A simplified schematic diagram of one of the tuning units is shown in Figure 14. The circuit is basically a two-stage tube-and-transistor amplifier. There is negative feedback from the collector circuit of Q-l to the filament of Y-l, which stabilizes the gain of the amplifier and the operating points of V-l and Q-2. There are two possible paths for positive feedback. One of these is the so-called Wien bridge network, which produces a maximum feedback at one frequency and gives the tuning unit its selective characteristic. The other is a resistive voltage divider, which gives the tuning unit its flat frequency or all-pass characteristic. When either positive feedback network is used, there is always an excess of negative feedback for stability of gain.
The one-third-octave response was synthesized by choice of the Q and the frequency separation of two tuning units in cascade. The resultant response is flat topped, as shown in Figure 2. When the
position, the two circuits are tuned to the same frequency, producing the narrow peak shown also in Figure 2. The resulting bandwidth is 8%, approximately one-tenth octave.
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