Digital Audio Selector Switch Circuit Sharing

Digital Audio Selector Switch Circuit: With the help of CMOS analog switches or multiplexers, audio signals can be switched digitally. Although this seems simple, the sound quality will inevitably degrade due to the noisy nature of CMOS switches.

Furthermore, the high on-resistance of these devices, together with the large parasitic capacitances typically present in CMOS circuits, results in high crosstalk susceptibility. The circuit presented here is a new method for digitally selecting one-tenth of an audio signal without any of the above disadvantages.

Digital audio selector switch circuit diagram

As shown in the circuit diagram, 10 input signals numbered 1-10 are applied to the bases of transistors T1-T10 through capacitors C1-C10 respectively. The bias voltage of the transistors is obtained with the help of R1-R10. Depending on the binary state applied to IC1, one of its outputs Q0-Q9 goes low. For example, if the input code is 0010, Q2 goes low, pulling the base of T3 to 0V, while the bases of all other transistors rise to nearly +15V.

Therefore, T3 acts as an emitter follower, while the other transistors are effectively reverse biased. The output rail of the transistor array is connected to voltage follower IC2, which provides the output signal of the digital audio selector switch.

Regulator IC3 is required only if the +5V rail is not available. If fewer than 10 channels are required for a particular application, the associated components can be omitted. If a mute function is required, simply shorting one input to ground will mute the output when the corresponding channel is selected.

The circuit can handle input signals up to 4 Vrms. The total distortion does not exceed 0.01% for frequencies up to 20 kHz. The crosstalk generated in this circuit is less than -80 dB. This value is achieved by paying proper attention to the layout of the actual circuit, the decoupling of the power supply lines (mounting C14 and C15 directly to the relevant pins of the op amp), and the use of good quality components.

The measurements shown in the circuit diagram were obtained in the prototype. All voltages were measured with the help of a digital multimeter (Zin=1M0) with respect to ground. The selected channel was digital 1.