I am new to this and trying to learn about designing analogue audio filters. I looked at a couple of tutorials online that gave examples about how to implement and simulate high and low pass filters. I used the internal plot tool to checkout the output using a decade sweep, but both my high and low pass filters exhibit high pass filter behaviour. The filters themselves are first order filters, where the only difference is I have swapped the position of the capacitor and resistor. I could provide the settings I put into the AC Analysis simulation command, but I believe the .ac dec 20 1 20k section describes that adequately. I hope I have shared enough info to make my mistake obvious to anybody that can help.

low pass filter

high pass filter

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    \$\begingroup\$ Did you remember to click the simulate button after you rearranged the components? \$\endgroup\$
    – Carl
    Jun 6 at 12:53
  • \$\begingroup\$ Hi @Carl. Yes, I even saved them as separate files and closed them and reopened them. If I change the resistance values it moves the frequency of the filter cutoffs, so I feel like I'm updating things accordingly in the application. \$\endgroup\$ Jun 6 at 13:03

1 Answer 1


Look what you have plotted in both graphs: -

enter image description here

And, of course both circuits will have the same input current no matter which position the R and C are in. You need to plot Vout.

  • \$\begingroup\$ Thank you for this. I was unsure about how to add a Vout type component onto the circuit, what is that control called in the application, please? \$\endgroup\$ Jun 6 at 13:06
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    \$\begingroup\$ Vout is just what I call the node that connects the resistor and capacitor so, plot the voltage on that node i.e, something like V(n002) but better still, add a node label to that node called Vout and, plot V(Vout) @RobHinchliff \$\endgroup\$
    – Andy aka
    Jun 6 at 13:09
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    \$\begingroup\$ Thank you for explaining this, LTSpice makes much more sense now. \$\endgroup\$ Jun 6 at 13:14

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