Re: Amplifone Design... Wonders me

On Tue, 9 Mar 2004 23:31:10 +0100, "mypearl" <mypearl@dds.nl> wrote:

>My concern now is why the amplifone deflection board is so sensitive to power supply ripple...
>
>I have never seen any home stereo amp with regulation. They have very good ripple rejection though... I don't suppose those are voltage feedback amplifiers makes a difference for PSRRR ?

I *believe* the voltage feedback amplifier, compared to current feedback does
make a difference. But I'm a software guy, so anybody more knowledgeable than
me, *please* jump in!

Here's my feeble analysis...(including the part I'm not so sure of).

Both amplifiers are running in voltage feedback mode. The difference is that
the audio amplifier looks at the voltage directly across the speaker, it holds
that voltage regardless of what the power supply is doing, therefore the power
supply does not need much regulation, just filtering.

The deflection amplifier needs to current control the yoke, it does this by
placing a resistor in series with the yoke. Since controlling the voltage on
the resistor controls the current through it, it also controls the current
through the yoke, like this:

(Fixed Font warning, ASCII art ahead)

          V -----+
                 |
                 )
                 ( Yoke
                 )
                 |
          FB ----+
                 |
                 /
                 \ Resistor
                 /
                 |
                ---
                 -

The power supply voltage is V, the feedback point is FB. The deflection
amplifier runs as a voltage feedback amplifier holding the voltage at FB to a
specific value, and therefore regulating the current through the resistor and
yoke.

Now here's where my lack of analog skills kick in and is something that has
always bothered me in my attempts to understand this circuit.

Let's say V is 25v, and FB is being held to 1v. Now let's say the unregulated V
voltage jumps to 30v.

The amplifier will hold FB at 1v, which will keep the current flowing through
the resistor, and yoke, constant. But here's my dilemma:

Since the Yoke and Resistor act like a voltage divider, and the voltage at FB is
held constant, then when V is at 25v, there is 1v across FB and 24v across the
yoke. Now when V jumps up to 30v, FB is still held to 1v, but now there is 29v
across the yoke.

My gut feeling is that increasing the voltage across the yoke from 24v to 29v
*will* have an affect on the display, even if the current is held constant.
Which leads me to think deflection amplifiers need to be run from regulated
power supplies, much like the design that WG originally came up with.

Is my thinking correct on this? Hopefully someone more versed in analog
electronics than me, can answer this question, it's bothered me since I first
started working on X/Y monitors.

-Zonn
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Received on Tue Mar 9 19:26:26 2004