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24.17.161.23
In Reply to: Shunt Attenuator Myth posted by Steve Eddy on January 17, 2007 at 17:47:22:
"The signal source sees an impedance equal to the sum of Rx and Ry and for a given signal voltage, a given current will flow through Rx and Ry. As per Ohm's Law, there will be a voltage drop (Vx and Vy) across each resistor as a function of the current divided by the resistance."Well really the signal soutce will see an impedance equal to the sum of Rx, Ry, and the total impedance of the next stage.
As you can see, the voltage drop Vx across Rx, the "single extremely high quality resistor," isn't even seen by the input that the attenuator is driving. Instead, the input sees only the voltage drop Vy across Ry, the shunt element, the one which the proponents of shunt attenuators claim doesn't matter so much in terms of quality.
However as can be seen, the quality of the shunt element matters every bit as much as the series element. And even though the voltage drop across the series element Rx isn't seen by the input, the current flowing through it must also pass through the shunt element so any deviation from the ideal of the series element will also manifest itself across the shunt element by way of Ohm's Law."No the the imput "sees" the current that passes through Rx ONLY. Current that passes through Ry is shunted back to the source or ground. Now I am not saying that quality parts are not needed, just trying to help you understand why some feel Ry can be of a lower quality.
Follow Ups:
Well really the signal soutce will see an impedance equal to the sum of Rx, Ry, and the total impedance of the next stage.
No. What it will see will be the sum of Rx and the parallel combination of Ry and the input impedance of the next stage. I'm assuming it's directly feeding the next stage rather than a stand alone "passive preamp" feeding some other component. In that case the input impedance of the next stage will likely be high enough that it can be ignored and the input impedance reduces to the sum of Rx and Ry.
No the the imput "sees" the current that passes through Rx ONLY. Current that passes through Ry is shunted back to the source or ground.
No. The input doesn't see any current per se. What it sees is the voltage dropped across Ry as a consequence of the current flowing through it. In other words, the signal, as far as the input is concerned, IS the current flowing through Ry, NOT Rx.
Now I am not saying that quality parts are not needed, just trying to help you understand why some feel Ry can be of a lower quality.
I completely understand why some feel Ry can be of a lower quality. And I'm simply trying to get them to understand why that feeling is incorrect and that the quality of Ry matters every bit as much as the quality of Rx.
se
But you do not "listen" to the voltage, you listen to the current as speakers are current driven devices.I will use the old water in a pipe analogy here. Voltage is the water pressure, current is the actual water flowing through the pipe. When you drink that water the pressure is not what you taste.
Voltage is electrical potential.
Current is electron flow.But of course as Ohms law clearly proves they are related as you can not have one without the other.
Actully I take back a bit of what I said at 6:30am this morning...
I was thinking only in DC terms, but since audio is AC there will be just as much current flowing through Rx as Ry. During each half cycle the current will flow from the respective negitive potential of the source. On one half of the cycle current flows from the source through Rx then Ry and back to the source. On the other half of the cycle current flows from the source through Ry then Rx and back to the source.
So you are right both resistors are of equal importance.
Well, I'm glad that got resolved.
se
twe: ""
But you do not "listen" to the voltage, you listen to the current as speakers are current driven devices.""At what point in this discussion did the type of speakers enter?
At the input to the amp, current is not necessarily the driver.
Seems you often do soon after me.So how many and what other names do you post under?
HowdyJohn (jneutron) doesn't use multiple monikers. He's quite earnest in his own way.
He only seems to show up here when Prop Heads is slow :)
Anyway let's try to keep on topic and away from the negative personal...
Thanks.I believe he was referring to stalking, as in how many others do I stalk, and who..
Could be wrong, but that was my take..
ted: ""
He only seems to show up here when Prop Heads is slow :)""He he..
Actually, I'm here lots, just not much to say. But when someone of tweekeng's talents tosses up a foible....:-)
But when someone of tweekeng's talents tosses up a foible....:-)
Toss up a foible? Is that something cats in "Joisey" do?
se
Quite the punster, are you!
twe: ""
So how many and what other names do you post under?""Que?? That somehow...has something to do with the correction to your erroneous statement??
twe: ""
Seems you often do soon after me. ""Again, que??
If your statements were accurate or on target, I would not have posted. If you have some sense that I seem to post immediately after you, then perhaps that is the reason why.
Cheers, John
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