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Components for building a high pass filter
Components for building a high pass filter
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Old 5th December 2016, 04:14 PM   #1
san1c99 is offline san1c99
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Default Components for building a high pass filter

Hey guys, I have recently started designing a ported box for desktop speakers I will soon be building. However, using Winisd I have noticed the speaker will exceed its xmax at full power. To limit over-excursion at low frequency I have added a first order high pass RC filter at 130hz.

Using an online high pass calculator I have found two different ways to make this filter. The first uses these components: Capacitor and Resistor, these are both high end audio components so building a high pass filter (HPF) with these would cost $10 each.

The second option is with these parts: Capacitor and Resistor. The difference is that these parts only cost $2 for making a HPF.

Since the resistor in an RC HPF is in parallel and the signal going to the speaker doesn't pass through the resistor I assume that a far cheaper non-'audio-grade' component can be used instead. I just want to know if using the low quality resistor in parallel will still have an effect on the quality of the audio signal or not.

One more question i want to ask is will the first order high pass filter shift the phase by 90 degrees? Thanks, Joseph
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Last edited by san1c99; 5th December 2016 at 04:25 PM.
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Old 5th December 2016, 09:28 PM   #2
AllenB is offline AllenB  Australia
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The fact that a component is in parallel does not mean it can't make a difference. If it were to modify the current draw through the series component then the signal available to the load is also modified. This is the method of a second order filter.

Resistor types are not very critical with speakers, eg the non-inductive quality is not much of a concern. The power rating could be.
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Old 6th December 2016, 01:16 AM   #3
san1c99 is offline san1c99
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Ok so despite being in parallel, the resistor will still have an affect on audio quality but this will be negligible.
Quote:
This is the method of a second order filter.
I don't quite understand what you are saying, do you mean my filter is a second order type or that a second order uses the principles you just mentioned?

Another question I had was if capacitors with higher values (farads) add more distortion to the signal? I think I read this somewhere and just want to confirm is this is true. Thanks, Joseph
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Old 6th December 2016, 05:21 AM   #4
PRR is offline PRR  United States
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Why do you have a resistor?

A loudspeaker "IS" a resistor.

If you compute your C-R for 100 Ohms, then throw an 8 Ohm speaker across the 100 Ohms, it becomes 7.4 Ohms for all practical purpose.

First compute a C for your nominal loudspeaker impedance. Say 8 Ohms, try 150uFd.

THEN remember a loudspeaker is not a *constant* resistance. It can be very "lumpy". Especially around bass resonance. Which is also about the point you are likely to run into Xmax trouble.

You can then use a somewhat small C. The rising impedance of speaker will semi-match the cap above resonance, and below resonance the cap will severely reduce speaker voltage. It is not uncommon to end up with even 1/3rd of the cap you compute from nominal speaker impedance.

BUT this added cap shifts all the speaker electromagnetic parameters. The result is not what your speaker calculator predicted.

A second-order choke and cap is a somewhat better way to cut bass excursion. Steeper, but mainly because loudspeaker impedance has less effect.

All in all, it is usually better to bass-cut BEFORE the power amp, rather than after. The impedances are higher (smaller caps) and more predictable. You are not making bass power that you will not use.
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Old 6th December 2016, 07:59 AM   #5
san1c99 is offline san1c99
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Thanks PRR for the information but I do not quite understand what you said about the capacitor. Firstly a filter before the amp is not an option for me and I prefer using a first order high pass filter because it produces a more desirable frequency response curve.

Now I am quite sure the program I use (WinISD v0.7) shows the effect of bass attenuation with the varying impedance curve taken into account. So I expect the real change to match the simulation.

But lets say that this is not the case and like you said, a smaller capacitor than calculated can be used. How would I determine the size of this cap? do I just get one of one third the capacitance and hope it works properly?
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