I apologize if this has been answered here before, but I can't find it.
Learning about Class A amps and thinking about building one from a kit to power some vintage Wharfdale speakers I recently acquired.
I understand that class a amps output approximately 50% of their output power with no input signal, but does this "bias" current actually go to the speakers? And is it 50% of the RMS or Peak value of the amp? I ask because my Wharfdales have an RSM rating of only 10 watts (20 watts peak) and I don't want to burn out the voice coils with my class a amp.
Learning about Class A amps and thinking about building one from a kit to power some vintage Wharfdale speakers I recently acquired.
I understand that class a amps output approximately 50% of their output power with no input signal, but does this "bias" current actually go to the speakers? And is it 50% of the RMS or Peak value of the amp? I ask because my Wharfdales have an RSM rating of only 10 watts (20 watts peak) and I don't want to burn out the voice coils with my class a amp.
If there is no input signal, all the power goes to heat up the heat sinks. That heating power is not related to the speaker at all, it only depends on the supply voltage and bias current.
The heating power (dissipation) in watts is the total supply voltage times the bias current. Say you have supply of + and - 15V, total 30V, and bias of 0.8A, you are wasting 24W as heat. Nothing goes to the speaker so it all goes into the heavy metal ��
As soon as you get signal, part of the current is diverted to the speaker so less heat wasted.
Jan
PS Soon there will be a lot of people here showing off their understanding with lots of math and complex stories and numbers, but the above is the basic thing. All follows from that ;-)
The heating power (dissipation) in watts is the total supply voltage times the bias current. Say you have supply of + and - 15V, total 30V, and bias of 0.8A, you are wasting 24W as heat. Nothing goes to the speaker so it all goes into the heavy metal ��
As soon as you get signal, part of the current is diverted to the speaker so less heat wasted.
Jan
PS Soon there will be a lot of people here showing off their understanding with lots of math and complex stories and numbers, but the above is the basic thing. All follows from that ;-)
Last edited:
You mean less power wasted as heat. Also less heat in the amp overall as some is now diverted to the vc.
A class A amplifier can be built as a single ended one, which has only 25% efficiency or as a push-pull which increases the efficiency to 50%.
If the amplifier gets to it's power limit, it produces clipping which results in square wave, a lot of harmonic distortion. That means, the power is shifted towards higher frequencies which can easily fry your tweeter (if it got any). So it's a very good idea to get a more powerful amplifier than your speakers are rated.
Single-ended Class A, and push-pull Class A, both have a maximum theoretical efficiency of 50%. In real life you will get a bit less efficiency than this. Minimum efficiency is 0%.ICG said:
Single ended can't have 50% unless you are "playing DC music".
But as jan.didden sad there is not much more to say.
But as jan.didden sad there is not much more to say.
SE is max 50% if you use a transformer. If you don't like transformers then use push-pull.
For Class A you can build amps which go anywhere between 0% and 50%, so 25% would be a self-imposed limit due to poor choices.
For Class A you can build amps which go anywhere between 0% and 50%, so 25% would be a self-imposed limit due to poor choices.
- Status
- Not open for further replies.