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Topic: Question about high pass filters (Read 5020 times) previous topic - next topic


Nov 08, 2013, 01:40 am Last Edit: Nov 08, 2013, 01:42 am by scswift Reason: 1
Hey guys,

I've got one of those WT5001-28P mp3 modules hooked up to my Arduino, and playing some tracks through a 12V 10W amplifier.  My speakers are 4 ohms, and kind on the small side.  One is 3" and the other 2.5".   They sound decent, but they're only good down to 100hz.  So when I played the unfiltered tracks which had a lot of bass, the speakers would rumble like crazy.

I figured the way to solve this was to use a high pass filter and filter out the frequencies below 100hz.  So, I went to this page:
..and having a limited set of caps on hand and a limited selection of resistors above 10K, I chose a 10K resistor and a 0.1uf cap which gave a cuttoff of 159hz.  Close enough.

I fitted the components between the module and amplifier, and they seemed to do the job.  But then I remembered that I'd seen speaker crossover designs that only used a cap... no resistor.  So I found the calculator I'd seen that on and punched in 100hz and 4 ohms for my speakers:


395uf.  Hm... well, I didn't have a 395uf cap on hand, so I couldn't test it but it got me wondering... why did I need the resistor in one filter but not the other?  Is it the inductor?

So I pulled the resistor I had going to ground off my first filter and suddenly the sound I was getting from the speaker got a lot louder, with more bass... but the rumble didn't return.   And now I'm wondering what the heck I've done.

That first calculator won't let me put in 0 for R, but if I put in 1 and set the capacitor to 0.1uf, it tells me the cutoff frequency is over a million hz, and I can assume as R gets lower that would only go up, so why am I getting any sound at all?   And why does it sound like what I might expect a 100hz HPF to sound like, with a lot of bass, but not so much that the speaker starts clipping badly?

I thought maybe the second calculator would give me the answer, so I adjusted the cuttoff frequency to see how it affected the capacitor value and to get anywhere near 0.1uf I have to make it really high, which again would suggest that I should not hear anything but the highest frequencies, if anything.

So what's going on here?  How is a single 0.1uf cap between a DAC and the ampifier input doing this?

(Note that this mp3 module does have PWM outputs, but I am not using those.)

If it helps, this is the amplifier:


Nov 08, 2013, 06:12 am Last Edit: Nov 08, 2013, 06:19 am by jremington Reason: 1
The first filter you linked, with a resistor, is for low-power electronics and not intended for speakers.

Stick with the speaker calculator. Speakers have considerable inductance that has to be taken into account and the filter calculator does that for you. The second order filter will work better and you can wind  your own coils. There are sites that will tell you how to do that. As for large capacitors, you need "AC" capacitors. You can make these from DC electolytic capacitors, by connecting two of them back-to-back, as shown below.

Here is everything you probably never wanted to learn about crossover networks: http://sound.westhost.com/lr-passive.htm#s8.0  Scroll down for coil winding info.


I didn't use the first filter with a speaker.  It was placed between the sound module and the amp. 


Nov 08, 2013, 07:38 am Last Edit: Nov 08, 2013, 07:49 am by jremington Reason: 1
Sorry, I didn't read the post quite closely enough. However, the first filter you posted is a low pass, not a high pass. What did you actually connect between the sound module and the amp?

Keeping in mind that the sound module output and the amp input have impedances that are probably mostly resistive, there is already a "resistor to ground" on the amp input, with value of typically 50 KOhms. The amplifier input resistor, together with a 0.1 uF cap coupling the sound module and the amp, makes a high pass filter. Assuming 50K and 0.1 uF, the low-frequency rolloff is about 200 Hz. With 0.2 uF, the rolloff would be about 100 Hz, which is what you wanted in the first place.


Nov 08, 2013, 03:21 pm Last Edit: Nov 08, 2013, 03:27 pm by UnoDueTre Reason: 1

So what's going on here?  How is a single 0.1uf cap between a DAC and the ampifier input doing this?

Here is a simple way to visualise the effect of a capacitor which forms a filter.
Note that for this explanation I'm simplifying things and ignoring things like phase angles.
Ok here we go....
A capacitor of fixed value will exhibit a varying impedance (think AC resistance) to different frequencies.
This impedance is called capacitive reactance  (Xc) and the relationship between Xc and frequency as follows:

So lets assume the capacitor is in seriess between the output of a DAC and an input of an amplifier and lets
call the output impedance of the DAC Z1, the reactance of the cap Z2 and the input impedance of the amp Z3,
the simplified equivalent circuit as follows:

So we can see that the cap (Z2) is in series with the DAC (Z1) and if you plug different values into the formula above,
you will see that the value of Z2 will increase as frequency decreases.
This total value of Z1+Z2 will form a voltage divider with Z3 and as frequency decreases, the value of Z1+Z2 will increase and create a greater and greater loss as frequency decreases.
You have just formed a high pass filter.

Now let's take an example where the cap is between the DAC output and ground:
(again Z1 = DAC, Z2 = cap and Z3 = amp)

Now since Z2 (Xc of the cap) increases with lower frequencies, the reciprical is also true, i.e. it decreases as frequency increases
and therefore Z2 will create a lower and lower impedance (to ground) with respect to Z1 and attenuate the high frequencies more.
This is the basis of a low pass filter.

Keep in mind however that this is a simple 2 pole filter and thus the attenuation versus frequency in both cases will be very gradual.
To increase this rate (or slope) one would use many stages of filtering and also use capacitors and inductors to further increase the slope.

Again I repeat that this is an over-simplification and I have ignored things like phase delays, pass band ripple and so on but I hope it serves to show exactly what the capacitor between the DAC and amp is doing depending on if it's in series or parallel with the two.


Sorry, I didn't read the post quite closely enough. However, the first filter you posted is a low pass, not a high pass. What did you actually connect between the sound module and the amp?

I just linked to the wrong page, I did actually construct a high pass filter:


Well I tried adding a second 0.1uf capacitor in parallel with the first and I do get more bass, but it seems to overpower the speaker a bit, so I guess a 200hz rolloff here is best, if that is indeed what I'm getting.

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