The Class - H Amplifier

[Workhorse]

Hi everyone,

Apart from class-D , the efficiency in Linear amplifiers could be obtained by converting a Class-AB amp into Class-H amp...

I mean to say in simple words "The Rail Switcher Amp" benefits from the elimination of wasted heat incase of class-AB , by splitting the power rails into 2 or 3 TIERS, and choosing the appropriate Rail according to the Voltage Swing at output.....So that the output transistor has less VCE across it to do the job with less wastage...

I have seen many pro-amp manufacturers Quote that Class-H reduces the current consumption from as much as 30% in comparision to same wattage amp in Class-AB, meanwhile, it also states that the heatsinking requirements are much less than class-AB amp.....

But their are many challenges that must be met in order to overcome various obstacles such as Switching noise, Spikes, Seamless Transition from one Tier to another and so on....

Below are the switching waveforms from a pro-amp in Class-H



regards,
K a n w a r

[amplifierguru]

Hi Kanwar,

Yes Class H may save some power /dissipation but at the price of complexity. The other aspect of these switching supplies is that the artefacts can get into the amp residual and degrade image/detail/timbre... PSRR in many stages falls away at HF.

You like a challenge... I'm a simple person (PC) and like simple sweet and detailed R2R Class AB. I'm not that desperate for power to go there.

Cheers,
Greg

[Workhorse]

Originally posted by amplifierguru
Hi Kanwar,

Yes Class H may save some power /dissipation but at the price of complexity. The other aspect of these switching supplies is that the artefacts can get into the amp residual and degrade image/detail/timbre... PSRR in many stages falls away at HF.

You like a challenge... I'm a simple person (PC) and like simple sweet and detailed R2R Class AB. I'm not that desperate for power to go there.

Cheers,
Greg

Hi GreG,

Yes you are right but, Class-H isn't meant for Studio or Hi-END HIFI, but its meant for PA, all these pro-amp guys[except me] use this Class-H to draw more with less wastage.
The complexity is very much high Dual CAPS , 2-4 Bridges , lots of switching asthetics involved....

regards,
K a n w a r

[CBS240]

You may be able to limit the affect of switching noise by just using the amp for slower sub freq., as this is where most power is anyway, filter and choke the noise out. Use another smaller amp for the rest of the audio band. By then you might as well go class D. HF operation is another matter with the ringing and such pesky oscillations.
I played around some with this type of circuit a while ago. 28V and 55V rails. I seem to remember using an EF stage instead of CFP to switch the output collector to second rail worked a little better. It was a bit smoother when I used the top EF to follow the audio from the pre drivers to keep like 12V above Vout when above 28V. This way I could use 1 pair of high current(lower voltage) transistors for output stage with min beta of 20 at 20A and not exceed its SOA or cause SB. Something about trying to get lots of juice without paralleling output devices. I used 3 pairs of TIP35/36C for the top rails because they are cheap and rugged. I was somewhat discouraged because I ordered some power devices and the TIP35's were fakes and had a dirt poor SOA at 30V and this was a problem. Guess I ordered a little too cheap! Anyway, I plan to get back to it someday.

[lumanauw]

Hi, Kanwar,

There are some interesting "efficiency" oriented approaches. If you are OK with complexity, you can build class H or G (3x complexity). Or something like Labgruppen/Chocoholic class D-A/AB-D.
For A-AB, The novel approach is Sano Hirota, and offcourse Nelson Pass has a very clever (and most likely will work practically) patent on this one.

Going to all classD can be a ticket too. Crown K1 is a good example of how efficiency can get along with quality (of pro amps standard)

[anatech]

Hi Kanwar,
Look at the Carver series of amplifiers. Your waveform looks like the first version. The switching transients will appear in the output for sure. In later versions, Carver had the commutators follow the waveform in a linear fashion. That worked much better.

Two things to watch for. The amps were so light that uninformed sound guys used them for highs and mids. The worst thing you could do. Because of the time constant, the commutators would "lock up". They stayed on and the amp would go into thermal runaway. These amps would hammer bass or full range all day long. The other problem had to do with Carver's AC pre regulation. Worked great on a proper circuit. They drew heavy current and blew up when run on long extension cords, and gas generators ? Don't even ask, don't go there.

The Lightstar would interest you. They used variable duty cycle to reduce a higher supply voltage to what was required. The frequency was very high, I can't remember off hand. I owned one of these Carvers too. It was the best sounding one they ever put out.

-Chris

[Workhorse]

Hi ALL......

Thanks for the great comments,,,,,.....

The Waveforms were from the PULSE Class-H amplifiers....

My own commercial amplifiers were still Class-AB not Class-H

I have studied many class-H pro-amp schematics from QSC, CREST, STUDIOMASTER ..the fact is that their Efficiency is good and they sound good but not as good as Class-AB amps.....
Class-H is only worth when power output is above 1KW level......150W Class-H amp is a piece of over complex box.....

Regards,
K a n w a r

[Workhorse]

Hi Anatech,

I have seen the BOB CARVER's Creation the Magnetic Supply based on Triac Conduction control, which acccording to me is a Flaw in its own way because in power amps the Peak to Average power ratio is very high and thus the Peak power demand cant be met with the Triac Phase angle conduction phenomena..

regards,
K a n w a r

[djk]

I have used and repaired many of the Carver amps with the triac circuit.

Chris is right about the low voltage line problem.

Other than that, the supply works fine. The M1.5 weighs in at 16lbs, puts out 1.2KW RMS (1.5KW on program material), sounds good, and has no heatsink other than the small 3.5H" X 17W" X 10D" chassis (pro version adds a small fan).

I think the later M1.0 is the best example of the Carver art. It runs on ±25V, ±50V, and ±100V, has good sized sinks, and (compared to the M1.5) 3X the supply capacitance.

The M1.5 used linear (cascode type) followers for the higher voltage tiers (as does Crest). QSC mainly uses a saturated switch. The M1.0 was an excercise in cost cutting, and used linear on the ±50V rails, and a saturated switch on the ±100V rails. An interesting wrinkle here was the use of only one switch for both channels of the stereo amp. Sunfire follows this same practice with their (switching regulator type) follower, only one for all channels.

The Sunfire sub amp is a bridge design and thus does not need ±V switching, just a single voltage. The Crown VZ amplifiers are also a bridge, and use only one saturated switch for the high voltage tier (warning, Crown pdf/zip is 6MB).

http://www.crownaudio.com/pdf/amps/m...schematics.zip

National Semiconductor has a patent on a bridge amp with a single linear (cascode type) follower (a big IRF FET).

http://patft.uspto.gov/netacgi/nph-P...S=PN/6,304,138

For hi-fi use I think the linear (cascode type) followers when implemented with a Schottky type diode have the most potential. You can bias the low voltage tier into class A, and have a smooth transition into a cascode class AB amplifer from the higher tier.

http://www.signaltransfer.freeuk.com/Gfig11s.gif

And of course, Nelson Pass.

http://patft.uspto.gov/netacgi/nph-P...S=PN/5,343,166

[Workhorse]

Hi DJK,
Thanks very much for the information on Rail Switcher amps....

What are your views regarding the sound quality of these Class-H amps , compared to Class-AB amps...

regards,
K a n w a r