GENERAL TECHNICAL DESCRIPTION Please refer to the block diagram of the AM 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 dB to dB 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 AM 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 dB when there is no activity on the channel. When activity is detected, the channel gain increases dB for each dB that the input signal level is above threshold. The channel gain increases for dB, until channel attenuation is unity (dB. 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 maybe 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 maybe used for amplification and tape recording. Each channel has connections for remote gain control via linear pots (K. Additionally, a single linear potentiometer is maybe used to control overall system volume. Linear to audio taper signal conditioning is provided by the AM to maximize control resolution. 2
Threshold setting (i.e. the input signal level at which the channel begins to come on) maybe accomplished in one of two modes Auto or Manual. In Manual mode, a front panel pot is used to adjust the minimum level at which the channels begin to come on. In the Auto mode, the ambient level is continuously monitored, and the minimum level at which channels begin to come on is set a few dB above ambient. With the AM set in the Auto Threshold mode, the chances of misadjustment are considerably less. Another feature of the AM that simplifies setup and operation is Last Mic Hold. With Last Mic Hold in the "On" position, the last active mic is held in the open mode until another microphone comes on. In this way, much better performance is achieved in marginal situations (i.e. very soft talkers, or talkers who turn away from the mic.