What range of capacitor values is appropriate for simple passive RC filters? - Page 1
Forum section: electronics / beginners

author: Electricolin
author_rank: Newbie
timestamp: October 26, 2021, 06:37:41 am
content: Hello users,Since this is my first time using this forum, and since English is not my native language, excuse me for any mistakes when posting/replying.As my first real electrical engineering project, I was instructed to design and build a amplifier that has a minimal output of 1W by a load of 4 ohms and a load of 8 ohms. The product that I have to deliver must include: 3 band equalizer (low, mid and high) and a volume control; but I run into some issues because I do not have many experience in this field yet.My question when designing a passive RC filter circuit, is the following:What range of capacitor values is appropriate for using in a simple passive RC filter circuit?At first I was going to use a capacitor of: 1uF and a resistor of 160ohms for my low-pass filter to achieve a cut-off frequency of around ~1000Hz (I didn't choose this frequency myself, it is what I got instructed by my teacher).The problem is, that I do not see any circuits online using capacitor values this high and when I search for the reason why to choose a higher resistor value to reduce the value of the capacitor, I don't really see the right answer that helps a electronics noob like me.Can somebody please explain this to me at a understandable level?Kinds regards,Colin

author: drdm
author_rank: Contributor
timestamp: October 26, 2021, 07:22:54 am
content: I usually go with resistors in the kohm range (10k is my go to) and then pick a cap for the desired frequency.

author: Circlotron
author_rank: Super Contributor
timestamp: October 26, 2021, 07:51:39 am
content: For a given type of construction, the greater the capacitor value the higher the cost. Resistor cost is generally constant, so higher resistance and lower capacitance equals lower cost.As the input frequency goes higher and higher the input impedance of the RC circuit gets closer and closer to the value of the series resistor. If this resistor is too low it might load the signal source too much. Also the signal source will have a certain output impedance and if the series resistor is much higher value than the signal source impedance the signal source impedance will have almost no effect on the -3dB frequency.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2021, 03:16:22 am
content: Film and C0G/NP0 ceramic capacitors are inexpensively available up to about 1 microfarad, but a maximum value of up to 0.1 microfarads is more typical. Electrolytic capacitors should generally be avoided for audio filtering because of their large voltage coefficient of capacitance which will create additional distortion; they can be used in coupling applications by using a much larger value than strictly necessary.

author: gbaddeley
author_rank: Regular Contributor
timestamp: October 27, 2021, 08:54:06 am
content: It depends on the source and load impedance, and the turnover frequency that you need. What are these in your case?

author: rstofer
author_rank: Super Contributor
timestamp: October 27, 2021, 09:30:17 pm
content: You can get pretty close to 1kHz with a 10k resistor and a 0.015ufd capacitorhttp://sim.okawa-denshi.jp/en/CRtool.phpI don't see the point in using low values as it just loads the previous stage but it really depends on what the downstream stage expects as a driving impedance. It it is another amplifier, there's no point in using really low values for the resistor.

author: TimFox
author_rank: Super Contributor
timestamp: October 27, 2021, 09:35:59 pm
content: In your original choice, the problem is not so much the capacitance value as the resistor value. Assuming the filter is in the middle of the amplifier (not between the output and the low 4 or 8 ohm loads), resistors between 1.0k and 100k ohms are easier to work with, using smaller capacitors, to avoid drawing too much current from the driving amplifier.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 27, 2021, 10:02:15 pm
content: If I may ask, when is it appropriate to use a filter (passive or otherwise), and when is it not? It's easy to say you want to put an RC-filter to filter out high-frequency components (ex: 50-ohm, 2 pF, f=1.59 GHz), but how do you account for its effects as it spreads to other circuits? I think you mentioned impedance as one thing to keep in mind.

author: Jwillis
author_rank: Super Contributor
timestamp: October 29, 2021, 03:50:39 pm
content: Your requirements of a three band equalizer will consist of three sections . A low pass filter , a band pass filter , and a high pass filter .The band pass filter will consist of a low pass filter in series with a high pass filter .Each filter can be calculated using fc = 1/(2*pi*R*C)fcis the desired cut off frequency .R is the resistor valueC is the capacitor value