ABSTRACT
The switching and routing of analogue signals is an important part of signal processing. Any electronic switching technique must face comparison with relays. Relays give total galvanic isolation between control and signal, zero contact distortion, and in audio terms have virtually unlimited signal-handling capability. For small signal switching the linearity of relay contacts can normally be regarded as perfect. Electronic switching is usually implemented with CMOS analogue gates, of which the well-known 4016 is the most common example. However, there are applications where discrete JFETs or MOSFETs provide a better solution. When an analogue gate changes state, some energy from the control voltage passes into the audio path via the gate-channel capacitance of the switching FETs, through internal package capacitances, and through any stray capacitance designed into the PCB. Opamps have non-zero offset and bias voltages and currents, and if not handled properly these will lead to thumps and bangs.
