Circuit Notebook 101 - A Video/Audio Switch-box

This switch-box is intended mainly for use when working /P, but of course it could be used in the shack too. It provides input signal selection and extra outputs to avoid the need to un-plug, change over and plug in cables. Fig.1 shows an external view of the Switchbox and Fig.2 the inside view during development. The circuit is shown in Fig.4.

It has two video inputs, each of which consists of two BNC coax connectors 'looped through' with a 'TERM' switch to terminate the input in 75R if required. The desired input is selected by a toggle switch which feeds the selected video signal to a video op-amp, IC1 (EL2020). The op-amp has a gain of x2 (defined by R5 & R6) and feeds two separate outputs, each through 75R sending resistors - thus providing unity gain from each output into a 75R load.

The switch-box has two audio inputs, each has two coax connectors (phono this time) 'looped through' to allow a signal to be tapped off. The desired audio input is selected by a toggle switch and fed to a unity-gain buffer amplifier, IC2 (NE5534). The amplifier feeds two independent audio outputs.

The switch-box is powered from 12 volts and contains a 12V to +/-5V dc-dc converter (Type NMA 1205 S) to provide the positive and negative supply rails. The usual reverse polarity protection diode is included in the positive input line. The total drain at 12V is 28mA


As this is an experimental unit, I have drilled the box for the connectors and switches, but the internal circuitry has been built on a plug-block, Rapid Electronics 'Protobloc 2' 34-0655 (similar to Maplin FD31J but with power line strips). I can then modify or add to the circuit without soldering or scrapping components. In fact, since taking the photographs I have added a 12V (7812) voltage regulator, so that the unit can also be powered from an unregulated plugtop power supply.

The video amplifier performance does not seem adversely affected by the stray capacitance between contact strips on the plug-block and has a reasonably flat response to 6MHz. Fig.3 shows the performance using a frequency burst signal (1 to 6 MHz), the upper trace is the input and the lower, the output.


One arrangement might be for a camera and associated microphone to be connected to one set of inputs, and a caption generator and sound ident to the other. Video and sound sources could then be crossed over independently and fed to the outputs.


Fig.1. Video/Audio Switch-box

Fig.2. Internal View showing the Plug-block

Fig.3. Input/Output Video Response

Fig.4. Circuit Diagram of Switch-box