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Old 28th January 2012, 08:32 PM   #1
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Default Power amp vs preamp topology

Greetings,

I have the following problem that I cannot fully understand.
In literature it is said that in a common preamp stage the topology for taking the output is as in attached figure top02.
However, when it comes to Power amplification they always put the output transformer in the place of Ra (plate resistor) like in attached figure top01. Is it possible to create a topology like the attached figure top02 (which is similar to a preamp stage).?

What are the differences between these two topologies??
They say that power amps must amplify also current (not only Voltage amplification). If do we amplify the voltage, the current is amplified also (according to the resistance), isn't it?

So, is it possible to have power amplification with topology in figure top01?

Thanks in advance
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File Type: png top02.PNG (21.1 KB, 104 views)
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Old 28th January 2012, 09:11 PM   #2
DF96 is offline DF96  England
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The resistor drops a lot of DC voltage, so requires a high supply voltage. It also absorbs AC power. You can afford this in a small signal stage, but not in a power stage. Despite this, it is sometimes used in low power outputs.
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Old 28th January 2012, 09:59 PM   #3
ryuji is offline ryuji  United States
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Quote:
Originally Posted by DF96 View Post
The resistor drops a lot of DC voltage
I dont know the state of mind in other designers, but in my signal gain stages i was counting on this, i assume im likely not alone. (brought 440v b+ down to 250 quiescent, from too high to proper operating point.
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Old 28th January 2012, 11:01 PM   #4
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Topology one will work fine as a preamp or amp. Topology two is probably intended to be parallel feed so you need a cap between the tube and transformer or the transformer and ground. It will then work as a preamp or amp.

The advantage of parafeed is that it is easier to make a high inductance transformer if the transformer doesn't need to have an airgap. Without the airgap the quiescent current would saturate the transformer.
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