Hello All,
I've seen amplifiers from various manufacturers that will run Class A or Class AB with an integrated switch on the face plate or back of the unit. The unit will operate in Class A at 25w/ch and then 80w/ch in AB mode, for example. I've searched the internet to understand how this is achived but only get sites that explain the different types of amplification. Is there a basic answer that one of the many gurus on this site can provide? I realize it's probably not a short answer but I'm very curious now that I'm gathering parts to build a Pass Aleph J.
Thanks In Advance!
I've seen amplifiers from various manufacturers that will run Class A or Class AB with an integrated switch on the face plate or back of the unit. The unit will operate in Class A at 25w/ch and then 80w/ch in AB mode, for example. I've searched the internet to understand how this is achived but only get sites that explain the different types of amplification. Is there a basic answer that one of the many gurus on this site can provide? I realize it's probably not a short answer but I'm very curious now that I'm gathering parts to build a Pass Aleph J.
Thanks In Advance!
There is no fundamental difference between A and AB operation. In many common "AB" topologies, going class A is just a matter of increasing output stage bias until nothing turns off at maximum desired power into desired load impedance, so the first part you need is some sort of switching in the bias circuit. Tends to involve an optocoupler.
At this point you are well-advised to also drop supply voltages considerably (e.g. by switching secondaries), otherwise power dissipation will go through the roof, power transistors quickly leave SOA and amplifier meltdown ensues. A bit more heatsinking and the odd pair of output transistors more than usual will definitely come in handy, which should help explain why you don't see this kind of feature on bargain basement amplifiers.
Note that while "AB" topologies can also run in Class A, it does not necessarily apply the other way round. Any kind of single-ended output, for example, is exclusively Class A as far as audio goes. (Things may differ for RF power applications.)
At this point you are well-advised to also drop supply voltages considerably (e.g. by switching secondaries), otherwise power dissipation will go through the roof, power transistors quickly leave SOA and amplifier meltdown ensues. A bit more heatsinking and the odd pair of output transistors more than usual will definitely come in handy, which should help explain why you don't see this kind of feature on bargain basement amplifiers.
Note that while "AB" topologies can also run in Class A, it does not necessarily apply the other way round. Any kind of single-ended output, for example, is exclusively Class A as far as audio goes. (Things may differ for RF power applications.)
Hi JSA1971,
The switch you're talking about, is switching the bias of the output stage. Class A mode requires higher bias, than class AB one. Some class A amplifiers are designed in a way that they run at high bias all the time, staying in class A up to, say, 25W output power, and then, having enough headroom in terms of the output swing, they smoothly operate in class AB at higher power levels - no switch required.
Cheers,
Valery
The switch you're talking about, is switching the bias of the output stage. Class A mode requires higher bias, than class AB one. Some class A amplifiers are designed in a way that they run at high bias all the time, staying in class A up to, say, 25W output power, and then, having enough headroom in terms of the output swing, they smoothly operate in class AB at higher power levels - no switch required.
Cheers,
Valery
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