I was in the process of posting today’s Ask Paul video on the difficulty of designing power amplifiers when I was reminded of the lengths some speaker designers were willing to go for bass.
I was thinking of Arnie Nudell’s Infinity Kappa series, the terror of solid state amplifier manufacturers the world over (though he wasn’t alone). Our power amplifiers of the mid-1980s had minimal overcurrent protection in an effort to not sound squeezed or to trip the unit’s circuit breaker every time a bass drum was struck. This worked fine for most speakers, but not the Kappa 9s and their siblings. Soon, PS amps with blown output stages came back for service in alarming numbers. All had been connected to Infinity Kappas.
Clever crossover design can force a power amplifier to deliver more watts where it is needed most. In particular, the bass regions. And that’s exactly what the Kappa series did. By lowering the input impedance of the speaker to below 1/2Ω, greater power could be squeezed out of amplifiers and converted to robust low frequencies. Tube amps didn’t care because of their output transformers, but many solid-state designs gasped for air when faced with such outrageous demands. Infinity (bless their hearts) included a “safe” switch on the rear of the speaker that limited impedance dips, but customers hated the switch because it robbed them of bass.
In the end, we were forced to redesign the amplifier’s protection circuitry to accommodate the Kappas—something we did on the fly in production—but for a while there it was touch and go depending on what music and volume levels users were at.
Ahhh. The good old days!