Re: Audio amp circuit help?

"default" <R75/[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> On Tue, 2 Dec 2003 10:22:45 -0800, DaveC <[EMAIL PROTECTED]> wrote:
>
> >Working on a VSP Labs MOSFET power amp:
> >
> >http://home.covad.net/~peninsula/Amp/VSPschem1.pdf
> >
> >Questions:
> >1. There are several adjustment pots on this PCB: "Open offset",
"Short
> >offset", and a circuit controlled by one of the TL072 op amps (in
the lower
> >center of the drawing) titled "Servo".
> >
> >What are these adjustments? If I was to guess, I'd say that the two
former
> >adjustments are for protection of the output stage from short
circuit. Is the
> >latter circuit for adjustment of the idle current in the output
stage?
> >
> >2. What does the circuit that includes a TL072 (in the lower left
corner of
> >the drawing) do? It includes a thermistor, which seems to indicate
some kind
> >of thermal protection. Does this reduce the input signal as
temperature
> >increases?
> >
> >Thanks,
> Looks like open and short offset are just offset adjustments for the
> input and output stages  - balance each separately?  Offset implies
> the zero adjustment (DC level at the speaker terminals, when there
is
> no input)
>
> There is an adjustment for BIAS that would be the idle current.
>
> Servo is confusing.  The "servo" amps I'm familiar with usually
> include the speakers in the feedback loop.  Sense the speaker sound
> with a transducer and apply that as feedback for the amp.  An
attempt
> to include speaker nonlinearity as feedback and correct for it.
>
> From looking at the servo circuitry . . . that looks like an
automatic
> gain control or limiter of some sort.
>
> Looks like there is a thermostat to shut down the output stage with
an
> over temp condition, and some LED's and thermistor to watch the
> temperature.
>
> Where'd you find the circuit, and what info have you got?
>
>
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The "Servo" loop is a slow integrator to keep the output average
voltage
near 0V. I've seen that terminology used before to indicate it is a
slow
out-of-bandwith-of-interest adjustment.

The thermistor and comparator (note the positive feedback round the
TL072) will shunt the input signal to ground by the zero Vgs
resistance
of Q20 (50 ohms max) when something gets too hot (probably the
output heatsink).

I'm with Ken on the "Open" and "Short" adjustments. There might be a
link
with the shutdown, in that if the previous stage has a significant
offset
and output resistance, or leakage current through a coupling
electrolytic,
you would have to tweak offsets for both cases of Q20 resistance.

For example, if there were only 1uA leakage not catered for, going
into
or out of shutdown would produce a 3.3V step on the output.

Regards
    Ian