Instruction manual

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filtering, helped define the signal peak amplitude. Even a fully phase-
corrected filter will exhibit overshoots, and a 7-pole “elliptic” filter, as
used in 708, can overshoot 3dB or more!
Other systems of overshoot control permit the primary low-pass filter
to overshoot, then isolate and re-introduce the overshoots to cancel
themselves in the signal path. The patented overshoot compensator in
the 708, on the other hand, pre-conditions the limited program signal
ahead of the filter so there is little or no tendency for the filter to
generate overshoots in the first place.
Input Clipper Diodes CR26 and CR27 form a “hard” clipper at the compensator input
and are biased to a point which represents 100%-modulation.
Presumably, the program signal has already been limited to this same
value, thus the two diodes rarely clip legitimate program waveshapes.
Clipping at this point is limited to those fast transients which somehow
elude the processing peak controller.
Phase-Lag and
Recombining
IC41A includes a phase-lag network which time-displaces the fast
leading and trailing edges of steep waveforms. This means that the
primary characteristic of a program waveform which would normally
excite filter overshoots is instead added to the waveform amplitude.
CR20 and CR21, also biased to the 100%-modulation level, “strip”
these displaced-and-added components from the program signal.
IC39A compares the “stripper” input and output, recovering the
stripped-off components. As these contain much of the program
harmonic (high frequency) information, we cannot afford simply to
throw them away. By recombining these stripped-off program
components with the stripped program signal in opposite phase, the
spectral integrity of the program is maintained. This 180-degree
displacement of certain program overtones is not discernible to the
listener, but is quite effective in inhibiting filter overshoots.
IC48A combines the stripped-off signal components from both the right
and the left channels. This is full-wave rectified by Q8 and Q9 to flash
the front-panel O’SHOOT COMP. indicator whenever filter
compensator action takes place.
LOW-PASS FILTER
The 7-pole, elliptic-function (Cauer) low-pass filter is an active version of
the classic L-C designs worked-out in Germany during the late 1940s
(probably with a slide rule!). The particular active configuration used in
the 708 is sometimes called the “FDNR” because each of the legs to
ground simulates a Frequency-Dependent Negative Resistance.
Referring back to the classic L-C design, resistors in series with the
signal replace series inductors, and each of the active circuits to ground
replaces an inductor/capacitor series-resonant element.