Input/Output Op-Amp Buffers: Inverted or Non-Inverted

[bartimaeus]

I understand that I need to remove the DC,

I'm not sure you do understand... think about what will happen when you turn POT1 up and down without a cap there...

I suppose that I don't, haha. I am certain that no DC will reach pot1 from my circuit. Should I be concerned with DC from the op-amp flowing into the circuit's output? I assumed that the high impedance of the output buffer would prevent anything like that.

[merlinb]

Should I be concerned with DC from the op-amp flowing into the circuit's output? I assumed that the high impedance of the output buffer would prevent anything like that.

Both the opamp's inputs must remain at 4.5Vdc all the time, otherwise it will amplify the DC different between them, which eats into your headroom. Without a cap you are allowing DC current to flow through R5 to ground, so the opamp will have to compenate by raising it's output voltage, so you will lose half a volt of headroom when the pot is at minimum. When you only have about 3.5Vpk headroom to start with, you can hardly afford to loose half a volt. Also the DC in the pot will make it 'rustle'!

[ElectricDruid]

The other possibility is to reference the bottom of the pot to 4.5V instead of 0V. That way, there is no DC across the pot, and the problem disappears. In this situation, you'd only need the cap if there was a possibility of DC coming into CircuitOut, but that's not your situation.

Tom

[bartimaeus]

Ok, I think I get it now! For this project I think I'll reference the pot to 4.5V to minimize the number of components need. Thank you both for all of the help, hopefully from here on I'll be able to tweak the buffer to any application.

[bartimaeus]

I've been working with this buffer, and it works great. Now, I want to add some passive RC LP and HP filters after "circuit out", prior to the potentiometer. Should I bias these filters to ground or to +4.5V? I'm thinking +4.5V, but I'd really appreciate if someone could confirm for me.

[merlinb]

Now, I want to add some passive RC LP and HP filters after "circuit out", prior to the potentiometer. Should I bias these filters to ground or to +4.5V?

Reference to ground, just like the pot.

[bartimaeus]

Thank you for the quick reply!

It's not shown in any of the prior diagrams, but I actually went with ElectricDruid's suggestion to reference the pot to +4.5V.

Here's a pic of what I'm currently working with, with the filters and pot all biased to +4.5V.



When I previously had the filters referenced to ground, and the pot referenced to +4.5V, I was getting weird results when adjusting POT3 (varying the highpass frequency).

[merlinb]

Oh, in that case you can do it as you have drawn it, yes.

[bartimaeus]

Great, thank you so much!