This 1977 patent caused me to buy my first piece of equipment based on engineering for better sound (ADVENT 300). Which is why I am a member of this group today.
Phonograph preamplifier network with infrasonic cutoff response
Patent #4,032,855
Holman, II June 28, 1977
A phonograph preamplifier network with infrasonic cutoff response. The preamplifier network is an active filter configuration with a high gain, high input impedance differential amplifier provided with an RIAA equalization feedback filter network coupled between the amplifier output and inverting input. An RC filter network is coupled between the inverting amplifier input and ground potential to provide a single, low frequency real axis pole. In addition, an input filter feedback network is coupled between the preamplifier input terminal and the non-inverting input of the amplifier, and further bootstrapped to the amplifier inverting input. The network provides a pair of low frequency off-axis complex poles. The RC filter and the input filter feedback networks are adapted to control the infrasonic frequency response of the active filter preamplifier network so that the preamplifier network is characterized by an 18 dB per octave roll-off response at frequencies below a predetermined infrasonic frequency, and a relatively flat response at frequencies in a range adjoining and above that infrasonic frequency.
Phonograph preamplifier network with infrasonic cutoff response
Patent #4,032,855
Holman, II June 28, 1977
A phonograph preamplifier network with infrasonic cutoff response. The preamplifier network is an active filter configuration with a high gain, high input impedance differential amplifier provided with an RIAA equalization feedback filter network coupled between the amplifier output and inverting input. An RC filter network is coupled between the inverting amplifier input and ground potential to provide a single, low frequency real axis pole. In addition, an input filter feedback network is coupled between the preamplifier input terminal and the non-inverting input of the amplifier, and further bootstrapped to the amplifier inverting input. The network provides a pair of low frequency off-axis complex poles. The RC filter and the input filter feedback networks are adapted to control the infrasonic frequency response of the active filter preamplifier network so that the preamplifier network is characterized by an 18 dB per octave roll-off response at frequencies below a predetermined infrasonic frequency, and a relatively flat response at frequencies in a range adjoining and above that infrasonic frequency.
