I think it was 'Myth Busters', the myth was not 'Busted'
True-class-A uses constant current (more or less). Some not-so-true-class-A modes vary the supply current, the judges are still out (if this is class-A or not).
ClassA is not constant supply current.
ClassA is a mode of operation that does not invoke crossover distortion.
Constant supply current is a continuous non varying current flowing from PSU to client circuit.
There are some ClassA topologies that draw a virtually constant supply current. These topologies are far outnumbered by the non constant supply current topologies that are available.
Designers and builders very rarely adopt the few constant supply current ClassA topologies.
Far and away the majority of ClassA circuits, irrespective of whether they are Single Ended, or Push Pull, or dual polarity, or single polarity, that we see are not constant supply current.
The most recent posting I am aware of that discussed this non constant supply current was made by me just yesterday. But there are many Threads discussing this myth and and showing the error in the understanding.
http://www.diyaudio.com/forums/pass-labs/212471-my-f5t-v3-build.html#post3026662
ClassA is a mode of operation that does not invoke crossover distortion.
Constant supply current is a continuous non varying current flowing from PSU to client circuit.
There are some ClassA topologies that draw a virtually constant supply current. These topologies are far outnumbered by the non constant supply current topologies that are available.
Designers and builders very rarely adopt the few constant supply current ClassA topologies.
Far and away the majority of ClassA circuits, irrespective of whether they are Single Ended, or Push Pull, or dual polarity, or single polarity, that we see are not constant supply current.
The most recent posting I am aware of that discussed this non constant supply current was made by me just yesterday. But there are many Threads discussing this myth and and showing the error in the understanding.
http://www.diyaudio.com/forums/pass-labs/212471-my-f5t-v3-build.html#post3026662
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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...??
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...??
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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.
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.
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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.
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.