03-06-12: Doggiehowser: Didn't Nelson Pass create the sliding bias thingamabob? :03-06-12: Doggiehowser
In actual fact my ex boss (Steven Deratz) invented the first sliding bias system back in 1978 and had a world wide provisional patent on it for a couple of years. But his system worked a bit like negative feedback, it monitored the signal and tried to adjust the bias up and down in real time which was a loosing battle because it was never in real time, just like global negative feedback.
I don't think Nelson ever had an auto bias system just an optical bias system which was on the S300/S500 MkII's, when he did Class A he went more for the real thing.
Later Krells plateau bias where it kick it up and stays for a while and gradually comes down is the next best. But think about it the initial (first) transient is not in class A because it lags behind like my ex boss's system (so it's class B "harsh") then if successive transients "lower" than the original follow all's good they are in class A, if higher they are in class B again, and if there is a quite passage the bias drops down again and it starts all over again.
This is why, I strongly believe that proper hard biased Class A cannot be by any auto adjustable type system.
Cheers George |
03-02-12: :Manitunc. Those amps don't use as much power when idling and don't get quite as not. My Dussun V6i, Musical Fidelity M3, Nobis Convigore and Classe 100 all run that way and sound pretty good. Manitunc:
If they don't run hot then they are not biased very high into Class A, maybe just a few watts depending on heatsink area.
"Ohms Law" simply put for the non technical to understand,
A 25 watt "only" pure class A biased amp needs X amount of heatsink area as the ML2's have I posted the link to.
A 100-150watt class a/b amp with 25watts of pure Class A bias needs up to 4 x the amount of heatsink area, even though the class A is the same as the ML2's, because the rail voltages are so much higher for the extra a/b watts, this quadruples the heat, even though it's still only 25 watts Class A.
Then there's the sliding (Plateau Biasing) Class A amps like Krell developed, but they were never as good sounding as their Pure Class A classics, like the old KSA50 KSA80 and KSA100 monoblocks all with fan forced cooling with no external heatsinks, just chimney's inside with fans at the bottom.
Cheers George |
This is correct the bias on a true SET amp increases and decreases the power (watts) and it stays class A. And then there's Class A1 with SET's that's another kettle of fish.
But what you said about Vick's amp was
"vicks,Your amplifier is 'single ended' with the KT88?"
And I said "no I think it's a hybrid of some sorts that acts like single ended."
Cheers George |
Yes James I believe these are class A most of the way, I know a friend of the designer here in Australia, Elson Silva, he builds "Cymer" amps here, he says there is just the same amount of fudging of class A amounts in the tube circles as well as solid state.
Cheers George |
I'd like to see the circuit when and if he publishes it.
Just noticed it also says it is run in Class A1, not Class A. Some tube gurus say the extra couple of watts gained from Class A1 is a backward step, rather a couple less watts and pure class A. I don't know never compared the two. It's getting good press and got all us talking about it.
I built a copy of the SET DHT Wavac MD805m mono blocks once no expense spared for transformers, complete with 300b driver and interstage transformers to the 805 RCA tube, the better I tweaked the circuit for sound, the closer I got to to one of my Class A water cooled solid state amps, in the end I stuck with class A solid state.
Cheers George |
I don't think they are single ended as they have series and parallel output tubes, this is said to behave like single ended without actually being single ended.
http://www.6moons.com/industryfeatures/audiopax2/4.html
Cheers George |
I believe the "power envelope" did nothing extra for the class A bias.
What I believe it did do was to aid in transient ability with a bit more short term current from the power supply. This way they saved on big expensive power supplies, and got a result that was better than using a mediocre supply.
I lifted this explanation from DIY audio, make of it what you will.
"The cheaper PE receivers used two ± power supply rails with a fast diode on the lower rails feeding the outputs, a polyswitch was in series with the upper rails."
Cheers George |
Thx Rx8man, my real name should have been georgehifi but for some reason Audiogon would not let me use this, so I went with the next thing I could remember.
Here is a more descriptive post of the self contained water cooled class A I built.
http://www.stereo.net.au/forums/showthread.php/37548-Living-with-Pure-Class-A-in-Australia?p=560887&viewfull=1#post560887
Cheers George |
Krells are great amps, just that we were mislead quite a bit by many manufactures even today about Class A.
eg: Zaphod1000 said he once measure Musical Fidelity's little A1 integrated (you know the one, you wanted to wipe your feet on it)in spec, at 2 odd watts class A, they quoted 20watts!!
And their big daddy of yester year the A370 at 12 watts, they quoted 185watts!!.
Cheers George
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Unsound: Georgelofi, I did see your link. I believe that at the time Mark Levinson was building those amps, he was charting fairly new territory, and those amps were overbuilt. Compare the size, weight & number of chassis, to a newer Class A amp capable of similar Class A power output:Unsound
This was what they needed just for 25w class A only , as they were still hot, after an hour or two idling, my friend owned a pair and I correctly biased them to spec for him, any hotter and they would have been unreliable.
Nelson Pass's favourite saying on how much class A bias heat sinks can handle, after 2hr warm up, is palms of hands on heat sinks with >35c ambient temp day and you should to handle the heat for 4 sec, this is the absolute max class A bias for those heatsinks. (brick layers need not apply for this test) soft wimpy tech hands or females only.
Cheers George |
Same topology Unsound, just different models in the same rang as the KSA250 range, all with partial Class A not even 30% of their full output into 8ohms.
Like I said the only ones to have high % of Class A compared to rms output, was the fan forced Krells of 80's.
And after that the only model to get near them with good amount class A % was the Plateau Biasing (Anticipator Circuit) Krells "S" models with their Xmas tree bias lights, but they never sounded as good from many a report.
Cheers George |
They still are not anywhere near pure Class A up to their rated outputs, as you think they are.
I built quite a few pure class A amps back in the 80's based around this Nelson Pass circuit
http://www.passdiy.com/pdf/classa_amp.pdf
but expaned out to 100 watts instead of 20 watts. And I can tell you for 100 watts of pure class A, the first one was bigger than your large sized coffee table, and weighed in at 120kg and still it got the heat sinks up to 60c after 3hrs and proceeded to get the dreaded class A thermal runaway, plus I had to change the power points to industrial ones because it started to melt the standard house ones.
The only way to keep it cool was to fan force x 4 the massive heat sinks, but this was too noisy. After this amp I built the same but this time I water cooled it, first one was self contained recirculated water with a quite pump heat sink jacket mini radiator and fan, it looked weird but worked a treat, wish I had photo's of it (still weighed in 60kg. After this one there were 2 more both water cooled but using the house water and draining into the garden.
This unsound is why I am confident in what I say about the pseudo pure class A amps out there, and why you should study the link to the Mark Levinson ML2 mono blocks I posted earlier and see what it takes to get 25w of A class, and they get too hot to touch
Cheers George |
Yes but they were only quite low in Class A
For instance the massive KSA-250 with all it's heatsink could only do 28.5 watt Class A into 8ohms and then up to 250watts of Class B into 8 ohms. And these ran stinking hot.
http://www.stereophile.com/content/krell-ksa-250-power-amplifier-when-class-class
Cheers George |
If you like pure Class A solid state, these Mark Levinson ML2 monoblocks are it if you can find a pair, note the amount of heatsinks just for 25 watts of class A (no a/B) and they get HOT!!!. Goes to show that others are stretching a long bow when they quote class a figures with pitiful heatsinks and quotes of 100 watts of class A. What they're really saying is a small amount of class A up to a point then class b the rest of the way.
Have a look at these beauties.
http://www.wtconcept.com/levinsonml2/
Cheers George |
