Electronic – Buffer stage for audio filter

active-filteraudiobuffercircuit-design

Can someone explain the purpose of this buffer stage? I understand how it works in the sense that I understand the cause of each reading on the oscilloscope but I don't understand why the buffer stage is designed this way. The circuit is a five band equalizer.

I thought the purpose of putting a buffer at the beginning of an audio circuit was generally to amplify the signal (I don't think my output is load sensitive) as explained here:

What is the reason for adding a buffer stage?

Here is the design of the buffer stage:
enter image description here

The corresponding voltage readings are simulated here (red is audio signal, green is amp input, blue is output of the entire stage):
enter image description here

To be clear, the audio input contributes a negligible amount of current so the 12V supply effectively goes through one giant voltage divider which reduces it to 2.4V which it adds as a DC offset to the audio signal which is needed because this is a single supply op amp.

But why would they design the buffer circuit so that the output is REDUCED?

The original design is taken from here where you can see the entire circuit:

https://electronicsforu.com/electronics-projects/hardware-diy/5-band-graphic-equaliser

Best Answer

It is a differential non inverting amplifier with a gain of ~1, (all 4 legs have the same resistance if you ignore R18)

R18 is acting as a 6V bias to keep the output and input between the 0 to 12V supply rails of the op amp, this mainly seems like they did not want to use symmetrical supplies for the op amp,

So the output voltage of the op amp is roughly signal in +6V

C3 is the input coupling capacitor, which removes any bias outside, and C4/R15 are to remove the DC offset this buffer has,