# Not HoWo - Series, parallel, RC low pass

#### Bryce Lynch

Oct 3, 2017
2
This isn't homework, I'm 47 & teaching myself, but it did seem like the best place for such a basic question. I'm having some mental block issue that's keeping me from understanding something that seem pretty basic and/or assumed.

If you assume, say, a basic RC low pass filter, why is the cap in series? I don't understand how the cap can impact the circuit. Resistor, then the union between, say, a speaker and the cap ... I don't understand how the cap can impact the signal the speaker gets?

I'm not talking about the property of a cap that makes it work with the resistor to perform a low pass ... I think I understand that. I think. What I don't get is how the cap can impact the circuit given the design.

https://en.wikipedia.org/wiki/Low-pass_filter#/media/File:1st_Order_Lowpass_Filter_RC.svg

I seem to be missing something very fundamental and/or have some hidden assumption I'm making ... but I don't know what it is.

#### Audioguru

Sep 24, 2016
3,653
The capacitor in a simple lowpass filter is not in series, instead the resistor is in series so that the capacitor to ground can cut high frequencies. The higher is the frequency then the lower is the impedance (AC resistance) of the capacitor.

A speaker is NEVER fed with a series resistor and with a parallel capacitor. Your RC lowpass filter can feed the high input impedance of a power amplifier that drives the speaker. Some cheap speakers use a series capacitor to reduce low frequencies going into a tweeter.

#### davenn

Moderator
Sep 5, 2009
14,179
If you assume, say, a basic RC low pass filter, why is the cap in series? I don't understand how the cap can impact the circuit.

well the cap is actually in parallel with the input (via the resistor) and the output
as per the diagram and the one you posted

have you read any tutorials on RC filters ?
here is one that explains the role of the capacitor

http://www.electronics-tutorials.ws/filter/filter_2.html

I'm not talking about the property of a cap that makes it work with the resistor to perform a low pass ... I think I understand that.

if you understand that ^^

then you have to understand this .... ----->>

What I don't get is how the cap can impact the circuit given the design.

anyway read that tutorial and see if you still have a question

cheers
Dave

#### Bryce Lynch

Oct 3, 2017
2
ty, both of you. I'm missing something basic, I think. I'm not sure if using a RC filter as an example was good.

Yes, I've ready that tutorial, and think I understand most of it. I've been working through the tutorials on that site, but I've missed something ... and I don't really know what it is I'm missing.

These two statements, from the tutorial, are relevant:
"Simple First-order passive filters can be made by connecting together a single resistor and a single capacitor in series across an input signal, ( Vin ) with the output of the filter, ( Vout ) taken from the junction of these two components."

"In this type of filter arrangement the input signal ( Vin ) is applied to the series combination (both the Resistor and Capacitor together) but the output signal ( Vout ) is taken across the capacitor only."

In that second paragraph, its the "input in combination but output across cap" that is related to my mental block.

I think (<-DANGER!) that the input is applied through the resistor and feeds the cap and Vout in parallel while the cap is feeding back through the junction to Vout? That union, and the signal flow, is what's got me stupid confused.

#### Audioguru

Sep 24, 2016
3,653
Your tutorial says that the series RC are "across the input" and the output is "taken across the capacitor only". Which means that they are a voltage divider. As the frequency becomes higher then the impedance of the capacitor drops which reduces the levels of the high frequencies. Is that clearer?

#### davenn

Moderator
Sep 5, 2009
14,179
These two statements, from the tutorial, are relevant:
"Simple First-order passive filters can be made by connecting together a single resistor and a single capacitor in series across an input signal, ( Vin ) with the output of the filter, ( Vout ) taken from the junction of these two components."

"In this type of filter arrangement the input signal ( Vin ) is applied to the series combination (both the Resistor and Capacitor together) but the output signal ( Vout ) is taken across the capacitor only."

In that second paragraph, its the "input in combination but output across cap" that is related to my mental block.

I think (<-DANGER!) that the input is applied through the resistor and feeds the cap and Vout in parallel while the cap is feeding back through the junction to Vout? That union, and the signal flow, is what's got me stupid confused.

lets draw the circuit a different way.......

and keep in mind that both the tutorial AND Audioguru have both told you that it is a voltage divider arrangement

Till you state otherwise, I will assume you know what a voltage divider is ?

is that clearer for you ?

Dave

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