If you are looking for a passive tone control for bass and treble control, here’s a simple one. Take note that because it is a passive circuit, it has no capability to boost signal. This circuit can only be used for high level source like car stereo speaker output. For headphone output, you may need to change some resistor values to decrease the attenuation of the signal. This will also modify the response of the circuit.
About the Circuit
The circuit is almost the same with what I have shared last time minus the transistor amplifier part. The output of the circuit should be connected to a high impedance input of the amplifier. Connecting it to low impedance input will affect the frequency response of the circuit. If the amplifier already have a volume control, you can remove the volume control of this circuit. If you want to use it for headphone out, you may need to decrease the value of R2, R3 and R4 for lesser signal attenuation. But do mind that changing these resistors will also change the overall frequency response of the whole circuit so you may also need to change the value of the other components.
Simulation
As expected, the circuit has no linear response. It has almost flat response when set at around 15% and not 50% as what you can get with active tone controls. It actually attenuates signal rather than boost it. In case of the 100% setting of bass and treble, it actually attenuates more on the middle frequency to have same effect like boosting the bass and treble. Because it attenuates rather than boosting, this circuit can only be used for high level sources like, speaker outputs or headphone outputs.
