I have always worked and listened to class “AB” amps where the class A was until about 1W rms and then after the amp switched to class B for higher level. Class A heats too much to get a reasonable power for me. Just by curiosity I checked the other day with a voltmeter while listening to music what was the power that I needed 99% of the time. I have realized that I do not play my amp louder than about 10Wrms (and this was very loud) in my living room.
Thus, I intent to build a class A / B amp :
Class A until about 15 Wrms / 8 ohms
Class B up to about 50W rms / 8 ohms
The class B would allow me to use the amp for party or else but most of the time I would enjoy class A. Is class A so good compare to good AB?
I thought it would be fun to know when the amp begins to go in class B while I listen to music like with a LED indication.
So I thought that one way to detect the level of music that makes the amp to go into class B would be to measure the AC current in one of the output transistor and compare it with the DC bias current in the same transistor. If the AC current would exceed the DC bias current then a LED would indicate class B operation. Using the usual output transistor source or emitter resistor to measure DC and AC voltages across it could be easy. I have built a prototype and it seems to work (although my actual amp switches to class B at about 1 W. Of course this circuit is only useful with a reasonable class A power. The advantage of that technique - compared to just measuring the AC output voltage and then calculating the power – is that it considers the load impedance since it can vary and also if someone change the DC bias current then no adjustment is required for the Class B detection circuit.
Any comments appreciated?
Thus, I intent to build a class A / B amp :
Class A until about 15 Wrms / 8 ohms
Class B up to about 50W rms / 8 ohms
The class B would allow me to use the amp for party or else but most of the time I would enjoy class A. Is class A so good compare to good AB?
I thought it would be fun to know when the amp begins to go in class B while I listen to music like with a LED indication.
So I thought that one way to detect the level of music that makes the amp to go into class B would be to measure the AC current in one of the output transistor and compare it with the DC bias current in the same transistor. If the AC current would exceed the DC bias current then a LED would indicate class B operation. Using the usual output transistor source or emitter resistor to measure DC and AC voltages across it could be easy. I have built a prototype and it seems to work (although my actual amp switches to class B at about 1 W. Of course this circuit is only useful with a reasonable class A power. The advantage of that technique - compared to just measuring the AC output voltage and then calculating the power – is that it considers the load impedance since it can vary and also if someone change the DC bias current then no adjustment is required for the Class B detection circuit.
Any comments appreciated?