23 am, Jim Thompson
00:49 -0700, NT wrote:
Hi,
I'm thinking about building my own audio power amplifier and
the 'novel' idea I came up with was to use one GIANT
transistor biased in Class-A operation to do the current
amplification (the voltage amplification will be done with
OPA134 OpAmps).
I'm thinking of using a BLF578XR MOSFET for the current
amplification but I'm not sure if it's suitable for LF (audio)
signals as it's designed for HF applications. The device is
rated at 1200W (!) so it should be more than capable of
handling the power requirements.
Do you guys think the BLF578XR can be used for this purpose?
Going back to Class A transformer output stages and assuming
you're talking about a transformerless amp: Do speakers commonly
handle much standing DC current?
Even if you shunt an 8 ohm speaker with a 1 ohm resistor as your
"pulldown", you still have 1/8 of the DC current flowing through
the speaker.
A choke would make more sense
For something that has a fixed DC _voltage_???
Run the drain (or collector) current thru a choke, then cap couple
to the speaker. Did that when I was 18, with one of those Delco
door-knob Germanium devices, for my car radio. I was fortunate
enough that my father-in-law-to-be ran a machine shop, and milled me
a fabulous heatsink ;-)
...Jim
Thompson
[On the Road, in New York]
Yep, thats the way.
I got the impression the OP wanted to keep it ultrasimple, in which
case one could just omit the output cap. Not ideal, but quite
workable.
The OP also wanted to run the thing as an emitter follower, which kind
of nixes the choke idea (although a choke would be the "simple" thing
to do if it were common emitter).
You could make a "current sink" with another transistor, biased like a
common collector but with no input. Then you'd need two heat sinks.
I didnt see the OP mention emitter follower anywhere. It would seem a
doubly poor choice of topology. Not only does one have the downsidse of
class A, but also you're stuck with either dc through the load, or even
worse efficiency. To the OP I'd suggest starting with an opamp driven
class B output, use a quick opamp to iron out crossover distortion.
Well, he talks about using an op-amp for voltage amplification, with a
giant FET for current amplification. That says source-follower to me,
and when I'm not thinking the phrase that dribbles off my finger-tips is
"emitter follower".
He specifically wants to use class A, probably because he's heard that
it's low distortion. It's certainly _an_ approach to use, and if you
have an educated ear it's probably a worthwhile thing to try at least
once, just to get an idea of how well/not-well it works.
