Using Caps For Frequency Attenuation Confusion

[SpringbokUK]

Hey!

I was recently reading about the role of caps in stompboxes on the Beavis Audio website and i came across some confusion. Apparently, by using a low value cap in the signal path, it will attenuate the lower frequencies. But then i find out that if you use a potentiometer along with the low value cap, you get a LOW PASS FILTER. Which is opposite to attenuation because your letting the lower frequencies through. I am very confused and i hope someone can help us out here.

Cheers! 

Liam

[Mike Burgundy]

google RCL filter and do some rreading, for example:
http://www.physics.unlv.edu/~bill/PHYS483/filters.pdf
It's all in *how* you position the parts. A cap is an infinitely high resistor ("open" connection) to DC, which is basically voltage with frequency = 0.
It becomes lower resistance at higher frequencies, until it is essentially a closed circuit for high enough frequencies.
Stick it in series: lets through only higher freq.
Stick it between signal and ground: bleeds away high freq.
How much at what frequency depends on values and surrounding resistors.
"resistance" varying with frequency is called impedance.
Hope this starts you off, and welcome ;P

[WaveshapeIllusions]

It all depends on whether the cap is in series or parallel with the signal. Think of a capacitor as a frequency-dependent resistor; the higher the frequency, the less resistance it has. At a high enough frequency, we can call it a short, no resistance. At a low enough frequency, we can call it open, there's effectively no connection.

Let's look at the lowpass filter first. Say there's 10k in line with the signal and 10nF to ground. At about 80 Hz (low E on a guitar) that 10nF cap looks like a 200k resistor. So very little signal is across it, most is on the 10k in series. Think of it like your volume pot just barely below 10. At 2000 Hz though, that cap acts like just an 8k resistor. Now more signal is going to ground through it than the 10k in series. This would be like you volume pot a bit below 5.

It only affects that frequency that way though. As the frequency goes from 80 Hz to 2000 Hz, our volume knob gets gradually turned down from 10 to 5.

What if we swap them though? What if there's a 10k to ground and 10nF in series with the signal?  Well, looking at the numbers from earlier, at 80 Hz that cap would still be about 200k resistance. Less signal would be flowing here, and more would be through the 10k to ground. In this case, the imaginary volume pot would be around 1, barely on at all. We'd call this a high pass filter. At 2000 Hz, it would still be about 5 though. That is what we'd call the corner frequency of a filter; which is where the volume is about halfway down.

To summarize: a capacitor can be seen a resistor that decreases in value as frequency goes up. Whether it is a low pass or high pass depends on where it is taking those high frequencies; either to the next stage (or output) or to ground.

I hope this helps explain it some. It is a bit more complicated than this, but we can ignore impedance for now.

[IvIark]

A capacitor in series with the signal, like an input capacitor creates a high pass filter with the input impedance of the following stage which is why selecting a lower value input capacitor will cut some low frequencies entering the circuit.

Then you have the RC high and low pass filter networks as described above where the cap is in series with the signal followed by a resistor to ground for a high pass filter, or a resistor in series with the signal followed by a cap to ground for a low pass.  You can calculate the RC network filters using this by Jack:

http://www.muzique.com/schem/filter.htm

[Pyr0]

Sorry to hi-jack the thread, but how about caps in op-amp feedback loops, how do you calculate the effect they have on a circuit ?

[WaveshapeIllusions]

In the feedback loop, everything is reversed in function. Since it's negative feedback, it removes signal. A cap passes more high frequencies back to the inverting input and thus removes more of that portion of the signal. I believe you can find the crossover frequency by using the formula for reactance (Xc=1/2*pi*F*C) and using the feedback resistor for Xc. There's a few easy calculators floating around.

[SpringbokUK]

Thanks for your replies guys. It's very much appreciated and i have a lot to think about now in terms of what i want to do and how i'm going to do it.

Many thanks again!

Liam

[SpringbokUK]

Just a few last questions actually. I'm a visual learner so this is how i'm painting this picture in my head.

I have my signal which goes through a 10k resistor, which is like my volume pot. Then the signal goes through a 10nf cap that goes to ground. The higher frequencies can only pass if in series with the signal. Instead the higher frequencies are forced to ground. Am i right in thinking this?

Or

I have my input which goes through a 10nf cap in series, which allows the higher frequencies through and then goes through the 10k resistor, which goes to ground.

I'm a little confused as to what the resistor is doing and how the position of it effects the circuit. what would happen if i had my signal go through a cap to ground and then through a resistor in series?

[PRR]

> my signal which goes through a 10k resistor, which is like my volume pot.

How is this resistor "like my volume pot"? Vol-pots usually go to ground on one end. This resistor apparently doesn't.

> what would happen if i had my signal go through a cap to ground and then .....

Stop there. Draw the WHOLE circuit. What is "my signal"? A weak hi-Z guitar? A 600 Watt amplifier?

A guitar facing a (say) 10nF cap will hardly be able to force any highs. It may also have some struggle with the later 10K resistor.)

A 600W speaker amp will barely notice a 10nFd on its output, and sing far past the audio band. (Actually many amps get mildly upset by caps this size, but the trouble is far outside the audio band, and the reasons are best left for a later course.)

[SpringbokUK]

well it's just that waveshapeillusions said 'think of it like your volume pot'. So that's what i did.

[SpringbokUK]

Unless he mean to think of the frequencies lowering the 'volume' as the frequency gets higher.

How i read things is like this. If someone say for example  'This does this and it results in this.' following with 'Think of it like this'. That's exactly how i will see it. So please, if you don't mind, don't think of me stupid. Just how i see things.

[SpringbokUK]

And i was using 10nf and 10k as examples because they are mentioned early on in this thread.

And i use a baritone guitar and a peavey triumph 60watt valve amp.

[chromesphere]

A way i remember what high and low pass filters do is simple to say "High pass filters -> the higher frequencies pass" or "low pass filters -> the lower frequencies pass".  Not your question but thought i would add that before i go on.

A capacitors resistance drops with increase in value and/or frequency.  Just think about that for a second.  ALso, try this calculator if you are not sure what im talking about here (it will help understand whats going on trust me).  Just plug in values and watch what happens: http://www.kusashi.com/reactance-c.php

Remove the resistor from the circuit for a minute.  If the cap is in series with the signal it will block the lower frequencies and pass the high frequencies. Thats what a cap does.  If its connected to ground, it STILL behaves the same. It will still block the lower frequencies and pass the high frequencies, except in this case, its passing the high frequencies to ground ie your losing the high frequencies to ground. 

I hope i explained that clearly enough. I only just understand the function of the resistor in pass filters, and it nearly did my head in, but it helped me to understand what i just explained above FIRST before adding in that stupid resistor   Also, you should have an idea of a voltage divider network or you wont get it.

Hope that helps,
Paul

[SpringbokUK]

Ahhh Thank you!


So if the capacitor is connected to ground then it will still block the lower freq but also the higher freq? Hope I understood correctly there.

[chromesphere]

In the example of a low pass filter, it will block the low frequencies but it will PASS the high frequency, sending them to ground.  Replace "sending them to ground" with "getting rid of them" in that last sentence.  The function of the cap stays the same wether its connected in series or connected to ground.

Edit: Remember that when the cap is connected to ground what appears at the junction of the resistor and the cap is your guitar signal that will continue on into the circuit or the output of the circuit etc.  What gets sent to ground is REMOVED from the circuit.

So, again in the example of the low pass filter, if the cap thats connected to ground is blocking low frequencies, that means its actually SENDING the low frequencies on into the effect or the output. 

If its passing high frequencies (to ground), its REMOVING the high frequencies from the circuit.

Hope i explained that clearly.
Paul

[SpringbokUK]

Ahhh cheers mate that's beast.

I'm planning to clone a colorsound overdriver is all. I have an obsession with overdrives. I have a few already. Got the Green Rhino, Black Forest (Which is a clone of the colorsound overdrive), and the Daddy-O. Not too keen on the Daddy-O. Sounds like a bag of sherbet but i couldn't complain for £20 to be honest. Haha.

Thanks to everyone again! I will PASS this knowledge onto my hands and get to work!

[chromesphere]

Awesome, hope i explained it well enought and welcome to the forum
Paul

[SpringbokUK]

Much Appreciated!

[ashcat_lt]

Replace "sending them to ground" with "getting rid of them" in that last sentence.  The function of the cap stays the same wether its connected in series or connected to ground.

Edit: Remember that when the cap is connected to ground what appears at the junction of the resistor and the cap is your guitar signal that will continue on into the circuit or the output of the circuit etc.  What gets sent to ground is REMOVED from the circuit.

Better to think of the high frequencies as being shorted.  It's not that they are being "dumped to ground" or that we are "getting rid of them".  They just find an easier way to complete their circuit through the cap than through the (presumably) higher resistance of whatever device comes after.  

Better yet to consider it as a voltage divider, since "everything useful is a voltage divider"*, per this thread.



* Sure wish I could remember where I got that phrase.  I think it was PRR.  All of this crap has made a lot more sense ever since!

[chromesphere]

Replace "sending them to ground" with "getting rid of them" in that last sentence.  The function of the cap stays the same wether its connected in series or connected to ground.

Edit: Remember that when the cap is connected to ground what appears at the junction of the resistor and the cap is your guitar signal that will continue on into the circuit or the output of the circuit etc.  What gets sent to ground is REMOVED from the circuit.

Better to think of the high frequencies as being shorted.  It's not that they are being "dumped to ground" or that we are "getting rid of them".  They just find an easier way to complete their circuit through the cap than through the (presumably) higher resistance of whatever device comes after.  

Better yet to consider it as a voltage divider, since "everything useful is a voltage divider"*, per this thread.



* Sure wish I could remember where I got that phrase.  I think it was PRR.  All of this crap has made a lot more sense ever since!

The OP seemed confused so i tried to simplify.  Voltage divider is lesson #2. Feel free to continue on from here
Paul