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Old 16th February 2011, 11:49 AM   #1
generg is offline generg  Germany
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Default deepness of outputstages

hello everybody,

when eating the a good soup for dinner I decided to ask you a question that is in my mind

- despite the good soup

normally is said more bias in the output stage, less distortion better sound.
Someone asked Mr. Pass what happens with the good sound when the bias of the devices goes down when paralleled.....and he said something like the sum of currents is important.

For instance Nelson uses 24 or 48 matched devices parallel...in his commercial amps:

the sum of the current is big......but who does the single device feel it neighbors? It has a slow current and should sound worse......

why is the sum of bad sounding single devices a very good sound?


I somebody is eating also a good dinner maybe he is willing to answer
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Old 16th February 2011, 12:21 PM   #2
flg is offline flg  United States
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Some parameters sum in benificial ways, some not. For instance, the paralleled outputs reduce the source impeadance of the amplifier improving damping factor. The individual transistor's Transconductance will be reduced at lower current but the total sum of all the output transistors goes up, reducing distortion.
Obviously you might be running more idle current with more transistors but, the heat load is spread out amongst them, and the total current available can be very high.
I'd like to hear the rest of this story myself. I have never built an amp with more than 3 pair at the output (Guitar amps don't count). 3 seems like enough if you bias the s%^&t out of 'em.
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Old 16th February 2011, 12:22 PM   #3
AndrewT is offline AndrewT  Scotland
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I think you must look at two different operating phases.
1.) When the push pull output stage is working within it's ClassA current limit. This also applies to a singled ended ClassA output stage.
2.) when the push pull output stage is working beyond the ClassA current limit.

I think you will find that bias current/device and total bias current will give different "output sound" results depending on whether you are in the 1 or 2 operating region.

Take as an example the ClassA output of a mosFET output stage that follows Borbely's recommendation. He suggests at least 500mA of total output bias and at least 100mA of bias for each output device.
500mA of total bias gives ClassA current of <=1Apk This is equivalent to 4W of ClassA power. A 5pair output stage with each device biased to 100mA will also allow <=1Apk of ClassA current.
Now consider what happens when you stay in ClassA or stray beyond ClassA (1 or 2) and change the numbers of output pairs.
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Old 16th February 2011, 12:32 PM   #4
flg is offline flg  United States
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I vaguely remember a discussion involving N.P. regarding this subject in the F4 thread. More than a year or two ago probably...
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Old 16th February 2011, 12:53 PM   #5
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I had the same question a while back.

Your guess is as good as mine, but here is my guess.

When you have fewer devices at higher bias levels the modulated current (with music) through each device is higher which also means greater changes in temperatures in the die. Transconductance also changes with changes with temperatures. I suppose this means another form of non-linearity and distortion.

On the other hand if you have more devices at lower bias levels then the modulated current through each device is lower which also leads to smaller changes in temperature in the die.

However it is not easy getting perfectly matched devices without a curve tracer.

So where does that leave us? I don't know.

Anyway, with my level of intelligence (total numpty), I would disregard everything I have said.

Edit: Sorry if I am repeating stuff already said. I am a slow typer.

I have a question that I am sure someone can answer. Why do you P-channel devices always (well at least it seems that way) have higher levels of input capacitance to their N-channel complement.

Last edited by Melon Head; 16th February 2011 at 12:58 PM.
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Old 16th February 2011, 01:16 PM   #6
Zen Mod is offline Zen Mod  Serbia
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Generg

I'm sure Papa will slap me if I'm wrong , but you'll find that deep deepness only in Papamps with common source output stage

meaning - current level by one pair is much more important in ,say, F5 than in F4 (considering that F5 isn't common source but common drain output )

that's just one side of disco ball

there is certainly more .....
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Old 16th February 2011, 01:18 PM   #7
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by Melon Head View Post
.......
I have a question that I am sure someone can answer. Why do you P-channel devices always (well at least it seems that way) have higher levels of input capacitance to their N-channel complement.
bad dope during doping process



chemistry and physics ........ what else ;

there ends what I know about that
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Old 16th February 2011, 01:48 PM   #8
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Yes chemistry and physics it all makes sense now
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Old 16th February 2011, 01:58 PM   #9
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Old 16th February 2011, 02:10 PM   #10
generg is offline generg  Germany
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We ask Zen Mods Papa.......


or better we ask first Nelson and then Zen Mods Papa!
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Last edited by generg; 16th February 2011 at 02:13 PM.
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