Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Last but not least

Following the past few day’s posts about audio amplifier efficiency, Class AB biasing, and Class A biasing, let’s wrap our little mini-series up with another topology most of us have not heard of, adaptive biasing.

The promise of adaptive biasing is a best of both worlds scenario: the efficiency of a Class AB circuit with the performance of a Class A amp. Sounds too good to be true, right?

The first time I ever heard about an adaptive biasing scheme was way back in the dark ages, the late 1970s. My very dear friend and one of the all-time good guys of audio, Nelson Pass, then of Threshold Corporation, had introduced the idea of what we called a sliding bias scheme, part of what later became known as the Stasis Circuit used in Nakamichi, Threshold and if memory serves correctly, even the Mark Levinson No. 33.

The core of this circuitry is covered in Nelson’s patent from 1975 titled Active bias circuit for operating push-pull amplifiers in class A mode.

Simply put, Nelson’s design raises the level of Class A bias in cadence with the rising input signal.

Recall in our discussion of Class AB design that a small amount of always-on power keeps small signals always on. In other words, we apply Class A (always-on) bias to the first 10% of the amplifiers output signal level, then switch over to the more efficient Class B for higher signals. Compare that to Class A operation which is always-on for any level of signal—always generating a shit-ton worth of heat (recall Class A amps are at their coolest when at full signal out).

What Nelson cleverly suggested was this. Take what we do in an AB amp where the first 10% of the signal is Class A and actively monitor the signal level. When any given input signal starts to exceed our 10% Class A bias, raise the limit from 10% to, say, 20% (or whatever is greater than the signal level), and continue on the path all the way up to 100%. Then back down again tracking the signal. The heat-producing bias is only enough to accommodate the signal, then goes away when it’s not needed.

Thus, we get the benefits of both worlds. Efficient, and sweet-sounding.

Why doesn’t every amp use this even today? (Nelson’s patents ran their course years ago). Well, as with any design there are problems as well as advantages and this post is long enough already. Ain’t nuthin’ perfect. (We used this for several models of amps though their model numbers and dates escape me).

In any case, a juicy piece of history I’d thought I would share.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Intl.

Our son played trumpet for much of his pre-teen and early teenager years and we had two dogs that used to howl when he played. However, when they would hear recorded versions of the same material, played back on my wife’s cellphone, they would also howl. So, our experience is different than Ed’s.

The dog gets it

When HiFi Family member Ed Spilka sent me the following note I just had to smile. How many times have I heard a similar story? Too many times to count.

And here’s the thing. It’s not just about vinyl. I have heard the same stories about DSD, vacuum tubes, and even good vs. bad cables.

I am sure the measurement folks will have a field day with this one.

“I wanted to share an interesting audio experience that happened the other day.  We were visiting a friend of my wife’s in San Antonio. She was showing us around their new house when we walked into “his” room which held Wilson Alexandria speakers, D’agostino amps, Berkeley DAC’s etc. You get the idea.

When he came home he invited us into his inner sanctum and we began to play. At one point we were A/B’ing between his vinyl collection and streaming on tidal/Qobuz with Sonny Rollin’s Way Out West. On one cut it is just the drummer and Sonny. When Sonny started blowing on the vinyl version, their dog began singing along—howling like crazy. As soon as we switched to the streaming version, the dog was silent, uninterested.

My wife pointed it out to us since we were too engrossed in “listening” to notice the obvious! It happened every time we switched back and forth between vinyl and streaming. Have you experienced that before?”

As I said, this has happened to me with animal reactions more times than I can count.

We might argue like crazy, but the dogs get it.