Re: High pass CLC T Filter component values
« Reply #20 on: June 21, 2019, 06:10:52 PM »
I gues a large number, including me, would disagree with you.
I have run a simulation with a load impedance step of 250R, from 1k to 50k. Please check the attached image.
As you can see, as load impedance increases, the resonance of the filter increases with it.

For the 50k load I have a 5.128KHz filter frequency.
For the 1k load I have 5,462KHz.

In my case and for the application of this filter, this difference in the filter frequencies is not important.

As you mentioned earlier, you can always use additional resistors to adjust the impedances.
Yes, but here I must know previously the source and load impedances to place the correct resistors, or not?



squarewave

Re: High pass CLC T Filter component values
« Reply #21 on: June 21, 2019, 07:32:56 PM »
You also need the correct source impedance to get perfect damping. AC response should be perfectly flat and then a sharp nose over into a perfectly straight slope of 18dB per octave.

Try this:

  https://www-users.cs.york.ac.uk/~fisher/lcfilter/

Select Butterworth > Highpass > 3rd order > 1000 Hz > 600 ohms

Results: 48 mH, 265nF

I love high/low cut filters. But you definitely have to get the damping right. Otherwise it's going to sound like a stuck mu-tron.

PRR

Re: High pass CLC T Filter component values
« Reply #22 on: June 21, 2019, 08:14:13 PM »
..................For example, I have a 27mH inductance ...........................

In audio we usually use Q near unity. Therefore the resistive impedance will be similar to the inductive (and capacitive) impedance. This, with practical limits on resistance, defines what is "practical".

Figure reactance of 0.027H:
20Hz - 3.4 Ohms
200Hz - 34 Ohms
2KHz -340 Ohms
20kHz - 3400 Ohms

So a 20Hz filter will probably need a "loudspeaker power amp" to drive it. (Also max current and DCR will matter.) A 4KHz filter will be near 600 Ohms, a happy zone for "passive" in a studio. A 20KHz filter will want to drive a several-Kohm load, and be awkward in a studio line.

From there: you can try "matched" resistive impedances, or try making source ~~5X lower load ~~5X higher. 120r source to 3k load will have little insertion loss, while 600:600 makes 6dB insertion loss. With offset impedances the two cap values will have to be offset, not the same.

squarewave

Re: High pass CLC T Filter component values
« Reply #23 on: June 21, 2019, 08:54:18 PM »
Figure reactance of 0.027H:
20Hz - 3.4 Ohms
200Hz - 34 Ohms
2KHz -340 Ohms
20kHz - 3400 Ohms

So a 20Hz filter will probably need a "loudspeaker power amp" to drive it.
Good point. Translation: 27mH is too small for low frequencies. Even if the last tap makes a much bigger inductor it's probably not enough for low-cut (20Hz 600 ohms = 2.4H). What are the taps exactly?

abbey road d enfer

Re: High pass CLC T Filter component values
« Reply #24 on: June 22, 2019, 12:16:45 AM »
For the 50k load I have a 5.128KHz filter frequency.
For the 1k load I have 5,462KHz.
In most (all?) cases, the corner frequency is not the only parameter that matters; what happens below and above is at least as important. IMO you can't neglect that.

Quote
In my case and for the application of this filter, this difference in the filter frequencies is not important.
Can you be more specific about this application?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Re: High pass CLC T Filter component values
« Reply #25 on: June 22, 2019, 02:16:56 AM »
You also need the correct source impedance to get perfect damping. AC response should be perfectly flat and then a sharp nose over into a perfectly straight slope of 18dB per octave.

Try this:

  https://www-users.cs.york.ac.uk/~fisher/lcfilter/

Select Butterworth > Highpass > 3rd order > 1000 Hz > 600 ohms

Results: 48 mH, 265nF

I love high/low cut filters. But you definitely have to get the damping right. Otherwise it's going to sound like a stuck mu-tron.

Thanks for the info, but my original question is based on a scenario where inductance values are already given.
If not I would choose the values already given by these online calculators.
« Last Edit: June 22, 2019, 04:09:22 AM by LazyTurtle »

Re: High pass CLC T Filter component values
« Reply #26 on: June 22, 2019, 02:34:43 AM »
In audio we usually use Q near unity. Therefore the resistive impedance will be similar to the inductive (and capacitive) impedance. This, with practical limits on resistance, defines what is "practical".

Figure reactance of 0.027H:
20Hz - 3.4 Ohms
200Hz - 34 Ohms
2KHz -340 Ohms
20kHz - 3400 Ohms

So a 20Hz filter will probably need a "loudspeaker power amp" to drive it. (Also max current and DCR will matter.) A 4KHz filter will be near 600 Ohms, a happy zone for "passive" in a studio. A 20KHz filter will want to drive a several-Kohm load, and be awkward in a studio line.

Thank you for the info.
Yes i took 27mH for the higher frequency. The multitapped inductor has higher inductance values that will be used for lower frequencies.

From there: you can try "matched" resistive impedances, or try making source ~~5X lower load ~~5X higher. 120r source to 3k load will have little insertion loss, while 600:600 makes 6dB insertion loss
Very interesting, so assuring a load impedace 5x bigger than source impedance will result in very low insertion loss.
I believe most modern audio mixers have output impedances much lower  than input impedance(typical value of hundreds of ohms for output impedance vs 10k or more for input impedance)...So, in case what I´ve understood is correct, this insertion loss problems then should not be a problem any more. Correct?

With offset impedances the two cap values will have to be offset, not the same.

But this would only be needed if I wanted a flat response, correct? What other side effects other than resonance or phase would have using equal value capacitances without matched impedances?
« Last Edit: June 22, 2019, 04:22:26 AM by LazyTurtle »

Re: High pass CLC T Filter component values
« Reply #27 on: July 01, 2019, 01:41:14 AM »
No additional feedback on these last questions?


 

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