Not using the entire potentiometer in a buffered volume pedal circuit?

[midwayfair]

The resistor going to Vb sets input impedance?

And would the power filtering in the greenbean schematic work fine?

Yes to both.

[brand0nized]

The resistor going to Vb sets input impedance?

And would the power filtering in the greenbean schematic work fine?

Yes to both.

DING DING DING! Woohoo!

[brand0nized]

Alright, here is the buffer schem with power filtering and the input impedance cap.



Why isn't the pot value so important?

[midwayfair]

Why isn't the pot value so important?

Output impedance is very small from an op amp -- and the input impedance of the next op amp stage is extremely high.

To think about this, remember that everything is a voltage divider. Just like your potentiometer. But the potentiometer isn't the only resistance. There's also the resistance at each audio pin of the op amp.

Let's take two situations for the output impedance of stage 1, and we're going to use a 10K pot, which is about as small as you usually see for volume controls.

In situation 1, our output impedance is, say, 50 Ohms. That's about right for some op amps. 50R is a mere 0.5% of our 10K potentiometer. This means that at full on the pot, we're losing less than 1% of our signal. Likely inaudible loss.

In situation 2, our output impedance is a 5K Ohms. This isn't an uncommon amount for a transistor being used as an amplifier (it's the transistor's collector resistance + a few hundred for the transistor itself). In that case, our 10K volume pot would only be twice as big as the output impedance! We could easily lose quite a bit of signal.

You can see that even a few hundred ohms would make the choice much more difficult. But in an op amp, the resistance is so small that even if you made that 10K pot 10 or 100 times larger, you're not changing too much.

[brand0nized]

That makes perfect sense! I've ordered 100k and 25k, so I'll just try both.

Also, pin 4 still needs to go to +9v and pin 8 still needs to go to ground right?

EDIT: pin 4 should go after the filtering?

[midwayfair]

Also, pin 4 still needs to go to +9v and pin 8 still needs to go to ground right?

EDIT: pin 4 should go after the filtering?

Look at the pinout in the datasheet again.

[brand0nized]

Also, pin 4 still needs to go to +9v and pin 8 still needs to go to ground right?

EDIT: pin 4 should go after the filtering?

Look at the pinout in the datasheet again.

Yup, just confirming.

[BaklavaMetal]

Alright, here is the buffer schem with power filtering and the input impedance cap.



Why isn't the pot value so important?

just one thing, your power supply portion of the schematic doesn't have any filtering. It has a small value resistor which can help reduce noise and interference, but for power supply filtering you would use capacitors. Every power supply has some ripple, that means that DC voltage isn't excatly a constant value but it fluctuates.That fluctuations can introduce noise.

Here's a picture to help clarify things
Have in mind that the numbers on the pictures are purely for demonstration, i haven't done any math, so don't take them as is.


Top left picture shows your power supply fluctuations, according to the picture, it fluctuates between 9 and 8.5V every milisecond. We need a cap here.
Why capacitors? Capacitors are devices used to store charge/energy. As such they need some time to to fully charge/discharge, as you can see in the top right picture.
The trick is to use capacitor which needs much longer time to charge/discharge than the actual time of your power supply variation. That way, when your PS starts dropping from 9 to 8.5V rather fast, capacitor discharges much more slowly thus making the voltage much more stable as you can see in the bottom left picture. We can say that when PS voltage starts dropping, the capacitors gives it's own accumulated charge to compensate.
Bottom right picture is the usual way of power supply filtering in pedals.

Hope this helps

[brand0nized]

I see, thanks for explaining filtering, makes a lot of sense.

Can I just drop in the 10u and 47u for filtering in my schematic? And keep the diode for polarity protection?

[BaklavaMetal]

yes to both.

[brand0nized]

Filtering added. Pin 4 after the 100u cap.

Is this correct?

[midwayfair]

Your power pins are wrong. Look carefully at the diagram in the datasheet.

[brand0nized]

Your power pins are wrong. Look carefully at the diagram in the datasheet.

Darn, I remembered incorrectly. Now to redo my layout.

Where can I add a trimmer to give a slight boost?

[midwayfair]

Where can I add a trimmer to give a slight boost?

You need more than a trimmer. You need at least a second resistor (if using a inverting amplifier) or a capacitor + 2 resistors (if using an inverting amplifier). Look here: http://en.wikipedia.org/wiki/Operational_amplifier#Negative_feedback_applications
(note that the wikipedia examples assume the op amp is running on AC, not DC. You need a large cap in series with the resistor R1 in example 1.)

[brand0nized]

Can I use this non-inverting example?

I'll read it a couple more times before I get how the voltage divider network increases gain...

Still need the huge cap to go to ground here, paralleling the resistor that goes to ground?

http://www.electronics-tutorials.ws/opamp/opamp_3.html