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PB2X8
DISTRIBUTION AMPLIFIER
OPERATING AND MAINTENANCE MANUAL
DESCRIPTION
The PB2X8 is an analog audio distribution amplifier with special features making
it ideal for a Press Distribution system. It provides two high quality XLR inputs
that can be individually switched or mixed into any of eight isolated line level
outputs. Each input may be individually switched for microphone or line level
operation with phantom power and low cut filters available for the microphone
inputs.
A calibrated test tone oscillator can be switched in place of either input for
system test and level checks.
A bargraph LED display meters either input, or any of the eight outputs. The
meter may be switched for OVU indication at nominal XLR line outputs of +4, +8
or +18dBm with the top position an effective headroom indicator. A headphone
output follows the meter selection for audible monitoring.
Each of the eight XLR outputs is a separate, independent and isolated amplifier
channel which may be switched to the A input, the B input or to the A+B sum to
mix both inputs. Each output channel has its own level control and a transformer
isolated XLR output, which will drive balanced or unbalanced 600 ohm and
higher loads at full line level. In addition, alternate output positions provide either
a 1/4-inch or a 3.5mm TRS jack with an attenuated output at -10dBu IHF level.
Internal jumper plugs can convert any of the –10 outputs to a nominal
microphone level at –50dBu. Either balanced TRS (Tip/Ring/Sleeve) plugs or
unbalanced TS (Tip/Sleeve) plugs may be interchangeably plugged into the
PB2X8.
A pair of 1/4-inch TRS jacks parallels the main XLR inputs to allow convenient
loop-thru connection into another PB2X8 for further system expansion. A ground
lift switch isolates the shields of the loop-thru TRS jacks to prevent ground loop
induced hum.
PURPOSE
Use your PB2X8 to clear out the usual forest of podium microphones by
distributing just an unobtrusive single or back-up pair of microphones.
Safely Isolate and distribute audio to all types of professional, semi-professional
and consumer grade recorders with no interaction or hum loops.
Easily match multiple level requirements and connector types.
Cleanly mix or switch both inputs into any or all eight outputs.
Efficiently distribute stereo, left, right and summed mono audio from a single
stereo microphone, IHF or 600 ohm line level source.
CIRCUIT OPERATION
Input signals applied to the rear panel XLR or TRS jacks are RF filtered by a
ferrite balun and capacitor network and limited by zener clippers. Low noise
balanced input instrumentation amplifier ICs A1 and A2 are individually switched
to boost microphone inputs by 40dB (MIC position) or provide unity input gain for
line level inputs (LINE position). A second MIC low-cut position on the two input
switches inserts a two pole active 150Hz low cut filter, A3 and A4, into the
microphone signal path to block wind noise and reduce room reverberation.
Input levels A and B are adjusted by pots R13 and R14, boosted l9dB by A3A
and A4A, high pass filtered, fed to the A and B distribution busses by A3B and
A4B and metered by positions A and B on the meter selector. Nominal A and B
input levels should be maintained around OVU on the +4 meter scale.
A 750Hz sine wave test tone oscillator, A5A and A5B, can be switched (via S5)
into either A or B input path just after the input level controls. The test tone
provides a nominal OVU +4 A or B internal buss level to the system. Turn down
the input pots when feeding test tone thru the system, set your XLR line output
levels using the internal meter (1 to 8), OVU LEVEL (+4 or +8) meter switch and
the individual screwdriver output level adjustments, then turn off the test tone and
turn up your input to match your nominal A or B input buss level to the test tone
OVU.
Each of the eight output channels may be driven from either the A input, the B
input or the A+B sum of both inputs. The summing switches are S6 thru S13
driving eight unity gain summing amplifiers (A6A-D and A7A-D) and the output
level pots M45...R87).
Eight 21dB gain output stages (A8 through A15) drive the eight output isolation
transformers (T1 through T8). RC networks across each output transformer
maintain smooth peak free high frequency response even if the line output is not
loaded. The transformers drive the XLR 600 ohm line outputs directly and also
feed their associated 1/4" or 3.5mm TRS output jacks thru resistive balanced
attenuator networks. Jumper plugs P1 through P8, located just inside the rear
panel and accessible by removing the top cover, convert any TRS output from a 10dBu IHF level to –50dBu microphone level. Tip/Sleeve plugs for unbalanced
loads may be inserted into the TRS jacks with no ill effects on balanced or
unbalanced line level outputs. Shield pin 1 of the XLR output connectors is
ungrounded to avoid hum loops. If necessary, ground jumpers may be inserted
into the PC board directly behind the #1 pin of the output XLR connectors.
The meter circuit, A16, drives the ten-segment bargraph display with a very close
approximation of VU ballistics. The OVU LEVEL switch, S14, sets the meter OVU
point to +4, +8 or +18dBm equivalent output. The meter CAL trimpot is R176.
Headphone amplifier A17 drives both sides of a stereo headset with an in-phase
signal from the mono input or output channel selected by the rotary meter select
switch. Internal trimpot R186 sets the headphone level. If necessary, readjust
R186 for a comfortable listening level at OVU. Use stereo headphones only;
mono phones short one output.
A5C and A5D form a 40kHz oscillator and voltage doubler to generate the
+48VDC microphone phantom power. Phantom power is applied to both HI and
LO input lines thru matched 6810-ohm resistors only when the input channel is
set for microphone operation and only if the rear panel phantom power switch
(S2) is ON. Regulated + and –15VDC power is provided by IC regulators A18
and A19. A separate +10VDC unregulated supply drives the LED bargraph meter
display. PC board jumpers W1 and W2 set the nominal AC input voltage to
117Vrms. Removing W1 and W2 and inserting W3 sets the input for 234VAC. A
1/2A fuse is required for 117V operation and a 1/4A fuse for 234V.
INSTALLATION
Open the Press Box and adjust output jumper plugs before mounting the unit
under other equipment in your system.
The 1/4" and 3.5mm TRS output jacks are shipped set for -10 dBu (.25V) output
level. If you would like to dedicate some or all of them to -50dBu (2.5mV)
microphone levels, remove the top cover (4 #6 side screws, 2 #4 top screws) and
move the selected jumper plugs (Pl-M). Mark the rear label!
Plug a stereo headphone into the front panel jack and run the test tone oscillator
to check headphone loudness. Adjust PHONES trimpot R186 if necessary.
The PB2X8 Press Box may be desk mounted on its suction cup mounting feet or
EIA rack mounted in 1.75inches using optional heavy-duty rack mounting
brackets P/N 21181-501. Remove the four #6 screws from each side to attach
the optional rack mount brackets.
If you are looping inputs thru to another PB2X8 place the rear panel loop shields
switch in the LIFT position on the sending unit only. This will prevent ground loop
hum via the AC line green wire ground. If you use a TRS loop in jack as your
main (or loop) input, be sure the shield switch is in the GND position. The XLR
input jack’s pin 1 (shield) is permanently grounded.
The wiring convention used on the PB2X8 XLR connectors is pin 2 HI, pin 3 LO,
pin 1 shield. TRS connectors, Tip HI, Ring LO, Sleeve is shield.
SPECIFICATIONS
Output Levels XLR: +4dBm nominal, +22dBm max
TRS: -10dBu (.25V), balanced or unbalanced,
-50dBu via internal jumper plug
Input Levels Microphone: -50dBm nominal from 50 to 600
ohm microphone
Line: -10 to +4dBm nominal
Distortion: 0.1% maximum THD, 20 Hz to 20 KHz
+20dBm level
Response: +0, -.25dB, 30Hz to 20kHz
Noise Mic. Inputs -124dBm EIN
20KHZ Bandwidth Line Inputs: -94dBm EIN
MIC Filters: Two pole active low cut, 150Hz
Phantom MIC Power: +48VDC, rear panel switch
Headphone Output: .2W into 600-ohm stereo TRS phones
Test Oscillator: 750Hz at internal buss levels equivalent to
nominal levels
Connectors: XLR type, 1/4inch TRS, 3.5mm TRS
Size: in. (mm) 17(432) W x 1.75(44.5) H x
10.5(267) D
Shipping Weight: 15lbs (6kg)
Mounting: Suction feet for non-slip desk mounting,
optional Rack Mounting Kit 21181-501
Power: 117/234VAC +/-10%, 47 - 63 Hz, 10VA
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