U.S. Patent No. 5,046,105 , 09/03/1991 (app. 04/30/1990) [DjVu196KB]
Audio signal equalizer having accelerated slope phase shift compensated filters
Bohn, Dennis A. (assignee Rane Corporation)

Abstract: A tone control shelving equalizer providing low shelf (bass) boost and cut, and high shelf (treble) boost and cut is disclosed wherein the active filter stages provide sharpened (accelerated) amplitude versus frequency response characteristics at the band transitions to leave the center frequencies uncorrupted by out-of-band effects of the tone controls while maintaining circuit stability and avoiding dropouts due to excessive filter phase shifts. Each high and low end band filter network has multiple poles and offset zeros located at a frequency offset so as to restrict the maximum phase shift of the filter to within a safe level not greater than about 90.degree.

U.S. Patent No. 5,157,350 , 10/20/1992 [ DjVu 378 KB]
Analog multipliers
Rubens, Harvey
Abstract: Class A gain control systems having extremely good linearity and wide dynamic range, and suitable for monolithic integration, are described in embodiments realizing several classical multiplier functions such as voltage controlled attenuators, exponentially controlled multipliers and four quadrant multipliers. Two identical current steering circuits, each comprising a matched, emitter coupled pair of transistors, or effective transistors, are fed from an actively linearized input circuit which derives the required well balanced signals from a single ended or balanced input source, and current mirror circuits are used to subtract unwanted common mode products from the output signals.

U.S. Patent No. 5,291,558 , 03/01/1994 (app. 04/09/1992) [DjVu217KB]
Automatic level control of multiple audio signal sources
Ross, Montgomery F. (assignee Rane Corporation)
Abstract: An ambiance channel produces an ambiance audio signal as a reference threshold such as from a noise microphone located away from the program microphones. Variable gain amplifiers in a plurality of program channels selectively gate open those program channels that have audio present exceeding the ambiance threshold. When the program channels are below the noise threshold and hence inactive, they are gated off and the background noise in the ambiance channel is shunted around the program channel to the output. A control function circuit adjusts the gain of each of the program channels that have been opened as an inverse function of the number of open mics (called NOM). In another disclosed embodiment, a programmed microcomputer receives as inputs the status signals that represent the gated open or gated off conditions of the program channels. These status signals are processed by a software program in the microcomputer that performs both the functions of gating open individual program channels relative to the background noise level, and setting the gain of each of the program channels according to the NOM function.

U.S. Patent No. 5,436,882, 07/25/1995 (app. 01/04/1993) [DjVu82KB]
Method and device for improving digital audio sound (class: 369/124.13; 341/118; 360/8)
Taddeo, Anthony R.
Abstract: An analog signal which forms part of a digital audio recording system is split into at least two signal paths in one of which it is delayed slightly before being recombined with each signal of the other signal path or paths. The delay can be effected by using a multistage filter in the delay line, which causes the signal in the delay line to be delayed for a period of time equal to a fraction of one cycle of the sampling frequency of the associated digital recording system. The slight delay eliminates or minimizes undesirable phase shifting which is created in the analog signal by the sampling frequency.

U.S. Patent No. 5,450,034, 09/12/1995 [ DjVu 118 KB ]
Reflected plate amplifier
Donn Werrbach (assignee Aphex Systems Ltd.)
Abstract: A reflected plate amplifier (RPA) for use with electronic audio equipment. The RPA comprises an input circuit for receiving an input signal, a vacuum tube, a plate current reflector, and an output circuit for delivering an output signal. The vacuum tube has a control grid coupled to the input circuit for receiving the input signal, and a plate for delivering a plate current responsive to the input signal. The plate current reflector is a transistor having an input terminal coupled to the plate of the vacuum tube for receiving the plate current, and an output terminal for delivering a reflected current which is responsive to the plate current of the vacuum tube and therefore responsive to the input signal, while the plate current reflector holds the plate voltage of the vacuum tube substantially constant for a wide range of the plate current of the vacuum tube.