Components for building a high pass filter

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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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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.
 
Ok so despite being in parallel, the resistor will still have an affect on audio quality but this will be negligible.
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
 

PRR

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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.
 
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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