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Old 18th May 2012, 06:04 PM   #651
sonnya is offline sonnya  Denmark
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If it is a differential output stage it is constant current - 100% when we are below class A
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Old 18th May 2012, 06:05 PM   #652
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For push pull class A the vector sum of the supply current is zero, any other class it is not.
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Old 18th May 2012, 06:06 PM   #653
sonnya is offline sonnya  Denmark
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Exactly!
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Old 18th May 2012, 06:30 PM   #654
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I can put in a 0,1 Ohm sensor resistor or measure the base current in the cap multiplier.
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Old 18th May 2012, 06:32 PM   #655
MiiB is offline MiiB  Denmark
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single ended Class A like the old Pass Aleph series does draw constant current. But the output current is limited to the quiescent current running in the output stage...true class A in its purest form...!! What we have here is capable of swinging more current than the bias, so It's class A-B. It will under load modulate the current draw on the supply lines.
I can be biased Hard, so it rarely leaves class A...but at the expanse or high current consumption and running hot...question is what is the optimum bias...does the high current bring any benefits....Think I remember some investigation that show and optimum distortion vise arround 120-150 mA...??

Last edited by MiiB; 18th May 2012 at 06:37 PM.
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Old 18th May 2012, 06:43 PM   #656
mikelm is offline mikelm  England
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In theory you can have an amp with constant current from supply and still give varied current to load but I agree that it is not that common.

You just need a single rail amp with an o/p cap and a CCS as the "load impedance" in the o/p stage. Its a very elegant topology precisely because the current is constant everywhere except between the o/p device & the load.

This is what makes the original JLH simple class A so elegant but of course the current is not constant in this design.

Last edited by mikelm; 18th May 2012 at 06:48 PM.
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Old 18th May 2012, 06:46 PM   #657
mikelm is offline mikelm  England
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Quote:
Originally Posted by MiiB View Post
Think I remember some investigation that show and optimum distortion vise arround 120-150 mA...??
I think that rather depends how you measure / assess the distortion.
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Old 18th May 2012, 06:57 PM   #658
mikelm is offline mikelm  England
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I would be very interested if anyone could demonstrate an amp design that when biased to 100 - 150mA had lower H10 - H20 than the same design biased into class A.

Last edited by mikelm; 18th May 2012 at 07:17 PM.
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Old 18th May 2012, 07:01 PM   #659
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I did some measurements. First i must say that i drive the amp on a lot of idle, over 2A.
I listened to Donald Fagens " Morf the Cat" on vinyl. The piece was " The night belongs to Mona". That is a very bass heavy piece. At idle i measured 0,67V over the 220 Ohm base resistor in the cap multiplier. When i listen loud i get 5V peak on my 8 Ohm 98dB speakers. That is 2 x 3.125W and they draw less then 0,7A. On heavy peaks the voltage went up to 0,69V so it is nearly constant current draw in my situation. I have to say that i filter my main speakers under 80Hz with 6dB octave so the amp has not much to do in the deep bass. My bass is active.
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Old 18th May 2012, 07:07 PM   #660
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by mikelm View Post
I would very very interested if anyone could demonstrate an amp design that when biased to 100 - 150mA had lower H10 - H20 than the same design biased into class A.
You know that is not possible
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