RF Technology Eclipse Series Operation And Maintenance Manual page 19

5 CIRCUIT DESCRIPTION
The outputs of the transconductance amplifiers are buffered by the darlington buffers
provided with the amplifiers (U402C, and U402D).
The output of each VCA is then amplified by U405B and U405C respectively.
The level of amplification of each VCA is adjusted in software in accordance with any
adjustments made to the LINE POT. The software converts the linear range of the
LINE POT into a logarithmic scale, such that if the LINE POT is wound down to zero,
the amplification of each VCA is reduced by 12db relative to its centre position.
Similarly if the POT is wound to its maximum position, both amplifiers increase their
gain by 12dB.
The outputs of these amplification stages are then attenuated. Analogue switches
U404A and U404Bare used to select which attenuation circuit is used for Line 2, and
U404D and U404C are used to select which attenuation circuit is used for Line 1.
If the resistive divider formed by R425, R426, and R439 is selected then the Line 2
audio signal frequency response is unaffected (it is Flat). If the reactive divider defined
by C402, R431, and R439 is selected, then higher frequencies of the Line 2 audio signal
are attenuated less than lower frequencies, i.e. Pre-emphasis is applied to the audio
signal.
Line 1 has an identical circuit.
The outputs of these pre-emphasis/flat frequency response attenuators are then buffered
by U405A, and U405D respectively.
The microphone input is amplified by U400D, after being limited by D400. It is passed
through a pre-emphasis network (defined by C404, R433, and R436), and is enabled, or
disabled by switch U403D.
The outputs of the Line 1 conditioning circuit, the Line 2 conditioning circuit, and the
microphone input amplifier, are then mixed (summed) and amplified by U407A. Its
output is, in turn, amplified by U407B, but the gain of U407B is either 2.7 or 27
depending on the state of analogue switch U403B.
The CPU is capable of injecting a clipped (saturated) signal into the audio path. This
can be achieved via the two digital outputs DEV_H_L and TEST_DEV. This is
currently not used.
The output of U407B is passed (signal LINE_INP) to the Line Level Sense circuitry
(sheet 3) so that the CPU can determine the input line level.
U407B's output is also passed to the limiter defined by D402, and D401. Resistors
R442, and R444 are used to "soften" the clipping, i.e. to "round off" the edges as the
voltage hits the clipping levels. This reduces the level of the lower order harmonics
produced.
U407C then buffers the output for mixing with the tone output circuitry.
RF Technology T50 Page 19