Operating instructions

13
Note that the Line inputs, and the TONE input, are protected by transils and fuses
against accidental connection to damaging voltages. The fuses (F300, F301, and F302)
are not user replaceable. They are surface mount devices and must be replaced by
authorised service personnel.
5.4 Line Input Processing Section (Sheet 4)
The two audio inputs are passed, after transformer coupling, to sheet 4.
In Sheet 4, the two Line Inputs are input to a
transconductance amplifier (U402A, and
U402B). A transconductance amplifier is a current controlled, current amplifier, i.e. it
amplifies input current, but its level of amplification is controlled by the level of current
that is injected into pin1 (or pin16). By converting a DAC output into a current, and
converting the input voltage into an input current, U402A and U402B are converted into
Voltage Controlled, Amplifiers(VCAs).
Two of the DAC outputs are converted to currents by U400B, U400C, Q401, and Q400,
and these currents are used to control the gain of the
transconductance amplifiers.
The input voltages are converted to current by the input load resistors R402, and R403.
The output currents are converted to voltages by resistors R420 and R424.
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 U403A.
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 signal into the audio path. This can be achieved via
the MORSE output. This is partially filtered by R446 and C421. C420 provides DC