I am working on a final project for my circuits class to design an audio amplifier. We are creating an equalizer circuit that has passive high pass, low pass, and band pass filters. Every where I search I find a design similar to this: enter image description here

What is the purpose of the resisters outside of the filters? For example, the two 10k resisters on top, or 3.3k resisters in the middle. They aren't part of the filters, are they?


Yes they are part of the filter. The top section relies on R4 and R5 and C1 for shaping the frequency response of the op-amp circuit. The 100k pot has no part to play in the filtering other than guiding the current that reaches the inverting input of the op-amp from either the actual input or the op-amp's output.

Maybe try researching Baxandall tone controls:

enter image description here

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  • \$\begingroup\$ So forgive my inexperience here but then how do I know the difference between the three filters? For example, which is the bandpass filter if there are no inductors? \$\endgroup\$ – AdamMc331 Nov 25 '14 at 16:02
  • \$\begingroup\$ As per the additional diagram in my answer, the bass is altered by the top 100k pot in your circuit and the treble is altered by the bottom 470k pot. The pot in the middle gives mid-range boost or cut - you can tell because it "mixes" bass and treble circuits with a parallel cap across the pot and a series pot from the wiper. \$\endgroup\$ – Andy aka Nov 25 '14 at 16:06
  • \$\begingroup\$ Can I ask an additional question, not found in the article - why is the potentiometer for the bass connected to a resistor while the one for treble is connected to a capacitor? \$\endgroup\$ – AdamMc331 Nov 25 '14 at 16:32
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    \$\begingroup\$ @McAdam331 One is a low-pass filter, the other is a high-pass. The resistors and capacitors sort of change places between the two. \$\endgroup\$ – John Honniball Nov 25 '14 at 17:38

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