John said:
Yes. "not too bad" in the strict sense of the phrase. That is, it is a bad
design, but not so bad as to be totally unusable. However, this does depend
on the definition of "good" somewhat. Assuming we give meaning to the term
"average", then this design can not in any reasonable way be classed as a
"good" or even an average design". It is a quick knock up, that serious
audio designers don't even give a second look to, well except when
commenting on how bad it is.
except for the biasing. Iq could have been set by a
pot+resistor from IC1 pin 4 to 7, and the DC offset trim should be
into the opamp, not fighting it.
A "good" audio amp, imo, will have at most, only one trim pot, and this is
to set the output bias.
The biasing/crossover of this type of circuit is potentially perfect,
in that both stages run at some idle current and signal makes one pick
up,
For very high speed, audiophile performance, this output device
configuration is very poor. The gates of the transistors are connected via
two much speed lag. This usually results in very, large shot through
currents when hit by 100n pulses.
but leaves the opposite side idling. As opposed to a lot of
circuits where conduction on one side actively shuts off the other.
DC bias on the output fets depends on their thresholds, not so good;
closing local loops on the fets (with more opamps) would be better.
Oh...a sure-fire recipe for disaster, if done correctly...It will generate
additional de-stabilising poles. That particular technique can be useful for
reducing LF. distortion, but it does it at the expense of BW, i.e. higher HF
distortion. I would be surprised if this "design" could stand having its HF
IMD compromised any further.
Driving the load from the drains means the amp has a very high
open-loop output impedance,
Well, not so high in this particular case. Assuming a nominal early voltage
of 30V, at 1A would be 30 ohms. Its highly variable though.
which makes it harder to stabilize.
Can be. This is actually a bit subtle. Depending on just what the actuall
compansation is. A correctly designed amp will have a stabilastion load
network, in which case, the stability may well be dominated by a higher
frequency unity gain point In which case, it turns out that the high
frequency UGF of a source follower and a drain ouput is the same.
Flipping things over, driving from sources, makes the crossover
biasing more interesting and costs swing.
At least it's not the same old 40-year vintage class AB thing.
Maybe it should be. It would then, presumably have the output inductor and
|| resistor with an output RC zobel network, that is pretty much mandatory
for stability under all load conditions. I would also suggest looking at a
direct zener clamp on the mosfets.
Unfortunately, I don't have the time to go over the other considerable
shortcomings of this er.. "design"
Kevin Aylward
www.kevinaylward.co.uk
www.blonddee.co.uk
www.anasoft.co.uk -SuperSpice
