs of the Presspower 2 or its expander module, the
mic/line switch selects the mic level or the line level signal buss and delivers it to a
buffer circuit which drives a whirlwind TRSP-1 balancing transformer. In mic output
mode a resistive H pad is switched into the circuit to attenuate the signal by 20 dB.
The H pad also provides a ground reference relative to pins 2 and 3 of the XLR, but
not connected to any other ground, which minimizes possible ground loop
problems. In line output mode, pin 1 of the output XLR is left floating. A 3.5 mm
mono jack is provided for each output.
The tone oscillator applies a 1 kHz sine wave to all outputs for level calibration
during setup. The tone level is +4 dBm or 0 VU in line mode and -43 dBm in mic
mode.
The headphone driver circuitry provides monitoring of the Presspower 2 output.
Each earpiece is powered by a separate amp circuit ensuring that Tip/Sleeve
headphone connectors (which short the ring) will not short out the entire signal. The
headphone amps are capable of driving 0 dB into 30 ohm headphones, which can
significantly reduce battery life. When only using battery power, turn the
headphone level full counter clockwise or unplug the headphones to conserve the
batteries. The taper of this level control is similar to the control on the inputs.
A 20 segment LED level meter with a 3 dB per segment resolution provides a
wide range of display with 1 dB accuracy. Two switches are provided to help
maximize battery life when using 9 volt battery power. Under battery power only
select dot mode to calibrate levels and use the meter on/off switch to turn the LED
display off when not required. Use the bar mode when the Presspower 2 is under
AC power.
The Presspower 2 has a versatile unipolar power supply allowing the unit to
operate from 120 VAC 60 Hz, 230 VAC 50 Hz, 4 internal 9 volt batteries or any
battery or regulated DC source from 12 to 36 volts. Any combination of the three
types of power can be used simultaneously. The unit will automatically select the
highest voltage, providing fail safe backup and making it easier to power in remote
applications. Under AC power a toroid power transformer is used internally for
superior performance and a voltage switch on the back panel selects 120 or 230
VAC. Nine volt batteries are mounted on the front panel in convenient quick change
holders. An external DC input (screw type) on the back panel will accept 12 to 36
volts DC, from either batteries or a regulated source. This allows use with camera
belt packs, car batteries, etc. At 12 volts DC with a -40 dB mic input and 40 dB of
gain, the Presspower 2 will ouput up to +8 dBm in line mode.
The power on/off switch turns both the AC and the internal 9 volt battery power on
and off. The external DC input on the back panel is unswitched. The AC LED
illuminates when AC power is present and is switched on. The DC power LED
functions as a battery check LED for the 9 volt batteries and illuminates when 12
VDC or more is present at the external DC power input. With no external DC, the DC
LED will light when the batteries are good and the power switch is on. When the total
series voltage drops below 27 volts the DC LED goes out and all four batteries
should be changed.
THEORY OF OPERATION
BLOCK DIAGRAM
