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Old 6th August 2008, 05:23 PM   #11
fab is offline fab  Canada
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Default high bias in push pull

I wonder what Nelson Pass would say about that....
Maybe he should reconsider his F4, F5 and X amps series based on some of you...

I made the experiment on low or no overall feedback design and increasing bias lowered the THD . Unless someone prefers to rely on high NFB as a cure all...

And I am not talking about the overall sound improvement due to class A...

Maybe you should read this article from Nelson "Leaving class A" http://www.diyaudio.com/forums/showt...62#post1553062 and reports your comments...

Or am I missing something here?
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Old 6th August 2008, 05:40 PM   #12
Mooly is online now Mooly  United Kingdom
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I am sure Nelsons designs do "what they say on the tin" and don't suffer from compromised layout, wiring and design flaws. Unfortunately many commercial products do to say nothing of D.I.Y. projects, and upping the the current won't neccessarily improve matters, it can make things worse in some cases.
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Old 7th August 2008, 08:23 AM   #13
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thanks to Zen mod,

i find "Krell KSA50" is nice for me. will it be easy to build? i don't need to much watts, 50 is more then enough for me as class A amp.

i am reading more about "Krell KSA50"...

i am worried that anythings goes wornd it could triger to class AB.

thank you.
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Old 7th August 2008, 09:04 AM   #14
Ipanema is offline Ipanema  Malaysia
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Just got a Marantz PM7200. It has a class A switch for quick comparison with class B mode. The difference between classA and classB is too little to be noticeble IMO, probably because the high NFB already reduces much of the distortion even in class B mode. So, increases the biasing doesn't always gurantee better sound. The overall design of the amp itself is more important.
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Old 7th August 2008, 09:10 AM   #15
AndrewT is offline AndrewT  Scotland
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Hi,
KSA50 is a push pull ClassA amplifier, 50W into 8r0.
It will automatically go into ClassB if you demand more than ClassA current from it, eg. 100W into 4r0.
The normal ClassA bias is 1.9A. The maximum ClassA output current is~3.8Apk.
The 2pair MJ15003/4 output stage can survive 2ohm loading, putting out peak currents of the order of 20Apk, way in excess of the ClassA maximum current. The transient peak current capability is probably of the order of 40Apk.
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Old 7th August 2008, 09:26 AM   #16
mikee55 is offline mikee55  United Kingdom
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Default I did discuss, before

Can I change Bias?

Mike
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If it don't work, I'll fix it in the mix! Or visit http://lsdp.proboards.com/index.cgi
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Old 7th August 2008, 02:38 PM   #17
fab is offline fab  Canada
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Quote:
Originally posted by AndrewT
Hi,
KSA50 is a push pull ClassA amplifier, 50W into 8r0.
It will automatically go into ClassB if you demand more than ClassA current from it, eg. 100W into 4r0....
Good reminder AndrewT that class A power is dependent of load impedance. And a speaker impedance varies a lot depending of the frequency of the signal...
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Old 7th August 2008, 02:41 PM   #18
fab is offline fab  Canada
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Quote:
Originally posted by Ipanema
Just got a Marantz PM7200. It has a class A switch for quick comparison with class B mode. The difference between classA and classB is too little to be noticeble IMO, probably because the high NFB already reduces much of the distortion even in class B mode. So, increases the biasing doesn't always gurantee better sound. The overall design of the amp itself is more important.
I can hardly understand how a bias switch can do a good job here. The thermal balance of the same whole amp will be different between 2 bias settings. This can be one reason (as many others like the conditions of the sound test done) why it does not sound different or better. It may even sound worse at high bias if thermal balance is modified. On my side, choosing the bias is done at the early stage of the design...

One can not simply take any completed amp and increase the bias with eyes closed. The conditions when the amp goes from class A to B must be carefully analysed. But, as long as the sound stays in class A power level then there is no output transistors switching artefacts thus there should be one problem to be put aside...
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Old 7th August 2008, 02:53 PM   #19
fab is offline fab  Canada
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Default Re: Sorry to butt in

Quote:
Originally posted by mikee55
...how do you know if you're in Class A anyway? An Led indicator type circuit would be nice.
Mike
I have one installed in one of my amp. Its my own design and it may not be perfect (depending on dynamic variation of DC offset at output and filtration of the very low frequency) but it gives a good idea.

See http://www.diyaudio.com/forums/showt...051#post703051

It should work in follower power amp output stage but not verified if other topology would...
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