I would like to connect a signal coming from a line receiver to several op-amp based filters in parallel. The basic topology (high-shelf) looks like this:


simulate this circuit – Schematic created using CircuitLab

What I don't understand is the meaning of R1. Why does the positive input have to be shunted to ground? Doesn't it act as a voltage divider since the inputs of the op-amp have their specific input impedance?

My question is, if I want to connect several of those in parallel from one output, do I need both R1 and R4, or is one time enough? Do I need them at all?


simulate this circuit


1 Answer 1


Yes, you can do this with little or no side effects. The resistor is not needed for your application because you are using an opamp buffer to drive the shared line. Just ensure the output of the buffer is nominally centred between the supply voltage rails.

  • \$\begingroup\$ So I could leave them out without side effects? How do I know if it's centered? Im coming from balanced line receiver, so how would I know that? \$\endgroup\$
    – jilski
    Jan 1, 2016 at 13:45
  • \$\begingroup\$ Just design OA3 appropriately to ensure its output is centred. \$\endgroup\$ Jan 1, 2016 at 14:17
  • \$\begingroup\$ If you are unsure what DC level may be present on the signal, you could get rid of it by a CR high pass filter: electronics.stackexchange.com/questions/208940/… \$\endgroup\$
    – Andy aka
    Jan 1, 2016 at 19:23

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