Class S

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Could anyone tell me if the Class S (Sandman) type amps are still in use today ? Are they any good ? And is it worth trying to make one ? If I understand correctly they work in a similar way to the Quad current-dumping technique.

I am looking to build my first dual-mono power amplifier. I have always liked the idea of the current dump and was intrigued by the Class S when I was younger. Truthfully I would like to go down the class A route but they are very inefficient, but good on a winters' evening ! I know that a well designed Class B sounds good, that's what I am using at the moment, but would like to know what the general opinion is out there. Am I going down the wrong path ?

Thanks

Gareth
 
AFAIK the class S was only/mostly used in low powered amps.

Tandberg Audio made this little 15W (25W according to some sources) amp named "Troll" before they folded their tents. I believe it was class S. It is quite rare today.

An externally hosted image should be here but it was not working when we last tested it.


For building your first amp, it would be best to stick to class B. Less expensive to build and run.
 
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AFAIK the class S was only used in low powered amps. Tandberg Audio made a 15W (25W according to some sources) one before they folded their tents.

An externally hosted image should be here but it was not working when we last tested it.


For building your first amp, it would be best to stick to class B. Less expensive to build and run.

Are they that bad ? Didn't Technics have a bash, though not for long ? What about the current dumpers ? Any ideas ? Or stick with Class B ? I have read 'Th eHigh Power Audio Amplifier Construction book' by @forgotten the name' sorry, but he is an American I believe. He mentions all the different output transistor/mosfet configurations to be used and there were three to choose from I think. I fancied a common emitter, again if I remember correctly, and the cascode design because of it's high bandwidth, but after reading about all the pro's and con's he mentioned I was a bit confused as to where to go which is why I wanted to go down the class A route (still would actually) and possibly the current dump.

Yours
Slightly confused
 
I think you will find that "Class S" is a trade name for a design, much like
"Class DX", not a category recognized by IEEE.

:cool:

Thanks Nelson, I thought wrong then. I thought it was actually a type like A, AB, B, D etc. In your opinion do you think I should go down this route ? Initially I wanted to build class A because of it's topology, transistors/MOSFET's being always on and thus no switching distortion as they are biased on and off. Then, having always liked the QUAD current dump I thought of trying one of those.
There is another reason I fancied a Class A and that is because I have no oscilloscope or acccess to one and I thought it may be easier to set up the Class A because of this. Am I thinking along the right road or should I think elsewhere. I would appreciate your honest answer here as you are the well respected Mr. Pass.
 
Exactly, Dr. Sandman's amp is elegant in a different way - it uses a passive bridge so that a (high power, relatively low-precision) Class B amp drives the majority of the load, and small, ClassA amp provides accuracy via a scaled, complementary output, and the two sum to high accuracy. Another way of looking at it (John Linsley Hood's view I think) is that effectively the big/dumb side drives the output so that the class A side 'see' a very high-impedance load well within its scope. It is not Quad-style 'current-dumping' but a related approach and also elegant.

I believe the principle got appropriated into Technics 'Class AA'
 
Yep.
It basically uses the small accurate amp to make the larger one "see" a high impedance load (so slightly different from what you described) via the bridge network. If you use even a basic push pull emitter follower output stage with enough bias to get round the crossover region then if you are only driving say 1K rather than 8R the distortion is very low.
Yes it was the class "AA" Technics that ripped off his idea. I hope he won...
 
Class S was was all about efficiency. It is quite similar to Class D. Some say it is exactly the same. Others call it soft switching as opposed to the hard switching of Class D.

Yes, I thought Class D was some type of ultra fast switching with high rail voltages, a kind of 'chopper' amplifier and is used in high-power subwoofers for instance (KEF etc) maybe because of the switching distortion, dissonant harmonics etc are kept up and out of the low frequency range of the sub's output. The design, Class D, was for efficiency apparently..could be wrong here though
 
Exactly, Dr. Sandman's amp is elegant in a different way - it uses a passive bridge so that a (high power, relatively low-precision) Class B amp drives the majority of the load, and small, ClassA amp provides accuracy via a scaled, complementary output, and the two sum to high accuracy. Another way of looking at it (John Linsley Hood's view I think) is that effectively the big/dumb side drives the output so that the class A side 'see' a very high-impedance load well within its scope. It is not Quad-style 'current-dumping' but a related approach and also elegant.

I believe the principle got appropriated into Technics 'Class AA'

That is what I was thinking (ish). The small power feeds th output under low output demand and then the class b side 'kicks in' when more power is demanded right ? To me this sounds like a good idea if implemented well, I have a book in the house by John Linsley Hood and he suggests ways of making it work and offering examples, by Technics (SE A100), of how others did it. Also he offers the QUAD current dumping technique which although similar is still different.

Still, no one has answered what I really want to know. I really want to go down the Class A route, pre-amp stages are Class A for instance, but it is wastefull of power. So I got to thinking on the Class S or Current Dump topologies. I have no oscilloscope and would like to know if I could build one of these designs easier than a Class B. I have another book by G. Randy Sloane (High Power Audio Amplifier construction book) and he is a big Class B fan. Myself I like the idea of Class A, no matter how wastefull it is, as all the output devices are always on-biased so no switching distortion etc etc. Do you see what I am trying to get at here guys ??????:(

Help please, an honest opinion would be much appreciated, thanks

Gareth
 
Seems that the Tandberg Troll wasn't a Class S after all. Perhaps it's the mystique of this rare little beast that made me think so. After much searching I found this schematic and it looks fairly normal.
 

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That is what I was thinking (ish). The small power feeds th output under low output demand and then the class b side 'kicks in' when more power is demanded right ? To me this sounds like a good idea if implemented well, I have a book in the house by John Linsley Hood and he suggests ways of making it work and offering examples, by Technics (SE A100), of how others did it. Also he offers the QUAD current dumping technique which although similar is still different.

Still, no one has answered what I really want to know. I really want to go down the Class A route, pre-amp stages are Class A for instance, but it is wastefull of power. So I got to thinking on the Class S or Current Dump topologies. I have no oscilloscope and would like to know if I could build one of these designs easier than a Class B. I have another book by G. Randy Sloane (High Power Audio Amplifier construction book) and he is a big Class B fan. Myself I like the idea of Class A, no matter how wastefull it is, as all the output devices are always on-biased so no switching distortion etc etc. Do you see what I am trying to get at here guys ??????:(

Help please, an honest opinion would be much appreciated, thanks

Gareth

Class A is really the way to go if you don't need loads of power..
I only really use class A in my own system these days since i have 92dbW speakers, 25w is enough to go bloody loud!
 
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Sandman has published a number of designs, the one I believe is usually termed Class S is similar in topology to the QUAD Current Dumper. But operates on different principles.......
As also mentioned here, it was realised with some modification, in Technics's SEA100 amplifier ca. 1980. JLH, in his title "Audio Electronics" gives quite a lot of space to discussion of this elaborate (for the time) version of the basic design. It contained a lot more bells and whistles than just the class S output drive system.
It's interesting that the SEA series didn't stay long in the market and was eventually superseded by the long-running SUV series amplifiers; another proprietary bridge class called Super AAA+ from Technics which was more reliable and IMHO, did sound a lot better than your average AB amplifier. An example I still have, has some good audio qualities. Importantly - unlike class A - they run cool in domestic use.

So, there is still life in the bridge A + B amplifier technology, Quads etc. if not just Sandman's class S approach in particular.
 
That is what I was thinking (ish). The small power feeds th output under low output demand and then the class b side 'kicks in' when more power is demanded right ? To me this sounds like a good idea if implemented well, I have a book in the house by John Linsley Hood and he suggests ways of making it work and offering examples, by Technics (SE A100), of how others did it. Also he offers the QUAD current dumping technique which although similar is still different.

Still, no one has answered what I really want to know. I really want to go down the Class A route, pre-amp stages are Class A for instance, but it is wastefull of power. So I got to thinking on the Class S or Current Dump topologies. I have no oscilloscope and would like to know if I could build one of these designs easier than a Class B. I have another book by G. Randy Sloane (High Power Audio Amplifier construction book) and he is a big Class B fan. Myself I like the idea of Class A, no matter how wastefull it is, as all the output devices are always on-biased so no switching distortion etc etc. Do you see what I am trying to get at here guys ??????:(

Help please, an honest opinion would be much appreciated, thanks

Gareth

Hi Gareth,

well I don't want to burst your dreams here but seriously, without having a scope AND spectrum analyzer I don't think you could succeed with what you have in mind. You might get things to work and amplify but you may likely never get even get close to Class A(B) performance without proper test equipment.
But by all means, the Class S topology is a very slick and elegant approach, I like. I took the freedom and attached the Dr. Sandman document for reference.

With reference to your question if you could build a Class S amplifier easier than a Class B, I highly doubt it ! :eek:

Cheers

Kay
 

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I have another book by G. Randy Sloane (High Power Audio Amplifier construction book) and he is a big Class B fan. Myself I like the idea of Class A, no matter how wastefull it is, as all the output devices are always on-biased so no switching distortion etc etc. Do you see what I am trying to get at here guys ??????:(

Help please, an honest opinion would be much appreciated, thanks

Gareth

My advice would be to build one the Slone's designs. If this is your first big project, start with one of the simpler Class B designs. Figure 11-4 is a good compromise between complexity and performance. It was suggested in another thread that the PCB can be improved upon, but it will work.

Class A is all good and well, but the power supply and heatsink requirements will eat up your wallet very fast. If you get it wrong, your amp melts down - with an even greater fiscal impact. Just how much better Class A is and how bad switching distortion gets, is another debate for another thread.
 
Somewhat related: Somewhere in the 80's I read a description of a prototype Sansui high efficiency power amp (80 to 90 percent efficient) using PWM signal tracking power supplies with a convention ab type output. I tried googling it, but haven't found anything yet - concept is similar I believe. Never saw one on the market, though.
 
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