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Browse Prior Art Database

High Isolation Function Switch

IP.com Disclosure Number: IPCOM000084016D
Original Publication Date: 1975-Sep-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 30K

Publishing Venue

IBM

Related People

Higginbotham, AF: AUTHOR

Abstract

High-electrostatic isolation between a high-level signal and a low-level, high-impedance input of a preamplifier is provided in a battery operated recorder.

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High Isolation Function Switch

High-electrostatic isolation between a high-level signal and a low-level, high- impedance input of a preamplifier is provided in a battery operated recorder.

One use of a preamplifier for both record and play is shown in Fig. 1. The position of the record and play switches corresponds to the record mode. One input is from a microphone and automatic gain control 1. Another input is a high- level signal along line 2. The output along line 4 is to a power amplifier.

During recording, a high voltage at the bias frequency is induced, by magnetic coupling, into the play winding. Stray capacitance across the play contacts from the play winding to the preamplifier input couples a bias signal into the high-impedance, low-level preamplifier input. This stray signal is amplified, shifted in phase, and summed with the bias signal to cause distortion of the record bias current. Extreme coupling can even cause oscillation.

A solution to this problem is to shield the play winding contact from the preamplifier input contact, by using two pairs of contacts and an intermediate signal land. The two pairs of contacts and the intermediate signal land are capacitively coupled to ground in the record mode.

In Fig. 2 the grounded land is placed on the underside of a circuit board for capacitively coupling the intermediate land to ground. This capacitance, C2, forms a voltage divider with C1, which reduces the magnitude of the voltage on the intermediate la...