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Class A vs Class AB

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2009 at 7:23pm
Ever seen a 100W light bulb?

Yes, I know you have, but do you realise it disipates 100 watts without a heatsink?

So do big valves.

Because Eddison was trying to develop the light bulb when he accidentally discovered the valve!

Therefore with big valves you can easily turn the wick up (the bias current that makes them conduct harder and become "class A" biased)

Not so easy with a sliver of silicon stuck to a little metal tab (a transistor: Bipolar or FET)

It needs a giant lump of finned aluminium to keep it "cool"

Therefore, if you're going to that much trouble, you're not going to waste it by having a switch that puts it into A/B, where a much smaller heatsink would have done.
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rhmbus Quote  Post ReplyReply Direct Link To This Post Posted: 04 Nov 2009 at 1:53pm
Originally posted by Analog Kid Analog Kid wrote:

This question still remains, so I will try again: is it not possible to design a HiFi power amplifier, either valve or solid-state, that can be switched between Class A and Class AB at the flick of a switch? Never seen this in a HiFi amplifier before, but in guitar amplifiers it is a quite common feature.
Marantz did one a couple of years ago ,
ray
many an old tune
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Analog Kid Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 2009 at 8:05pm
Originally posted by Graham Slee Graham Slee wrote:

Forcing Hi-Fi amps to get hot by moving the bias into class A quarters can help get rid of the "transistor sound"...


That made me think: if you can get rid of the "transistor sound" from a solid-state power amplifier by making it operate in class A mode, then class A valve power amplifiers must sound extra valvey. Tongue



Edited by Analog Kid - 14 Nov 2009 at 12:54am
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