Question on PS for class AB vs class A
I have a question about the powersupply of a class AB amp. I was reading an article form Peter van Willenswaard (a very respectable audio engineer in Holland and perhaps also elsewhere) about class D amps. He discussed class A, B and AB in the intro of the article. He stated that a class AB amp (with little bias, 20-50 mA) puts higher demands on the stability of the power supply than a class A amp. Did not quite understand that part... I always thought that a class A amp (with its constant high ciurrents) would be more demanding?
I guess it depends on what you mean by stability. A classic
push pull Class A design continuously draws about twice the
rated output of the amp on average, but the instantaneous
current draw does vary with delivery to the speaker.
Certainly this means that the rails of the power supply will be
more constant with the Class A amp, but the supply will need
to be more robust to do this job.
To get the things clear I calculated an example. I am not an expert so I just can hope I´ve got the Things right.:)
Every "real World" PSU has an Impedance, in my exampe it has 1 ohm . The ideal Voltage is 50 volt.The class a amp has 500 Watt dissapation and a signal of 50 Watt.
So we get
500Watt at 50Volt gives 10 ampere
50Volt/10 ampere = 5 ohm
1 ohm for the Psu 5 ohm for the amp means
8,3 Volt lost in PSU 41,7 Volt for the amp
Whith Signal of 50 Watt we get 550 Watt for the amp whith means
550 Watt at 50 Volt gives 11 ampere
50 Volt/11 ampere = 4,5 ohm
1 ohm for the PSU 4,5 ohm for the amp means
9 Volt lost in PSU 41 Volt for the amp
That means a Voltage drop of 0,7 ohm when the Signal comes.
In the class AB amp we calculate 50 Watt dissapation and 500 Watt Signal
50 Watt at 50 Volt gives 1 ampere
50 Volt/1 ampere = 50 ohm
1 ohm for the PSU 50 ohm for the amp means
1 Volt lost in PSU 49 Volt for the amp
Whith Signal of 500 Watt we get 550 Watt for the amp like it is calculated above. That means we get a Voltage Drop of 7,3 Volt whith the Signal v.s 0,7 Volt in the class A amp and I think that is the hurting Point.
Assuming you have to care much more to get a low impedance in class AB amp PSUs.
:drink: :drink: :drink:
I ignored that the currents and oter things are changing whith the voltage drop, but I think it is easyer to understan ( and much easiear to calculate) so, and the general meaning is the same.
For the record, Class A amps need a BIG power supply, BUT is it fairly continuous with current at different audio levels, so absolute regulation is not so important. Class A-B, B amps can, in theory, use a smaller power supply, BUT the supply will have very different current demands with different audio levels. IF the bias regulation in the amp is not buffered well enough from the supply, then problems with the power supply can actually change the bias setting.
Thanx for the answers (have to do some pondering on some of it, learning all the time).
Btw in most (even highly regarded) 80-100 watt AB amps you see one 200-300 VA trafo and maybe betweem 20000-40000 uF capacity.
I just had an Idea for Power regulation for a Pre amp, widh I would like to discuss.
Imagine a given pre amp circut and paralell for example a Resistor whith the same resistance to it.(of cours in line whith the Power Rating of the PSU which might has to be increased). This would lower the voltage drop of the supply in the different Situations no Signal vs. Signal. So the current should be more constant.
What do you think of that????
The voltage change will be half of the situation before.
You'll double the power use also.
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