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Class A

Discussion in 'AV Pre-Amp/Processors & Power Amps' started by fabio1605, Jul 5, 2004.

  1. fabio1605

    fabio1605
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    Got a button on my amp... marantz pm7200... sayin class A sound dont change what is it?
     
  2. karkus30

    karkus30
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    It changes the bias on the output transistors. Most amps spend their time in class B it makes them more efficient and powerful. Class A gives lower distortion, but reduces the power of the amplifier and hence the power output to your speakers. It usually gives a warmer valve like sound, but it depends on how revealing your speakers are.

    You can notice another difference ; when it in A class the transistors are functioning all the time which means the amplifier runs very hot when its not being fed a signal and actually gets cooler when you start to play something.

    Pure class A is very rare and usually results in very low power amps that require highly efficient speakers.

    Looking at your amps spec it runs at 95W RMS in B and around 25W in A
     
  3. fabio1605

    fabio1605
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    thanx for that makes a lot of sense.... i noticed the class a 25watts thing and thought seems a bit low but volume size it was exactly the same.... i knew it got in class a mode i dint relise it cooled down whilst playing

    Thanx for your help i will bare it all in mind... theory wise when its on full volume class a would reduce distortion then...

    Thanx again for your help
     
  4. karkus30

    karkus30
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    Volume is a strange thing and doesnt work out the way you might think.
    You might think that double the volume would be achieved by doubling the output, but it doesnt work like that. Its logarithmic, so to get double the volume requires 10 times as much power. So if you took your power rating as 25 Watts, instead of 50 Watts you would need 250 Watts. So you can see that the difference between 25 and 95 isnt so great. But having more power available does mean theres a greater reserve of current to cope with big bass bits and other powerful dynamics.
     
  5. bobbypunk

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    Note: PM7200 must be turned off by button on front panel (Power off) before changing mode to class A or it will not change mode!
     
  6. fabio1605

    fabio1605
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    what do you mean... i gotta turn it off before changing it to class A mode....
     
  7. Dean

    Dean
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    Yep it says so in the manual too. Just got mine yesterday, nice amp and weighs more than my old Marantz KI. Need to run it in but already the sound is impressive out of the box. :devil:
     
  8. alexs2

    alexs2
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    As karkus says,amps that run in Class A are relatively uncommon,although rare would be underestimating the numbers.....Krell,Levinson and Gryphon have long used Class A,and Musical Fidelity are no strangers to it either.

    Very few of these amps are low powered,with the last of Krell's non sliding bias amps running to 300W of Class A with massive heat output,but utter stability into any current speaker load,even now.

    certainly well worth trying the Class A setting,and being sure to give the amp plenty of time to reach operating temperature,as if it's in any way similar to an old Krell,the sound will change as the amp warms fully.
     
  9. dmurfet

    dmurfet
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    I have just recovered a set of kit Class A monoblocks from my loft and I am using them to power my front channels. (Sage Audio Supermos1 - I can't find anything on the internet about them!)

    Although they claim to be 100w Pure Class A - and I am using the Yamaha 750SE which is also 100w - they are massively more powerful.

    The story is not so clear though with the sound. Voice seems softer and more realistic, the bass is unaffected since my sub is active anyway and I have smallish SF speakers. I have had to crank the sub quite a lot to keep up with the fronts and now as a result I am not sure the system sounds balanced. They do run warm, but not hot - at least not after 3 hours of use. I am not convinced they are better - probably because of age - but they are different!
     
  10. dmurfet

    dmurfet
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    :suicide: :suicide: :suicide:

    OMG!

    I disconnected the monoblocks for two reasons -

    they sounded a bit overpowering even when I ran the autosetup on the processor

    one of them has a fault that makes it crackle and hiss a bit especially when you are messing round the back with the cabling

    Now the Yamaha sounds *very* shut in - balanced yes and in some ways nicer but the midrange now sounds like it is in the next room. Imaging is better now however (my SFs are very good at that IMO).

    I can see me forking out a staggering amount on some amplification that will clout my speakers in a controlled way in trhe near future..........

    Anyone for some "well travelled" Pure Class A monblocks? :cool:
     
  11. alexs2

    alexs2
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    If they get warm rather than hot,and assuming the heatsinking isn't truly massive(and I mean as in surrounding the 2 sides of a 20" deep amp),they probably aren't running in Class A up to 100W,but are biassed into Class A for the first 20W or so.

    Running 100W amps in full Class A requires a massive power supply,and either fan cooling,or serious heatsinking as in the Krell KSA80,which wasnt fan cooled but used huge and rather sharp heatsinks.

    The midrange on a good set of Class A amps,regardless of power,can be superb however.
     
  12. dmurfet

    dmurfet
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    These use the sharp, dangerous heatsinks on the rear of each, not down the side. You can touch the heatsinks and not burn yourself n- that's what I mean by warm. They do draw a lot of current though I think since they utilise cooker mains cable which is completely inflexible (and very short which does not help!)

    Anyhow I have now scared myself at what 20 year old power amps can do - wonder what todays are capable of - I did recently hear the Krell showcase 5 / Lexicon and some very expensive other kit and because it was all so controlled, it was difficult to comprehend how good it really was - obviously that is my desired result - best I do some more work!
     
  13. alexs2

    alexs2
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    If you're into making amps,and in particular Class A,you could take a look at www.passdiy.com where you'll find some superb homebuilt examples of Pass Labs Alephs and Zens....the circuit diagrams etc are available there also....a great example of a company giving something back to the community that it sells to.

    Also some amazing creations on the gallery there.
     

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