Extra-rare object and part of the attempts of Infinity to broaden their products range, beyond loudspeakers.
The Infinity DSP Switching Amplifier (or SwAmp) was the earliest Class D amplifier to have been made available to consumers : 1976.
However, knowing the technology and reading the original 1976 brochure, one is compelled to see some links between Infinity and that other brand who made Class D first available to consumers. Yes : Sony.
Said brochure praises V-FETs at length and also admits that :
"at least one major company has been at work for several years on a switching amplifier which will undoubtedly be on the market soon - and which we expect to be a superb instrument.".
That would be the Sony TA-N88B, officially launched in october 1977 but technically ready by late 1976.
Also, Infinity says they were on to Class D development for six years and "approached the Switching Amplifier as a project in team design, assigning portions of the problem to experts in such disciplines as circuit design, power supply design and thermal analysis.".
This would put us in 1970, but I can't help seeing Sony there :)
Whether in joint-development (I doubt it) or in part-licensing, I don't know (yet) but recording engineer & expert Roger Nichols seemingly was in the loop.
The Infinity DSP Switching Amplifier, like the TA-N88, is a PWM (Pulse Width Modulation) amplifier which chops AC input at the same rate as a TA-N88 (20Khz) with a 40Khz clock linking the switching voltage regulator and power transistor control logic.
The PWM's pulse switches at 500Khz speed - see the diagrams and TECH description for the details.
The SwAmp has left and right gain controls, outputs for one pair of loudspeakers and backlit scripting and knobs in blue disco style.
The "display rate" eludes me.
More images and details about these non-loudspeakers Infinty products here.