puzzling grounded input

Never mind... I think it has something to do with the input impedance...

Michael

--- Only incidentally.

Think about it like this:

At the top of the pot the input signal will be at its maximum amplitude since that's connected to the output of the signal source, and at the bottom of the pot it'll be at 0 volts, since that's ground. Anywhere in between it'll be somewhere between max and 0.

What you have with a potentiometer is what looks like two resistors in series with the slider at their junction, like this:

VIN>---+ | [R1] | +------->VOUT | [R2] | GND>---+------->GND

Since it's a 10k pot, let's assume the slider is located half-way between the two ends, like this:

EIN>---+ | [5k] | +------->VOUT | [5k] | GND>---+------->GND

Then, if we relabel the circuit for convenience:

E1 | [R1] | +---E2 | [R2] | GND

we can say:

E1 * R2 1V * 5kR E2 = --------- = ---------- = 0.5V E1 + R2 1V + 5kR

If you solve that equation for the slider being located anywhere along the element and remember that both sides of the pot have to add up to

So what is it?

It's a volume control! :-)

JF

A volume control is a variable voltage divider. If the bottom end of RV1 was not grounded, RV1 would just be a variable series resistor, and would have very little effect on the volume.

Oh...

That's all I thought it was...

Ah...

Thanks John and Peter!

Michael

yeah, like everybody says, and you're on the right track with your input impedance idea; tacking the far end to ground stabilizes the input impedance so that the input signal is well behaved with respect to where you set the pot at. on the other hand, if you just had a variable resistor in series, like with the thing ungrounded, the input signal source seeing a wildly changing impedance both in value and in reactivity (from mostly resistance to mostly capacitative) will very likely change its frequency response etc, unless it's nicely designed to ignore that kind of thing.

One quick question regarding the labeling conventions of the resistors in the schematic. Why are some labled this way: 1K5 or 2K7

Is that 1.5K , 2.7K ohms? Makes sense, I just haven't seen that before.

Scott

Exactly right. one reason for this is it makes printed schematics easier to read.

Look at C5 if I scale the schematic to "125%" the '.' almost disappears, by using a letter instead of a '.' the values are more easily read, (they could have labeled C5 100nF, I don't know why they didn't)

they should have used vector graphics instead of pixels for the schematic then it would scale better - but that's a different matter

also the font they chose has the '.' skewed to the left which is unfortunate.

bye.

Jason,

Thanks for the clarificati>>