General Technical Description - Lectrosonics AM6 Operating Instructions Manual

GENERAL TECHNICAL DESCRIPTION

Please refer to the block diagram of the AM6 for the following discussion.
The microphone preamp is designed around an ultra-low noise, dedicated preamplifier IC. This IC provides noise
and distortion performance previously available only in discrete preamp designs. The preamp is fully balanced and
RF protected. Phantom power (15 Volts) is jumperable on each channel to accommodate both dynamic and
electret type microphones. The gain of the preamp is variable via a rear panel trim pot from 6dB to 55dB of gain.
This adjustment of gain range allows the preamp to accept signals from mic to line level, while optimizing the
preamp gain for any input signal level.
The microphone preamp feeds a low noise Voltage Controlled Amplifier (VCA). The VCA implements the automatic
function, as well as allowing remote level control of any channel using a linear potentiometer.
The AM6 has two modes of operation for each channel; Auto and Direct. The operational mode is selected via the
front panel Auto/Direct switch on each channel. In the Direct mode, the channel is always on. The Direct mode is
analogous to a standard channel in a non-automatic mixer.
In the Auto mode, each channel is attenuated 15dB when there is no activity on the channel. When activity is
detected, the channel gain increases 1dB for each 1dB that the input signal level is above threshold. The channel
gain increases for 15dB, until channel attenuation is unity (0dB). After the channel attenuation reaches unity, no
more gain modulation occurs even if signal levels continue to increase. This 2:1 gain modulation gives a smooth
transition from full attenuation to unity gain, unlike the choppy action of automatic mixers that use gates (i.e.
instantaneous change from full attenuation to unity gain).
The presence or absence of signal is determined by the Speech Filter and Log Amplifier section. Speech signals
are filtered to emphasize the voice band, and converted to decibels by the log amplifier. The signal is then
compared against the instantaneous value of the Fixed Threshold plus the Variable Threshold, and the appropriate
channel gain is derived. The Variable Threshold introduces a threshold offset proportional to the signal to prevent
sound from the system loudspeakers from turning on unused microphones.
The level of individual channels is adjusted by the front panel Channel Level control. Each channel is jumperable
to tone/notch or no tone/notch. Typically, channels using microphones as their source would be jumpered for notch
filtering and tone control. Line level signals, such as recorded music, would normally be jumpered to bypass the
tone controls to give flat frequency response reproduction. Note that when a channel is jumpered for bypass, the
volume of that channel will not be affected by the Main Level control. In this way, the volume level of a line source
may be preset using the Channel Level control, and will not be disturbed if the system volume is adjusted via the
Main Level control.
The signals are summed, and their volume is controlled by the Main Level control. The signal is then sent to the
Notch/tone controls section.
The balanced line outputs are low impedance (100 ohms) line level drivers that may be used for amplification and
tape recording.
Each channel has connections for remote gain control via linear pots (10K). Additionally, a single linear
potentiometer is may be used to control overall system volume. Linear to audio taper signal conditioning is
provided by the AM6 to maximize control resolution.
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