Scott Dorsey wrote:
Kevin Aylward wrote:
Scott Dorsey wrote:
input transistors. That's, dare I say, just plain daft, and don't
work.

Its like nails scratching nails down a blackboard to us analogue
designers. There are so many reasons not to try this. Indeed, in my
current designs, there arnt any, its all current sources.

(Note that by input I was referring
to the negative-going input on the differential pair that is used
for feedback, but of course if you offset one side of the
differential pair, you offset both of them)

You also offset the output, which don't do very nice things to the
speaker.

No. There is a capacitor in the way. That's the point. You want
that offset on the capacitor.

One doesn't use output capacitors for power amps nowadays. Amps are DC
coupled. Don't know the last time I saw a single supply amp. Probable in
Dr. Who's TARDIS.



To further clarify, if you offset the + input, the negative input
will follow. If you offset the - input, you have to force the +
input to follow. By itself, it will stay just where you set it.

To do what you suggest requires having the + input as usual, that is
connected to Ov in order to ensure that the output is also zero.

No, the + input is also floating above ground.

Not if one is using anyone of the 100's millions of DC coupled power
amps one doesn't.

There is a DC blocking
cap between the input jack and the + input. If you raise the voltage
of one input, you raise the voltage of the other.

The
bypass capacitor is then offset with a voltage and fed via a
resister. The junction of the resister and cap would then be
bypassed solidly to the 0v reference point to minimise distortion
and hum loops. Since we now have two caps, it would be simpler to
just wire them in series back to back without the bias circuit at
all.

There is _already_ a bias circuit.

Not when we are first designing the circuit there isn't. When we do, we
need to make sure that bias voltages are very clean. This requires
capacitors.

We aren't _adding_ anything here.

See above.


hint: http://www.anasoft.co.uk/EE/index.html

Sorry, I tried to look at the "very low distortion amp" circuits,
but the resolution was very poor and they were pretty much
unreadable,

Strange, it aint that good, but its certainly readable on my system.

My problem is that I couldn't read the numbers on the parts, either on
the screen or printing it out. So I can't say things like "what is
the base on Q1" set to, because I can't tell if that is Q1 or Q7.

Oh...point taken.

However, the design is easily understood from inspection by anyone that
understands circuit design. There is no rocket science here. All the
blocks are standard stuff. The only bit that might need explaining is
the somewhat novel second feedback loop on the output transistors on one
of the circuits. Oh..and that some of the diodes are zenors for
simulation purposes.


Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
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