Sorry Jon,
I'm stuck on google groups for a little while - I can't believe people
actually use it full time or that google could make an interface this
bad. (I suspect it is very fine for simple threads) anyway...
Cripes. Google didn't even show the thread when I'd looked,
a day ago or so. And it had been around for at least 24
hours by then. Used to be the case that google groups would
show the posts within an hour or so. Doesn't seem to be
true, anymore. If not, there is no possibility of having a
discussion very quickly via google. It would greatly
lengthen out the interactions. Maybe that's on purpose, now,
to cause people to find some other solution?
Don't be. I was just explaining myself, not complaining
about your usage.
A little experience will get you into the right ballpark when
estimating what you could expect for distortion. It is basically the
same "rules" as you would see with op-amps - the more linear it is to
start with the better. Higher bandwidth stages generally mean you can
use more negative feedback to eliminate distortion - but the lower the
final gain the more instability is likely to become a problem. And bad
circuit layout can increase distortion (and even more so hum and
noise) easily by a factor of 10.
As for how low you need distortion to be one rule of thumb (I forget
the reference) is to be clearly audible the message must be 20db above
the background noise and to be inaudible distortion has to be 20db
below the background noise - which pretty much sets "low" distortion
for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has
a high dynamic range and you (allegedly) want a distortion figure at
least 20db below that. So a 60 db signal range 0.0001% (or 100PPM).
The you start getting into all kinds of trouble with power output /
dynamic range of the amp etc and you relies that it is all a
compromise anyway. You do the best you can within the restrictions of
the job description.
Understood.
Yes and No. All the published circuits are made by people who want to
sell transistors,
A concern I care not the least about. My _real_ preference,
were I to impose it on the design, would be to use ONLY
PN2222A BJTs for all the active devices. One part. That's
it. Why? Because I've got thousands of them.
Literally. Something like 22,000 of the bastards. I give
them away like popcorn to students at schools. Got them
_very cheaply_. So if I were pushing something, I'd be
pushing a 10W PN2222A design, use signal splitting approach
probably (because it's the only way I think think of, right
now), and distribute the dissipation across lots and lots of
the things.
What to go there?
not audio systems, power supplies or transformers.
Got it.
As a result the power supply is often assumed to be regulated, which
is not true in this case, or the power supply is treated in a very
perfunctory manner that is not at all compatible with good design.
In this case you have the voltage you need for the 10 watts, plus
voltage drop for the driver circuitry and output stage , plus ripple
voltage, plus whatever is required for transformer regulation and
mains regulation. When you add it all up you might find that a chosen
transistor/component is actually not at all suitable for the job. Back
to the drawing board. Change this change that recheck everything again
etc.
In this case, though, there is nothing particularly
remarkable about the rails. Taken across the entire span,
even, doesn't exceed the maximum Vce of a great many BJTs. So
no real worry there. But I see some of where problems may
arise. Luckily, at this level I can side-step worrying about
that part and get back to learning about amplifier design,
yes?
If you do the power supply first you have the figures needed for your
worst case already. It saves time and makes a better result (no
tendency to comprimise to save all the calculations already done).
Well, does this mean we should hack out the power supply
first? I'm perfectly fine with that and can get back to you
with a suggested circuit and parts list if you want to start
there. We could settle that part before going anywhere else
and I'd be happy with that approach, too, because to be
honest I don't imagine it to put a horrible delay into
getting back to amplifier design. So I'm good either way.
I wasn't going to prompt, but it is close to the sort of thing, I
think, you should be aiming for . As someone has already noted (I
would attribute you if I wasn't on GG, I'm sorry) it has been drawn up
for a single supply, rather than a more common (for this size /
configuration) split supply.
I had assumed we'd be using a split supply.
I had assumed a speaker would be hooked up via a cap to the
output, so DC currents into a speaker coil would be removed
from any concern. But I was also holding in the back of my
mind the idea of tweaking out DC bias via the speaker and
removing the coupling cap as an experiment to try. And if
so, I'd pretty much want the ground as a "third rail."
(Playing just a bit upon the Chicago parlance about the once
dangerous rail in their transit system.)
(I don't really care too much about arguing about
Correct. Theory says it does nothing. I practice the theory but have
the occasional heretical belief about that.
Actually, I think I've read that theory says it is _better_
to be removed. The reason seemed pretty basic, as it's
easier to get close to a balanced current split; and this, I
gather, lowers 2nd harmonic distortions produced in the pair
-- notable more on the high frequency end I suppose because
gain used for linearizing feedback up there is diminishing
and can't compensate it.
In other words, it's not neutral. It's considered to be
better if I gathered the details. Then even better, the
current mirror enforces the whole deal and you've got about
the best to be had.
Of course, mostly just being a reader means I have no idea
which end is up. So I might have all this wrong.
and better still both R3 and R4 should be
This would provide more differential gain.
_and_ improve distortion because the currents are forced to
be balanced in the pair, yes?
In the right configuration it would reduce the common mode signal gain
of things like mains hum and supply ripple (you mentioned power supply
isolation before).
Yes, that's how I thought about it.
Also, from another (what do you call it branch? thread?) you were
discussing boot-strapping R6. This is not done so much as amplifiers
get bigger but a BJT configured in the same way as the replacement for
R5 is very common. I'm leaving the details to you - perhaps there is a
way to reduce component count without affecting performance. (I am
hoping this is what you wanted "nutting it out for yourself")
Yes! I don't want things handed on a platter. But I also
don't want to have to rediscover all of the ideas by making
all of the mistakes, either. This is the kind of "pointer"
towards something that I like a lot. It gives me a place to
think about something, but leaves me some reason to have to
do so and that helps me own it better.
One general truth about learning is that you don't present
someone with a problem so out of their depth that they have
no chance at it. Doing that means they fail, they feel like
a failure, and it causes a student to just want to go away.
They lose motivation, usually, in cases like that. On the
other hand, providing no difficulty at all merely means
repetition of what they already know and they grow bored from
that, too. Finding the sweet spot where a student is faced
with interesting problems that are not already known, but
perhaps within reach of grasping at with some effort, is the
key. Then it can be fun, educational, and motivate.
That's what you just did for me.
I assume the input impedance of that example
Okay.
Yes, but you should probably think of a whole passive network to
filter out low and high frequency - (think what happens if you amp is
operated near a source of RF)
Well, every trace picks up like little antennae. All kinds
of trace voltages appearing here and there. Not good.
So. Can you make an audio amplifier that can withstand a
microwave oven environment and deliver good performance while
irradiated with 1kW banging around in there?
Depending on transistors layout etc it might not be needed, but more
often it is the size that is the question.
I was thinking it helped locally linearize the VAS section
and that such would be "good" most anywhere. But I am just
taking things without having worked through them on my own.
So...
Okay. Thanks.
Why would/should you use them?
I'm still thinking about that. In general, I was thinking
about them because of the "little re" that is kT/q based in
each BJT, and varies on Ie. Since Ie is varying around, I
was thinking about something fixed there to overwhelm it and
"make it knowable" for the design, I suppose. Maybe that's
all wet, given your query. I'll toss the idea off the side,
for now.
Thanks for that. I'm just glad to be able to talk to someone
about any of this, at all. So please accept my thanks for
the moments you are offering.
Is there a way you could post a schematic of where your thinking is
and what you would like to discuss - there is no need for a complete
circuit.
Yes. I can use ASCII here, for example. But before I go off
into the wild blue with this, do you want to focus on the
power supply first? Or just jump in on the amplifier?
Jon