ATI Audio PB2X8 - Manual (Page 3)

Audio Technologies Inc. 856-719-9900 [email protected] www. audio.com
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.