Last but not least

June 6, 2021
 by Paul McGowan

Following the past few day’s posts about 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.

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21 comments on “Last but not least”

  1. Having read the posts the last few days, all this talk about heat is a bit like talking about cars in terms of their exhaust fumes. Both are the waste product of their inefficiency.

    “Why doesn’t every amp use this even today?” The main reason may be because 20+ years ago manufacturers were designing better class A/B amplifiers so other than for a few audiophiles, they became redundant.

    Class A/B is still inefficent compared to class D. You can discuss fuel efficiency as much as you like, but a Tesla has no fumes as all, and is worth more than most of the rest of the US auto industry put together. Class D audio and electric vehicles are taking over their industries because of their efficiency, but it doesn’t mean you have to like them.

    1. “Tesla is worth more than most of the rest of the US auto industry put together.”

      You are referring to its market capitalization, which only reflects the inflated stock value, not the intrinsic value of its operation. Tesla is still a small company in terms of its revenues; and while the company is profitable, those profits do not yet derive from sales.

      1. It’s valuation reflects the fact that it has a massive lead on the huge US auto market, but more importantly the best battery technology and production on the planet. No one comes close to the range of the Tesla. Richtea correctly points out issues with the UK infrastructure, and a bigger problem is making the vehicles as demand is far greater than supply. I see these as teething problems that will get sorted, and have improved a lot in the 2 years we’ve had an electric car, just as the battery life was a major issue with mobile phones and now is almost a non-issue.

        I said in my earlier post a few days ago that I saw heat a fundamental issue in home audio, solve that with Class D or ultra-efficient Class A/B and you open up a world of possibilities for consumer-friendly design without compromising sound quality. That revolution started at least 15 years ago.

    2. Steven, I realize you are not a scientist so maybe you should leave the science to those of us who are. Just because something emits an exhaust does not mean that it is inefficient. The condensing boiler that makes the hot water to heat my house and make domestic hot water for bathing wand washing burns natural gas and is 95% efficient. There is so little heat energy left in its exhaust that the exhaust is vented through PVC pipe that I can touch without any fear of burning my hand. Both electric and gas car are inefficient because the are both mechanical contrivances that are load with frictional losses.

      1. I calculated that my 42kW Nissan Leaf, based on 132,000 Btu from a gallon of petrol, does the equivalent of about 140 miles per gallon. I don’t know any petrol cars that do that. Like A/B amplifiers, petrol cars consume fuel when sitting in traffic going nowhere, our old car idled at 800 revs, whereas electric cars consume nothing (if you have the aircon off).

        The Plinius SA-103 has been in production for many years and you can select Class A or A/B bias. Stats: Power/Current consumption: 485W/2.1A, class-A Idle. 92W/0.4A class-A/B Idle/Standby. People run these bridged at 400w Class A per channel. I remember reading a review saying they are so good in A/B that the Class A option is pretty pointless, especially given the heat they produce. A friend had one and they are just a huge massive lump.

          1. My Volkswagen Golf did that and I bought it in 2010. It did not always work. What they didn’t tell me was that I was killing people with NOX. The Nissan has an eDrive thing. You drive on one pedal only and it brakes automatically. Apparently it’s because it can brake more efficiently. My wife just loves it, especially the Bose stereo. There are massive tax breaks for going electric, we get next to no tax breaks for anything else. Maybe there should be tax breaks for buying a Class D amplifier.

            1. Steven,
              I haven’t owned, or regularly driven, a motor vehicle for just over 10 years now, therefore I have decreased my carbon footprint greatly.
              Also, pleased do not be disparaging of “huge massive lumps”…at 193cm & 123kgs I consider myself a huge massive lump.

  2. Good points in Steven’s post. I always thought hi-fi was about a journey to the best sound quality but now it seems we are being driven to be green. I’ve nothing against being green, it’s a very worthy cause, but implementation needs to be planned and structured. I read the other day that new technologies can be accepted at a much faster rate than we might expect. Electric vehicles are rapidly gaining ground! Unfortunately here in the U.K. we don’t at present have the charging infrastructure to support this which will give rise to a lot of problems. I expect the situation is similar in other countries. Imagine you plan a touring holiday. You can’t charge at home so you have to plan a ‘charging’ route. Charging isn’t as quick as filling with petrol. When you arrive at the charger it’s already in use or it’s out of service due to malfunction or vandalism. The connector might not fit your vehicle, they’re not all the same. Think RCA to XLR, perhaps you’ll need a crossover cable, hope the pins are wired correctly. Everything’s good but they won’t accept your payment method. There’ll be other problems I haven’t thought of, there always are. I appreciate this is a rather ‘glass half empty’ post but in the rush to be green and efficient I think we risk tripping ourselves up.

    What has this got to do with hi-fi, not a lot, but it might fuel the urge to stay home and listen to some more music. Going green, what about this idea. Why not power your hi-fi from solar panels, all that mains borne noise and DC issues gone in an instant.

    1. Solar chargers and Ap-Fi (as opposed to Hi-Fi) need a lot of electrical isolation. MPPT chargers are noisy brutes… So you’d still need to implement power conditioning to eliminate the (un)harmonic noise on the supply.

  3. My Denon POA-4400 monoblocs, from 1993, were 30watts cont.
    ‘A’ class & 160watts cont. ‘A/B’ class (20Hz-20kHz)
    Fantastic little ‘straight wires with gain’ they were.

  4. I believe the new Classé mono blocks are designed like this. The first 10 watts are class A? I’m guessing. Is the new BHK 600 going to be Class A for the 10% of its power rating?

  5. Several days ago I asked a question about Audio Research “Enhanced AB biasing”. It may have started with their 200 W mono blocks that was incorporated into the third version of the VT 100 amp. So I’ll ask the question again. Is enhanced AB a takeoff on Nelson’s patent?

  6. First 5%-10% watts class A, and then the rest (90-95%) class B is what most modern power amps do nowadays.
    As for Class A : “adaptive biasing” : check out the NEW Krell integrated K-300i and Duo 300 XD amp.

  7. I love my high-bias Class A Pass xa100.5s–sweet, tubelike. In standby mode they don’t use much power or generate much heat. They take about a half hour after turning on to sound their best. They are not very efficient, dissipating about three times their rated output power, so their heat sink fins get quite warm. According to Pass, high-bias Class A provides lower distortion at all power levels (low to high), improves the ratio of low order harmonics to high order harmonics, reduces intermodulation distortion and have a more consistent output impedance. Pass amp lineup includes Class AB amps, but their most expensive, best sounding are the high-bias Class A’s.

  8. Perhaps we should all have class A amps which are part of the hot water system in the house, a water pre-heater add on. Mine do a great job of keeping the cats warm. Does anyone comment on the non-ecological production of the battery pack in their electric autos?

    1. I’m just waiting to see what happens when those batteries need replacement. How does that affect the value of the vehicle? By then the batteries may be cheaper to produce. But will those savings be passed on to the consumer or retained by the manufacturer?

    2. Class A helps warm up the listening room in the winter…LOL

      I tend to listen to my audio system for longer periods during the winter months than in the summer months when the days are longer and I spend more time outdoors. In Spring and Fall my windows are open, so there is natural ventilation to keep the listening room from getting too warm.

      I have a house solar panel system so I have a clear conscience when it comes to power consumption.

  9. Back when I was first learning about AC conditioning my AC conditioner was a Furman ELITE-15 DM I conditioner. It had meters for voltage and amperage.

    My amp is a Class D (very good sounding) amplifier. When I contacted Furman to see if the amperage gauge was working, they told me to try a few things other than my equipment. Until I did? They thought the unit might be defective. It was drawing almost nothing above idle amperage… while playing music.

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