Circuit routes audio signals between equipment

The audio-signal router in this Design Idea can automatically redirect cable connections between the audio equipment and an amplifier/receiver for dubbing or playback purposes without causing distortion or noise to the signal (Figure 1). It can be useful for older-model amplifier/receivers and any type of tape-deck-playback or -recorder equipment. The circuit uses four Maxim 4606 quad-SPST (single-pole/single-throw) CMOS analog switches to accommodate as many as eight playback and record signals for two pieces of audio equipment. Each MAX4606 contains two normally open and two normally closed logic switches. These switches have a maximum on-resistance of 5Ω and an on-resistance match of 0.5Ω between channels to minimize distortion. They can also handle rail-to-rail analog signals.

The circuit consumes little current and therefore requires no heat sinks on the power supplies. Because this circuit targets use with audio signals, it needs positive and negative supplies to prevent imbalance of the audio signal. It also needs a 5V supply for the logic input (VL), which lets you control the switching of the logic gates. Figure 2 depicts the power-supply circuit, which requires a 10V-ac, 300-mA input. You can connect the enable input to a set/reset circuit or a flip-flop to toggle the internal switches from normally open to normally closed or from normally closed to normally open.

Assume that you have a cassette-tape deck and a reel-to-reel tape deck, in which the cassette deck connects to Tape 1 and the reel-to-reel deck connects to Tape 2 or another input in your amplifier/receiver. You want to record from the reel-to-reel to the cassette tape, but you can instead only record from the cassette deck to the reel-to-reel tape. When you apply a logic level, you will be able to switch your recording and playback direction to the target equipment with a single push of a button.

When wiring the circuit, you should shield and ground all wires from the equipment to the input switches and out to the equipment with reference to the audio cables’ return path. This approach minimizes noisy signals due to a lack of proper grounding from one channel to another. Most of the circuit uses SMD-technology devices and can fit into a 2.5×3.5in. enclosure and minimal installation space.