GB505646A - Improvements in or relating to low-frequency amplifiers with negative retroaction - Google Patents

Abstract

505,646. Automatic volume control. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. Aug. 9, 1937, No. 21916. Void. Convention date, Aug. 11, 1936. [Class 40 (v)] Volume range expansion is effected, by means of an automatic adjustment of the effectiveness of the negative feed-back path of an amplifier. This is done in some arrangements by including in the feed-back path an impedance having a non-linear voltage current characteristic, e.g. a resistance with high positive or negative temperature coefficients. In one arrangement, Fig. 2 (not shown), a lamp with a positive temperature coefficient is connected in series in the feed back circuit which also includes adjusting resistances. In another arrangement, Fig. 4 (not shown), the feed-back voltages are developed across a circuit comprising a resistance with a negative temperature coefficient. As shown in Fig. 3, the feed backpath includes a reactance 17, which may be inductive or capacitative, shunted by a large electrolytic condenser 16 in series with a resistance 14 having a positive temperature coefficient. At high levels the frequency characteristic of the feed-back path is dependent mainly on the impedance 17 whereas at low levels the impedance 17 is effectively shunted by condenser 16 to emphasize the low frequencies. An, alternative tone-correcting arrangement comprises an impedance designed to act as a resistance at the middle frequencies and as a reactance at high and low frequencies in place of the resistance 6. In another arrangement, Fig. 1, the feed-back path includes an amplifying valve 3 the gain of which is adjusted in accordance with signal strength with the help of a diode using the same cathode and producing a bias voltage by rectifying signals fed to it over a condenser 9. The feed-back voltages are developed across resistance 6.