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After the 20dB pad, the signal is amplified 20, 30, or 40dBm by the microphone preamp.
This low noise preamp provides 20dB of gain and adds only a few dB of noise. When
used with the 20dB pad for unity gain, the unit generates 116dB maximum signal to noise
ratio. Therefore, with 20dB of headroom the AS8X4 provides 96dBm “working” signal
to noise ratio.
Following the microphone preamp, the signal is distributed to 4 sections - Iso1/2 driver,
Iso3/4 driver, headroom circuit, and cue buss circuit.
The Iso outputs are broken into 2 sections Iso 1/2 and Iso 3/4.
The signal passes through a receiving op-amp to a 20dB T pad that is user selectable
(internally) on or off. Each group (1-2 and 3-4) therefore can independently have the T
pad engaged, allowing one to send hotter signals down longer lengths of cable. With the
mic preamp at 20dB of gain and the T pad engaged and attenuating 20dB, unity gain is
preserved. This is the factory default setting. A mic to line level conversion (20-40dB of
gain) is done with the T pads off. By utilizing various combinations of settings, the
overall gain structure can be set from 0 to 40dB in 10dB increments.
After the T-pad, signal is input to Lundahl 1581XL transformers which balances the
signal and presents it to the screw terminal outputs. This PC mount transformer offers
excellent performance with low distortion and high noise immunity.
Following the mic preamp, the audio signal is also distributed to the headroom LED and
cue circuits.
The headroom circuit displays the amount of headroom left before clipping. 0, 12 and
24dB headroom LEDs visually indicate headroom and show signal level problems
quickly.
The cue circuit buffers (isolates) the signal. Each momentary switch on the front panel
places the signal from that channel on the cue buss that is then sent to the monitor. There,
it may be listened to on headphones and metered in dB by the VU meter.
Description of controls
AS8X4 unit front panel:
Each channel provides a phantom on/off switch, a pad switch and a monitor button.
Engaging the phantom on/off places 48VDC on the + and – terminals of the Direct
IN/OUT terminal strips, and lights the phantom LED indicator.
Engaging the PAD switch attenuates input signal 20dB allowing it to accept line level
signals.
The monitor button is momentary style and when pressed, that channel’s audio will be
sent to the headphone jack and VU meter on the monitor unit.
Rear Panel: