The EPC780 is an example of a really great MOSFET amp (that you never, ever want to have to fix). One of them doesn't taste very good and the other I'm mildly allergic to This subject has been discussed to death, and the short answer is "it depends".Ī BSS EPC780 and a lot of Pass Labs amps both use vertical MOSFETs in their output stages, but a BSS EPC780 and a Pass Labs amp are about as similar as an apple and a cucumber. He has made his name now worldwide and you must give him credit for that but that doesn't or didn't change my point of view of preferring JLH. I got the feeling D.self enjoyed this ( correct me if I am wrong ) and there is no denying he is a very intelligent man and designer so give him his due but diplomacy was/is entirely missing from his personality. This caused a big split in the UK with neither side willing to give ground ,I doubt even several decades later anything has changed but as I said before magazine sales of EW went through the roof and the letter pages were crammed with letters. It certainly did on EW -lasted for years -and years with on one side JLH & several other designers including a well know Italian designer and the "opposition " which had at its head -who else but D.Self who if he is reading this must agree he was -well ferocious in his discrediting and technical permanent attack on all things Fet in relation to audio power amps. I also use Fets & Mosfets and follow JLH,s design philosophy but you will find this subject generates a great deal of energy expended arguing the case between the two types of active devices. These have a very low turn on voltage beginning at just 150mv or so. Lateral FET's are another one to look at and those are my favourite devices. I like using fets and the fact they draw little gate current makes driver stage design easier imo. The voltage needed to turn the FET on is quite high compared to a bjt (around 4v vs around 0.6) The impedance of the gate is very very high indeed. In fact at DC the gate can literally acquire enough charge to turn the mosfet on simply by it 'floating' and not being connected to anything at all. At high frequencies a bit of current is needed to charge and discharge the gate capacitance. The mosfet is voltage driven and the gate draws virtually no current at all at DC and low frequencies. That can be pretty high in high current output stages, in other words the driver stage has to supply lots of current. A major difference is that the bjt is essentially current driven meaning you inject a current into the base/emitter junction to turn it on. Bipolar (bjt) transistors and MOSFET's are very different in operation.
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