small question about high-pass filter

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marcusriley
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small question about high-pass filter

Post by marcusriley » Wed Feb 12, 2020 12:05 am

Hello All,

I have been experimenting with the basic single-order low-pass and high-pass active filters, like the ones below.
Image
Image

I notice that the high-pass filter has a pretty consistent "bleed", regardless of the cutoff frequency. Even when being fed a sine wave, I'm hearing the bleed, so I'm not sure if this has to do with harmonics. Does anyone know what causes this? Can anything be done?

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Grumble
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Re: small question about high-pass filter

Post by Grumble » Wed Feb 12, 2020 4:45 am

The first filter is actually a high cut-off filter, but it acts as a low pass filter.

The bleed depends on the quality of the capacitor and the value of the resistor : The capacitor should have a low leakage current (high dc impedance) and the resistor as low as is convenient.

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Re: small question about high-pass filter

Post by jorg » Wed Feb 12, 2020 9:26 am

These filters have a very gentle slope (6dB/octave). Through the high pass, you will get "bleed" which is proportional to fin/fcutoff. For example, if you feed it 100Hz and the cutoff is 1KHz, about 10% of the input voltage will "bleed" through (about -20dB) - which is clearly audible. If you set the cutoff to 10Kz, you still only get 40dB attenuation.

All of that assumes your capacitor is "perfectly linear." If your capacitor is, say, X7R ceramic, it will introduce massive distortion (especially at modular signal levels). DIstortion equals harmonics, which will "bleed" through a highpass much more readily than the fundamental frequency.

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Grumble
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Re: small question about high-pass filter

Post by Grumble » Wed Feb 12, 2020 11:42 am

http://sim.okawa-denshi.jp/en/

This is a site with online calculations of various circuits, for your filter question choose the filter tool section. :mrgreen:

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Re: small question about high-pass filter

Post by ashleym » Thu Feb 13, 2020 6:45 pm

From “electronics tutorials “?

If so have you tried the active filters further down the page? How did they compare? If you take the sage advice above, did you notice the same difference in capacitors in active and passive configurations?

You don’t have to answer but I’m sure there are people here who would like to know.

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Re: small question about high-pass filter

Post by The Real MC » Thu Feb 13, 2020 9:29 pm

Depends what you are driving the circuit with. With that design, the output impedance of your source will impact the cutoff frequency. That was conveniently left out of the transfer function of the RC circuit.

I'm an EE with many college level design books and I have never seen an filter architecture like those before. There is probably a good reason.

I have seen many website circuits that are just plain wrong.
I haz a wrench, to fix da synth.
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Re: small question about high-pass filter

Post by ashleym » Fri Feb 14, 2020 12:12 pm


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