Technical & Editorial

AVR – Audio Video Receiver – Build Quality: Part III – Component Choices for a High Performance Design


Low Distortion Switching with SSI CMOS Switch Blocks and Separate Op-Amps

In an SSI implementation, a unit gain op-amp buffer always follows the two-channel selector switch to prevent any current flow in the switch. This prevents any distortion from occurring at the switch as a result of current flow through the CMOS switch.

Often, in an SSI implementation, an additional extra op-amp stage, wired as a buffer, is connected to each of the RCA input jacks. The buffer's output is connected to SSI selector switch and provides a low impedance path to the selector switch's input. The low impedance reduces the switch distortion that occurs even when no current flows in the switch. Without the buffer, the component connected to the RCA jack, which has an unknown, likely higher, output impedance, would drive the switch directly.

The total number of op-amps for input buffering function (one per stereo input jack) far exceeds the number that could be integrated on the AVR LSI chip.

Replacing the solid state switches with relays in an SSI implementation eliminates all the electronics associated with the two-channel input selection subsection. Pre/Pros have so many switching functions it is cost prohibitive to replace them all with relays, in contrast with a two-channel preamp that often uses relays exclusively. The only Pre/Pro schematic I have seen using exclusively relays is the Bryston SSP-3. The SSP-3 has 34 relays (stereo pair in each) including relays for anti-thump protection at the output.

The complex diagram below shows an SSI implementation of a Pre/Pro. This could also represent the block diagram of a stereo preamp ignoring the 7.1 input.