| CPC H03F 1/0211 (2013.01) [H03F 3/45475 (2013.01); H03F 2203/45116 (2013.01); H03F 2203/45522 (2013.01)] | 13 Claims |
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1. A preamplifier, or preamp, comprising of a number of electronic components, including:
a. a power supply circuit, preferably battery-powered with the voltage evenly split about the system ground into plus and minus supply voltages, preferably with circuits disconnecting the primary power, such as one or more batteries, from the rest of the circuit when the preamp is not in use; and
b. a differential operational amplifier, or opamp, preferably a linear integrated circuit, preferably a micro- or low-power single or dual-opamp with a quiescent supply drain of 1 milli-amp (mA), or less, the first of the opamps being dedicated to a variable-gain preamp circuit, and if such power supply is not simply two battery cells with a center-tapped system ground between them, the second of the two opamps being dedicated to the power supply; and
c. a variable-gain feedback circuit connected to the first opamp, which sets the gain of the preamp, preferably connected between the minus input of the first opamp and its output, with the plus input of the first opamp being dedicated to the input signal, this feedback circuit having at least four embodiments of slightly different circuit connections, each of which approximates increasingly closely an ideal semi-log gain characteristic in which the log of the gain is proportional to the linear change in feedback control, but at any gain set by circuit, the amplified signal response is still linear; and
d. two or more fixed resistors, preferably 1% or better tolerance, used and connected in the feedback circuit; and
e. one or more gain-control potentiometers, or pots, used and connected in the feedback circuit as gain controls, either analog or digital, preferably all with the same total resistance value, preferably multi-turn, grouped in k sets of n number of pots, where k is 1 or more and n is 1 or more, each of the number n pots in a single k group being connected in parallel with each other with all the CCW terminals at a first connection and all the CW terminals at a second connection of the parallel circuit, with the pot wipers not being connected to either of these first or second parallel connections, so that there are k groups of n parallel pots; and
f. a mode switch of 2 poles and k throws, preferably make-before-break (shorting), which throws connects one of the k groups of n pots to the feedback circuit by their outer CCW and CW terminals without connecting to the pot wipers; and
g. a pot selection switch of 1 pole and m number of throws, preferably shorting, part of an m-throw switch with 1 or more poles, preferably where m equals n, the m number of throws preferably equaling the number of n pots in each of the k groups of pots, in which each throw connects in order to the wiper of just one pot in each of the k groups, meaning that each throw connects to the wipers of k pots, with each of the pots being in a separate group of k pots, the order of the throws 1 to m or 1 to n being the same as the order of positions of the 1 to n pots in each k group, wherein the pole of the pot selection switch connects k and only k wipers of the pots to the feedback circuit, such that while n pots may be connected in parallel in the feedback circuit and the wipers of k pots may be connected to the feedback circuit, one and one pot only can control the variability of the gain of the circuit.
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