# Parallel RC as a filter?

I was looking at an amplifier schematic and saw a curious portion that I realized I wasn't sure what its purpose was.

simulate this circuit – Schematic created using CircuitLab

At first glance I thought I saw a low pass filter, but then I said wait no, those are in parallel. What's that capacitor doing there? Is it smoothing the audio source, acting as some kind of noise filter? Is there a name for this kind of device? Would any assumptions need to be made (like whether the audio source has a constant current, or constant voltage or neither) to determine the function of this device? I was operating under the assumption of direct current and audio was encoded as time varying voltages, but if that's not the case, would the circuit do anything different? I saw some things thrown around online like smoothing capacitor, so on an audio signal would it just smooth out sharp peaks? Would this be make it sound nicer, like how a sine wave sounds different than a square or triangle wave?

What would the difference be if I skipped that and connected the audio source directly to my NODE1 there?

• high pass filter..kind of. R1 isn't usually there so it weakens the high pass part a bit – DKNguyen Sep 5 '19 at 14:19

It looks like a first-order passive high-pass shelving filter.

Its role is to attenuate low-frequency content of the audio signal without cutting it off. Mainly used to reduce (de-emphasize) excessive bass as a form of coarse equalization via bass-trebble (tone) controls.

Some features:

1. Attenuation of low-frequency content is: $$\\alpha = 20 \log \big(1+\frac{R_1}{R_2}\big)\$$. In this case approx 5 dB attenuation.
2. Attenuation of high-frequency content is 0 dB.
3. There is a transition band in-between that goes from $$\f_z = \frac{1}{2\pi R_1 C_1}\$$ to $$\f_p = \frac{1}{2\pi (R_1 || R_2) C_1}\$$, which are the frequencies where the zero and the pole of the transfer function of the filter are located. In this case, transition goes from 1,205 Hz to 3,617 Hz.

There are active and higher order variants of the high-pass shelving filter that allows you steeper transition bands if you need them, and also help avoiding loading effects.

Simulate it with a swept sine.

Assuming the source is low impedance and the following circuitry is high impedance, it's a 1:3 voltage divider at DC, and passes high frequencies unattenuated. There's a zero at $$\\frac{1}{2 \pi \mathrm{(3.3nF)(40k\Omega)}} = \mathrm{1200Hz}\$$, and thus a pole at $$\\mathrm{3600Hz}\$$.

So, it's an equalizer (or emphasis) network of some sort.

It is a high pass filter and I believe it is part of tone control. You can adjust the high frequency response of the amplifier. It might help to see more of the circuit.