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Twenty five cent improvement

Paul's Posts — 16 February 2012

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Twenty five cent improvement

One of the cleverest solutions to amplifier designs came to our attention through our old friend Nelson Pass, then of Threshold.  We always referred to it as the “sliding class A bias circuit” and what was clever about it is it took less than $0.25 in parts to work.

All traditionally designed power amps are class A to some extent.  Class A simply means that the output transistors are fully on and drawing power from the wall, even if there’s no audio signal.  Most amps are only class A for the first 30 watts of power and then they switch to what we call Class B where the transistors only draw enough power to make your speakers work.  This type of circuit is called Class A/B.

A fully Class A amplifier constantly generates tons of heat – most of it wasted.  In fact, class A amplifiers create most of their heat when they are just sitting there without a signal – and less heat at full output power.  They are very inefficient devices but great sounding.  Nelson figured out how to have both Class A performance and efficient operation – and he did it for less than a quarter’s worth of parts.

Using a simple resistor and a diode in series with each other, he connected one end to the amplifiers output and the other end of the two parts to an internal circuit (used by all amplifiers) that adjusts the bias level.  What this did is made the bias go up and down in synch with the output music signal.  So, remember the 30 watts of class A every amplifier has?  This amount of power tracked up and down with the music so for any given signal, there was always class A performance.  But when there was no signal, there was no heat being generated.

I think Nelson patented the circuit and came up with a clever name.

But to this day it’s the best twenty five cent improvement I’ve ever seen.

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About Author

Paul McGowan

Paul McGowan is the CEO and co-founder of PS Audio Inc. a Boulder Colorado design and manufacturing company of high-end audio products and services. McGowan has been designing and building high-end products for nearly 40 years. Hobbies include skiing, music, hiking, artisan bread baking, kick boxing and cooking. He lives in Boulder Colorado with his wife Terri and his 4 sons.

(7) Readers Comments

  1. hahax

    February 16, 2012 at 6:21 am

    The variable bias is fascinating but I thought that while most amps operate in class AB, few of them ran as much as 30 watts classA. I thought most of them ran only a couple of watts class A.

    • February 16, 2012 at 7:18 am

      Depends on the amp – what most engineers do is turn the bias up enough to eliminate what we call crossover notch distortion which is eliminating the crossover between the upper voltage and the lower voltage of the system. You are correct that in the most minimal example only a few watts is required. Most high-end amps use a bit more so you cover the inner detail spectrum of the music and use the bias to enhance the way things sound.

      • Soundminded

        February 16, 2012 at 11:02 am

        Early solid state amplifiers suffered terribly from crossover notch distortion which is why the best manufacturers began to specify maximum THD at rated power and at any power level below. I think the invention of power mosfets made it a lot easier to design class AB solid state amplifiers with low crossover notch distortion.

        One way to listen for crossover notch distortion is to connect a very efficient speaker to the amplifier and put your ear right up to it with the volume very low. If you can’t hear it that way chances are you won’t ever hear it at all.

        When did Pass invent this? Must have been a long time ago. JVC had a receiver RX500 that switched between class A and AB in 1984. There may have been others earlier. IMO amplifier efficiency should be one of the last considerations in choosing an audio amplifier for a home stereo system.

        • February 16, 2012 at 11:10 am

          I’d agree mostly – but a true class A amp is a real beast for heat so that would always be a consideration for me.

          MOSFETS are in general better than bipolars but buggers to bias since each one has a different bias point and you either wind up matching them (which is a nightmare if you ever have to replace one) or come up with an auto bias circuit based on a current mirror like we did in an earlier design. I do like their sound better.

  2. Frank LaFond

    February 16, 2012 at 7:09 am

    Nelson Pass is such an enigma to me. While running several successful companies over the years, he’s tirelessly been the patron saint of DIY, sharing his intellectual property freely where most others hoard theirs. He spends countless hours educating from his articles in Audio Amateur,… to the passdiy.com site to participation on DIY audio electronics forums. I consider Nelson one of the true treasures of the audio world.

    • February 16, 2012 at 7:16 am

      And we all admire him for it. I have taken my lead from Nelson on openness in design for years – I think it’s good to share, educate and let the chips fall where they may. Even though I get a lot of flack for suggesting patents are restrictive and damage more than they help, I still believe it.

  3. hahax

    February 16, 2012 at 9:20 am

    Nelson is one of the special people. He’s so creative and yet also so open with his ideas. He designs single ended amps and I was once told something that I felt was very negative about them. I was curious to confirm this and wrote to him and he simply confirmed what I was told, even though he designed and sold single ended amps. Of, course he said there were other positive aspects of single ended. And coming from someone like Nelson I had to totally respect both parts of his reply.

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