Help understanding RC Power Filter

Thread Starter


Joined Aug 20, 2012
My boss told me to add a RC power filter to my LM324 op amp power input to reduce noise input/output. The op amp power input has very low impedance, so it accepts noise and will pass high noise through to output. Adding a capacitor in order to get rid of output noise makes sense to me, why in the world place a resister before the power in pin?

He wants me to add a resistor first (50 Ohm) and then place a capacitor after, then comes the input. Resistor is in series with LM317 and LM324, cap is parallel with these and connected to ground.

I do not understand this at all so I assume I am missing some concepts.


Joined Feb 19, 2009
The RC filter is similar to the LC low pass filter. Rather than an inductor, the resistor is the impedance, preventing surge currents. The capacitor works with the resistor, if you use a filter calculator, you can find the cutoff point of suppressed frequencies.

Using only a capacitor in parallel with the power source is a way to lower the effective source impedance, typically to keep the voltage steady in a high power system. The resistance in that case is the internal resistance of the battery or power supply. The resistor also helps the capacitor supply current to the circuit, rather than back to the power supply, when the voltage sags.


Joined Dec 20, 2007
An LM324 is very old and is very noisy. Many newer audio opamps have much less noise.
An LM317 regulator also produces noise but most modern opamps ignor the noise on the power supply.


Joined Dec 20, 2007
TL071 single, TL072 dual and TL074 quad audio opamps have noise about 5 times less than a lousy old LM324 and the bandwidth is to 100kHz (the LM324 has trouble above only 2kHz).
The LM324 has horrible crossover distortion but the TL07x opamps have only 0.003% distortion.

OPA134 single, OPA2134 dual and OPA4124 quad audio opamps have noise about 10 times less than the lousy ones, a bandwidth to 150kHz and distortion of only 0.0008%.

There are many more good opamps.


Joined Dec 20, 2007
The LM317 voltage regulator is as good as most voltage regulators.
Most opamp circuits do not need a voltage regulator because the opamp ignors fluctuations of the supply voltage.


Joined Nov 30, 2010
AG is talking about PSRR...Power Supply Rejection Ratio or SVR...Supply Voltage Rejection ratio. Here's a datasheet of a TL071 that shows 70 db to 80 DB reduction of the power supply noise (depending on exactly which version you use).

That isn't a perfect, "ignore" but it helps a lot. Just about any regulator chip used to keep the amp safe from over voltage events, combined with the 70db or better Power Supply Rejection, makes most applications like this very easy to keep the background noise down.