Amplifier Extension Circuit
This circuit is designed to serve as an extension for (pre-) amplifiers, allowing users to increase the number of inputs available. With the advent of MD-recorders, DVD players, and similar devices, owners of older amplifiers have faced a persistent shortage of inputs. This switchbox addresses this issue effectively. Moreover, it enables users to loop audio outputs from devices like DVD players and video recorders to the audio system, eliminating the need to power on the TV. This feature is particularly convenient when the audio setup is located some distance away from the video system, allowing users to solely listen to audio from sources like DVD/MP3 players.
Compact Design with Bistable Relays
The circuit employs two bistable relays, each equipped with two changeover contacts. This configuration ensures the circuit’s compactness and eliminates the necessity of applying significant force to rotate a switch shaft. Operation of the relays is facilitated by three small buttons (S1 to S3). These relays, operating at 12 V, function seamlessly with a 9 V battery, offering a practical and efficient solution.
To minimize power consumption almost entirely, several differentiator networks (C1/R2, C2/R5, C3.R8, and C4/R11) were created to produce essential pulses for the relay windings. Each relay is equipped with both SET and RESET windings. The third stereo input is directly linked to the output through the normally-closed contacts of relay Re2. The remaining two inputs are routed through Re1 to the normally-open contacts of Re2, and then to the output.
Input Selection via Resetting Relay Re2
In order to exclusively select the third input, it’s necessary to reset Re2. This specific function is achieved through the small circuit centered around T3. When S3 is pressed, T3 conducts for a duration of several milliseconds due to C3/R8/R9, ensuring the relay switches. R7 promptly discharges C3 when the pushbutton is released. The advantage of differentiator networks lies in their exceptionally low current consumption (<25 μA) even if the pushbutton is held down, with only the initial pulse surpassing 0.5 mA. This low current consumption implies a prolonged battery life, potentially lasting several years, provided there are no issues such as leaks.
Input Selection for First or Second Input
For the selection of the first or second input, Re1 must be set or reset using S2/T2 and S1/T1 respectively. When S1 or S2 is pressed, D1/D2 and the circuit surrounding T4 generate a set pulse for Re2, ensuring Re1 is chosen. An emergency power supply feature is implemented with a 1000-μF electrolytic capacitor parallel to the battery, supplying power during battery exhaustion. Each 9 V pulse draws nearly 10 mA. The relays utilized are standard types compatible with Siemens’ common V23042-B2203-B101 (now Schrack).
R13 limits the short-circuit current (via C5) upon initial battery connection. The three pushbutton connections are strategically positioned on the PCB for convenience, optimizing space. It’s important to note that using a slide switch creates a close contact and consequent low current flow. For testing convenience, the power supply (marked ‘+’) is available next to each connection. Alternatively, if the pushbuttons are mounted on a panel, a single common ‘+” connection suffices. Unfortunately, the depicted PCB is not commercially available in a ready-made form.