The output transformers are valves between 2 voltage rails. Say, + and - 10 V.
In A, B and A/B those rails are fixed values. Classes G and H vary those rail voltages. Carver and NAD got famous on G. The idea is that minimizing the difference between the rails and raising them only when needed minimizes heat of the output transistors.
NAD continues to dabble with their hybrid Class D amps today.
Wikipedia is your friend:
H is good.
D is a muddy mess.
My Sunfire sig 600 is class H, and smokes much if not most of the newer amps at lo local shop. They are sick of me showing up with it, and besting much of their high class amps. Im thinking's soon they will ban people from bringing in their stuff for speaker auditioning.
It’s all in the implementation. Making a generalization about amps by class is . . . well . . . unfounded. It’s like saying all blond(e)s are dumb. Every one I’ve met is different.
I’ve heard the AHB2 (class H) in my own system (with Janszen electrostats), and it sounded great.
I’ve heard Apollon amps made with PuriFi 1ET400A modules (Class D) in the same system, and the sound was great also.
It’s true that some early Class D didn’t sound so good. Development has been intense, and it’s paid off. The good ones are more than competitive with Class A and AB. Choice then becomes a matter of individual taste.
@arcticdeth arcticdeth "D is bad, H is good". That is the analog vs digital opinion just played on a different field?
Just to clarify: I only wanted to know the basic differences and general assumption. And like with the old analog vs digital, it is likely 'cheap D is better and cheap H' (here: $100 CD player beats $100 turntable).
Yep, attempt to get to Class D efficiencies of manufacturing as well as power usage without the switching issues.
If the linear amp is kept in Class-A, then the efficiencies over a Class A are really outstanding. AFAIK, they make no claim that H is better sounding than A or A/B, but as good, with much higher efficiences.
The point made elsewhere, that it’s all in the implementation, is really important.
It may help to understand that due to FTC rules, most amplifiers have to be heavily overbuilt. FTC rules require pre-heating at (I think) 30% of the rated power for an hour before testing for noise, distortion and output. This is a situation never encountered in music. If you have a 300 W amplifier, you don't play music at 100W continuously. To meet the FTC requirements an amp must have excessively over-built heat sinks, which adds so much to manufacturing and transportation costs. Anything that reduces the weight of an amplifier not only makes it cheaper to make, but greatly reduces the carbon footprint.
Absolutely. This seems to be what the NAD Hybrid amps are doing, but they have very very little information out in the public domain.
Yamaha's EEEngine amps also have this type of design.
One engineering difficulty about Class H though is the voltage rails have to anticipate the signal peaks. There's some interesting discussions online about Yamaha's implementation, distortion, and fixes. Class G solves this problem by having multiple rails available all the time, but this adds to the possibility of distortion when those rails switch.
My Soundcraftsmen A400 became scary good, after removing the wimpy power cord and hotwiring a 10 ga power cable I built to replace it, and, creating a 2ndary ground. Took this amp to another level. Power cords make a difference. Anyway, having owned many Soundcraftsmen products, I kind of know the story of Paul Rolfes, who was the chief engineer at Soundcraftsmen, and happens to be credited with the development of the signal-tracking multiple-rails power supply, as well as other patented circuits, which led to his development of Class H. When MTX bought out Soundcraftsmen, it was quite soon that they shut down the line, which was imo unfortunate, as the products ( for me, specifically power amps ) were quite good. Just some history. Enjoy ! Always, MrD.
@kraftwerkturbo , The A400 ( and A400 Pro ), S800, S840 and S860 all use the PCR circuitry, but I am fairly certain they were not categorized as Class H ( there were other models as well ). The A-5001 and A-5002 are Class H. Some differences in sound between the models, but a fully up to date ( re-worked and overhauled, after many years ) unit, imo, can easily hold it’s own with what is available today. With my dozen or so SC units owned, I never experienced a failure, but there are reports out there that differ from mine. There are some interesting readings on the net about the SC products. I hope this helps. My best, always, and Enjoy ! MrD.