Hi,
we know that a class B amplifier's theoretical efficiency calculates to π/4 when fully driven with a sinusoidal signal. Add the power consumption of the previous stages and the bias, we get the minimum power the PSU has to be designed for.
If we redesign the power stages for class G or H with, say, rails of +/- ½ Vcc and +/- Vcc, what power needs the PSU be designed for then?
Best regards!
we know that a class B amplifier's theoretical efficiency calculates to π/4 when fully driven with a sinusoidal signal. Add the power consumption of the previous stages and the bias, we get the minimum power the PSU has to be designed for.
If we redesign the power stages for class G or H with, say, rails of +/- ½ Vcc and +/- Vcc, what power needs the PSU be designed for then?
Best regards!
Pretty much the same as for class B, at least for traditional class G with 2 sets of rails. If you integrate the power of fullscale sine output over time, you should find that the parts where the amp is using the full +/-Vcc rails is accounting for the bulk of output power. So not a great deal less, really. This setup mainly helps efficiency well below maximum output power.
Things get a little more interesting when you've got a tracking switch-mode supply or DC/DC setup. Efficiency would be getting an awful lot closer to 100% territory then, depending on specifics (voltage headroom kept, tracking decay time, signal frequency).
Things get a little more interesting when you've got a tracking switch-mode supply or DC/DC setup. Efficiency would be getting an awful lot closer to 100% territory then, depending on specifics (voltage headroom kept, tracking decay time, signal frequency).
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> class B amplifier's theoretical efficiency calculates to π/4 when fully driven with a sinusoidal signal
OK, so do the simple math. pi/4 is 0.785 or 78.5% of input goes to load, 21.4% goes to amplifier waste. 100W of supply power is 78W of output. Or normalized to 100W output, we need 128W of supply.
Now suppose a magic amplifier of 95% efficiency. 100W of supply makes 95W of output. Or normalized, 100W output needs 105W supply.
The difference 128W or 105W is usually trivial to the supply buyer. Transformers and stuff are usually offered in 30%-50% increments: 100VA, 150VA, 200VA.
I'm guessing G runs say 85% efficient. So 100W out is 120W power. The main point is the reduced *losses* meaning less stress in the power devices.
That's at FULL power. A straight B will have losses at part-power even higher than at full power. G can reduce this significantly, by not using full voltage when not needed.
OK, so do the simple math. pi/4 is 0.785 or 78.5% of input goes to load, 21.4% goes to amplifier waste. 100W of supply power is 78W of output. Or normalized to 100W output, we need 128W of supply.
Now suppose a magic amplifier of 95% efficiency. 100W of supply makes 95W of output. Or normalized, 100W output needs 105W supply.
The difference 128W or 105W is usually trivial to the supply buyer. Transformers and stuff are usually offered in 30%-50% increments: 100VA, 150VA, 200VA.
I'm guessing G runs say 85% efficient. So 100W out is 120W power. The main point is the reduced *losses* meaning less stress in the power devices.
That's at FULL power. A straight B will have losses at part-power even higher than at full power. G can reduce this significantly, by not using full voltage when not needed.
The bottom line is that the peak, or maximum power draw of class H is nearly identical to AB, while it’s average consumption is much less. So the tendency in mass produced equipment (professional or otherwise) is to only size the transformer or power supply for the typical average current demand, which is about half as much as the typical demand on class AB. What you end up with is less sustained power capability (lower RMS at clipping) than a comparable class AB amp because the supply voltage falls that much further under a given load. A load which is essentially the same for both at maximum signal. One may argue that it is not needed because musical peaks are supposed to be short. But the 1% of the time you need full power for more than 20 milliseconds at a time you will notice it and miss the punch the old power hog had. There used to be class H and G amps on the market that had full size transformers in them and could produce as much sine wave power as their class AB counterparts, but they are long gone. All such models have been discontinued in favor of cheaper ones. Ones that could get close are disappearing and their prices have almost doubled in the last 5 or 10 years. Touring grade amps have all gone class D or some variant of it, and models that can produce full power for more than 20 milliseconds at a time, safely, are outrageously priced.
Of course if you’re building your own class G you just use the same VA rating toroid as you would have with a normal amp, just with the extra voltage taps. Then you get a cooler running amp that can put out full power. It may not be marketable anymore, but your rig will sound better and have more punch than your competition’s, who bought his amp(s) in the music store.
Of course if you’re building your own class G you just use the same VA rating toroid as you would have with a normal amp, just with the extra voltage taps. Then you get a cooler running amp that can put out full power. It may not be marketable anymore, but your rig will sound better and have more punch than your competition’s, who bought his amp(s) in the music store.
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