TL Audio VTC - Manual (Page 41)

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PC331 - Channel Module Input. CD1331 refers.
PC332 - Channel Module Equaliser.
CD1332 refers.
PC333 - Channel Module Fader.
CD1333 refers.
Channel Module Overall circuit diagram: CD1350.
See also the Channel Block Diagram in the specification section.
The PCBs are interconnected with short, flat jumper cables. The mute switches and faders are wired
to plug-in headers for ease of service replacement.
3.4. Replacing Mute switches.
When replacing mute switches or their internal LED, check the position of the LED locating tab. The
switch body should always be fitted so that the tab is at the top of the switch i.e. pointing away from
the fader. This orientation is important for correct LED polarity and secure fitting of the switch cap.
When replacing the LED, check that the pins are correctly aligned and push the LED fully down into
the switch to avoid intermittent operation. The cap should be fitted with the two small slots towards
the top and bottom of the switch body. Take care not to lose the small diffuser when removing or
changing switch caps.
3.5. Input Circuit Description.
The VTC console is a hybrid design, using valve technology where its inclusion is considered to
enhance the sound quality of the console. Valve stages are included in the mic/line inputs, the
monitor (tape) inputs, and the mix amplifiers for the groups, L+R and Mix B busses.
The valve stages for the channel inputs are located on PC334 at the rear of the console. The stages
are similar for the mic/line input and monitor path, consisting of a voltage amplifier with local
feedback to control the gain. The valve biasing and operating point have been chosen to introduce a
subtle warming of the sound, with very low distortion at normal operating levels, increasing to a
THD of about 1% prior to clipping. The distortion is predominantly low order harmonics. The signal
level can be monitored by the “signal” and “peak” LEDs on the channel strips, illuminating at
approximately -30dB and +18dB respectively.
The valve effect will be enhanced by routing the sound source through the mix amplifiers, whilst
retaining a high integrity signal path. The console has not been designed to offer “guitar amplifier”
extremes of distortion, for which an external effect will be required.
3.6. Input Stages (CD1331).
The line input is attenuated by R28,R29,R30 and R30, whilst phantom power is applied to the mic
input by SW1. Selection of mic or line is then by SW2, with first stage amplification by the low noise
pre-amplifier IC1 whose gain is controlled by RV1. If replacing SW2, note that it is a make-before-
break switch, indicated by the blue plunger. The zener diodes D1,D2,D3 and D4 protect the input
stage from over-voltage transients, such as may be caused by plugging a microphone in or out with
phantom power switched on. The channel path offers High Pass filtering and Phase Reverse by SW4
and SW3 respectively, before the Flip switch SW5, and routing off the module to the valve stage and
insertion point.