"DaveC" wrote in message
al.net...
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,
--
DaveC

At a quick guess I'd go for input short-circuit/open-circuit adjustment,
output overload protection, and output stage high-temp protection.

Ken

On Tue, 2 Dec 2003 10:22:45 -0800, DaveC 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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On Tue, 2 Dec 2003 10:22:45 -0800, DaveC put finger
to keyboard and composed:

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".

"Offset short" appears to be a nulling adjustment for a zero input
signal. That is, the output of the differential pair should have no DC
offset when the input is shorted. "Open offset" would appear to be a
similar adjustment for when the inputs are disconnected.

The servo TL072 appears to be sensing the presence of DC at the
speaker terminals. The time constant appears to be set by C28 (0.1uF)
and R75 (1M), which is of the order of 0.1sec. Perhaps the circuit is
some kind of automatic bias adjustment.

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?

The TL072 is operating as a comparator with hysteresis. It shorts the
input signal when the temperature exceeds a certain threshold.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

"default" wrote in message
...
On Tue, 2 Dec 2003 10:22:45 -0800, DaveC 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