CircuitWerkes Sub-03a, Техническое руководство

Страница: 8 / 10

Theory of Operation
Incoming audio is buffered and level controlled by U9a and its associated components. Three nodes are
fed by this first buffer section, the audio input clipping indicator, the input audio presence indicator (U6b),
the 35Hz notch section (U9b) and the main 25Hz notch section, U9d.
The audio input clipping and presence indicators are basically identical except that the clipping indicator is
driven directly from the first buffer's output and is set to a level just below the point where the input buffer
overloads. The audio present indicator is fed from just after the input gain control pot and is comprised of
opamp U6b (operated as a comparator), reference-set pot R53, a yellow LED and associated
components. When the audio level exceeds the preset comparator reference level, the comparator changes
states and illuminated the LED. The reference level pot is set to illuminate the LED when detection
amplitude is ideal.
The 35Hz notch section is a modified twin-tee circuit, the exact center frequency of which is set by multiturn
pot R24. Buffer U9b feeds the 25 Hz detector section and a secondary 25Hz notch filter, U9c, (of the
same design) which in turn feeds the balanced output amps comprised of U5 and its associated
components. Another comparator-LED circuit hangs off the end of the output amp and is set to illuminate
when the audio output level approaches clipping.
The main 25Hz filter section feeds the 35Hz detector section while the 35Hz filter feed the 25Hz detector.
It is necessary to remove the opposite frequency tones from the audio feeding each detector.
The outputs of the two detector sections feed through hash filters (to eliminate any extraneous falsing). The
hash filters are simple RC constants on Schmitt trigger inputs of U3a (25Hz detector) and U3d (35Hz
detector). The outputs of the hash filters feed an AND gate (comprised of U3c and U3b) directly and the
relay driver circuits for their respective relays through an RC delay filter. If both tones are detected
simultaneously, the AND gate circuit fires the combo tone relay and defeats the relay drivers of the
individual tone outputs. If only a single tone is detected, its relay output folows after a brief delay.