Re: Amplifone Design... Wonders me

Zonn wrote:

>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.
>
>

No, the yoke voltage doesn't change, the drop across the output
transistors changes.

>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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>
>

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Received on Wed Mar 10 04:08:14 2004