Class G and H Audio Amplifiers
One design that has been out for eons and still catches us off guard is the rail shifting Class G or class H boost supply amplifier. Most purists agree that this concept is ideal for PA work but not high fidelity audio. To a certain degree I must agree but then this would imply that class D is also in the same boat. There I disagree, they are in the same boat but some very high quality amplifiers are class G, H as well as class D. What is this class stuff anyway?
Classes of amplification – a layman’s guide
When I studied way back in the dark ages we only knew of class A, class AB and class C. Class A is when the entire wave is amplified, the output stage could be a single ended stage or a highly biased current hogging push pull but the bias was such that no switching between the top and lower halves of the wave was amplified as a separate component. (see crossover distortion). Huge current draw and inefficient. Definitely not ideal for battery operated equipment.
Class B is theoretically an amplifier which amplifies the entire wave form in the VAS or voltage amplification section but the power output section amplifies the positive and negative excursions with exactly the same gain, but separately. Meaning one transistor would amplify the top half and another, the lower half. The caveat to this arrangement is switching noise and the non linear operating region (of both tubes and semiconductors). Of course this would come from the PNP and NPN output devices requiring a Vbe of about 0.6 to 0.7V to start drawing current. A very “disturbed” sounding amplifier indeed. Of course if we then bias these output devices to overcome this barrier, the Vbe, to a level which causes conduction, switching noise could be vastly reduced.
And then we have class D, requiring possibly a pulse width modulation circuit to drive the output devices which are either on or off. High efficiency and battery friendly. Cell phone technology screams for class D.
Which is the best though?
In my own experience, class A for the high notes – less drive required to the mid and tweeter, class AB or D for the lower notes. Bi-amped of course or if the wallet is thick, tri-amped. Efficiency also plays a huge role. And yes, you get more “bang” for the buck with Class D.
What about class C. Ah, the old class C radio transmitter used for Morse Code. Yes, again switching on and off. The transmitter (ahem, tube stage most probably) is biased to off, highly negative to the control grids, and when it gets a CW (continuous wave) it has sufficent drive level to get the bottles in the final stage to output enormous gobs of power. If you have never heard of diode switching now is also the time to see just how ingenious these little devices are. As pushing the key down to see what happens.
So in radio work class C is the way to go? Not really because now we have SSB or single sideband, possibly USB (upper) or LSB (lower. If you have ever owned a SSB CB set you would have seen this. But more conventional in amateur and maritime radio. Class AB is used in the final stages. A rig could be rated at 1500W peak envelope power but the final stages draw 3000W. On a ship the power supplies for these transmitters can easily dissipate up to 5000W and one can hear the loading take place on these supplies at full power. Get your hands on a couple of 3000V DC supplies from a mothballed ship and build yourself a healthy PA system. Don’t kill yourself though, I have been burnt by touching the live anodes of a class C transmitter. No fun!
For today’s lesson we look at class H (or G) audio – manipulating the power supply.
What the hell’s this?
The Class G / H audio amplifier uses a power supply which has in the more simple arrangement a split rail (+), (-) high voltage and a (+), (-) low voltage e.g. +50V, +25V, 0, -25V, -50V as above. What we have here is a class of amplifier which is designed to run at the +/-25V rails at lower powers, in this case about 35W into an 8 Ohm load on the lower supply rails. If the waveform is increased to a value which is above the threshold of the high or low side pinch-off voltage of the supply diodes the higher powered transistors then conduct current, supplying the higher rail voltages through the loudspeaker, allowing for more headroom.
What are the differences then between class G and H?
It’s all in the power supply. Class G has a passive arrangement, the supply rails remain fixed at, as in this case +25V for lower powers and +50V for higher outputs (or -25/-50V rails).
In class H the headroom is increased by applying a higher rail supply, now active, to a single pair of output transistors. By far the most common arrangement used in this technique is by bootstrapping, a capacitor is charged to boost the supply rails for a duration defined by the headroom required. This a more complex and expensive arrangement.
A typical example of the class G design is found in the Sanyo STK412-430 chip.
I note that this chip is often referred to as a Class H audio output IC but in my own mind (MHO) this is a class G chip which allows for switching between 27V and 57V rails.
The TAS2559 – Class D Class H Boost
The above is a 5.7W high quality audio amp running class D with class H boost.
As can be summarised by the above in my opinion there is a great deal of ambiguity in the use of the terms Class G and H. Class G can comprise of more than two fixed rails, sometimes 3 or 4 rails of + and – supply whereas the class H definition is more around a supply rail boost. Confusing? Right you are.
In a broader sense…
I like the idea of a supply which can be instantaneously varied to increase headroom – this would be a high speed pulse width modulated supply which can be called upon to increase the rail supply in a linear fashion.
Unfortunately in a broader sense I’d like to think that class D has made the other classes all the more obsolete. “Yeah right” say the purists. Perhaps we need to look at how audio amplification has evolved to a point where I truly cannot hear the difference between Class A and Class D yet in my mind Class A is supposedly superior. Audio Nazis will lambast the different classes only based on what they think they hear – I have heard many high end systems and to be honest (my opinion), I cannot hear the difference. Speaker systems, yes but not a well designed amplifier. One wire amplifiers haven’t been invented yet. Not at the time of this writing.
But having said that…
Look back at the article on the JVC 5042, a lowly midrange, el-economo home theater amplifier. Boy, that thing pumps! And it uses the Sanyo 412-430 chip. Class G or Class H? I’d say class G. I don’t hear switching noises either.
Class G vs AB Headphone amplifiers – Texas Instruments
I read up on Rod Elliott’s research and article after I had compiled this blog post and found it to be right on the button in every aspect. Strange that this continuous ambiguity still exists between the G/H classes. A lot more technical input but a great read.
Of historical value (1956): The ARRL look at Linear amplifiers in AM and SSB