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csl113 22nd June 2006 01:06 PM

class H design
i want to design a class H amp, a high power class H amp, but i don't kow how to control the transistors that boost the suply rail

i have found only one complete schematic of a class H amp: qsc-isa750

does anyone have more schematic that i can inspire, or a book about the design of class H amplifiers....???

the output transistors will be 2sc5200/2sa1943 ,and the suply rail should be + / - 50,100,130V

SpittinLLama 22nd June 2006 06:42 PM

Do you have a link to the QSC schematic? Is this Class H or Class G? Sounds like you are talking about Class G (2 or more supply rails) in your question.


csl113 23rd June 2006 01:30 AM


On the ISA 750 and ISA 800Ti, class H complementary bipolar output circuitry increases system efficiency by........

i am talking about class H not class G, there is a big difference, but both have multiple supply rails

class G is basicly 2 amplifiers in one,a lower voltage amp and a higher voltage, and both are usualy class B, but the smaller one could be class A, and the larger one could be in most casesc class C

and class H is basicly only one amp, but it has multiple voltage rails. as the output gets bigger, and near the first rail, it automaticly switches to the second rail for the duration that the output is above the first rail, this is like boosting the power suply at only ONE amp

for exemple the qsc as the signal is near 55V at the exit it switches to the second rail at 110V (it boost the suply rail) , and the Vce at the output transistor remains a max of 55 or 60V even it has the rais of 110V. this efectivly lower the number of output devices down to almost half and also the heat disipation, and is you have 3 rails you could in theory use only about 35% of the output devices necesary for the max voltage rails. but in practice as bipolar transistor suffer second breakdown at high voltage ,and this reduces the SOA,you will use ~only 25% of output transistor necesary for a given power,and the cost alone of output transistors+heat sink is greater than that of the rails switching MOS and the components that drive them

here is the schematic(i could not attach it,because it is too large file size)

Workhorse 23rd June 2006 04:06 AM

Hi Folks,

Have a look at this Thread:

K a n w a r

oPossum 23rd June 2006 04:54 AM

Schematics of old Soundcraftsmen Class H amps

SpittinLLama 23rd June 2006 05:53 PM

Everything I've read in books identify Class G as a two or more supply rails and the supply steps up and down the rails as needed. Class H is an improvement over Class G such that the number of rails goes to infinity and the supply now becomes a smooth signal. The supply is always only the few volts above the output needed to keep the output stage transistors from hitting saturation. I have seen efficiencies in the mid to upper 80% using a Class AB amplifier with a supply that tracks the output signal (Class H) with just enough head room for very low THD. Douglas Self identifies the different classes this way in his book 'Audio Power Amplifier Design Handbook' for just one example. Maybe I am still too knew to these boards and don't understand the lingo. Topic has probably been beat to death here but I am an infrequent reader.

Thanks for the links to other threads on Class H. I will check those out.


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