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Help needed: Linkwitz-Riley frequency response curves
Help needed: Linkwitz-Riley frequency response curves
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Old 18th November 2019, 05:23 PM   #51
yannikab is offline yannikab  Greece
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Join Date: Aug 2019
Hey remember me? Long time no see. I 've had the LR crossover delivered for a while now, but only got the chance to tinker with it now. Here's a frequency response graph from HOLMImpulse, for your viewing pleasure. The crossover point I requested was 60Hz.

Help needed: Linkwitz-Riley frequency response curves-png

Blue line is my audio interface recording itself with a loopback, as a baseline indication. Besides the frequency splitting, the crossover offers 13.6dB of gain which is pretty welcome. I think I 've come across this gain value in the past so I am assuming it's directly related to and a specific result of using the NE5532 opamps.

Actual crossover frequencies are 61Hz for the left channel and 62Hz for the right, so they got it right. I asked them to not go under 60Hz so they made sure the components satisfied that requirement. Just for fun, I 'll be measuring the capacitor values because I think that's where the small discrepancy between left and right channel comes from.

Just for fun, I hooked up a 10K resistor to each of the low pass and high pass outputs, connected the resistors, and measured the summed output:

Help needed: Linkwitz-Riley frequency response curves-png

As expected, the frequency response of the summed output is pretty much flat. Gain loss of 6-7dB is due to the resistors of course.

What can I say, it works. :-)
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File Type: png b.png (29.3 KB, 73 views)
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Old 19th November 2019, 05:23 PM   #52
yannikab is offline yannikab  Greece
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Join Date: Aug 2019
Hello again,

I discovered this nice checkbox on HOLMImpulse's interface labeled "Distortion". I picked THD and these are the results for loopback, lowpass, highpass:

Help needed: Linkwitz-Riley frequency response curves-loopback-png

Help needed: Linkwitz-Riley frequency response curves-lowpass-png

Help needed: Linkwitz-Riley frequency response curves-highpass-png

Could someone explain what these additional lines indicate? They look quite irregular which makes me wonder. Especially the loopback curve.
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File Type: png loopback.png (22.0 KB, 24 views)
File Type: png lowpass.png (26.1 KB, 24 views)
File Type: png highpass.png (22.3 KB, 25 views)
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Old 19th November 2019, 06:51 PM   #53
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by yannikab View Post
...13.6dB of gain...I am assuming it's directly related to and a specific result of using the NE5532 opamps.
The NE5532 is a very nice op-amp that can be configured for any voltage gain from a rather useless zero (in inverting mode) up to x100,000 (i.e. +100 dB), its open-loop gain at DC and very low frequencies.

Perhaps your 13.6 dB is specific to the specific circuit used to achieve the Linkwitz-Riley response. The generic Sallen-Key active filter topology ( Sallen–Key topology - Wikipedia ) has unity gain in the pass band, but sometimes needs awkward values of R and C.

An alternative, very similar-looking topology, keeps the frequency-determining C's equal to each other, but tweaks the voltage gain of the op-amp above unity to achieve the desired frequency response; one gain for a Bessel response, another gain for a Butterworth response, and so on.

It's a lot easier to buy two 10nF 1% caps than to find one 10nF and one 14.7 nF cap, so the equal-cap-value implementation is much more practical in many cases.

Since your Linkwitz-Riley filter is two cascaded second-order Butterworth filters, one possibility is that the gain of each Butterworth stage is half of 13.6 dB, or 6.8 dB. I haven't researched that at the moment (in a rush), but that does seem a wee bit high from memory, particularly for a low-Q Butterworth response.

Another plausible explanation is that multiple Ebay sellers are all selling copies of the same circuit, probably published in Elektor or Wireless World a few decades ago, and the original circuit just happens to have had 13.6 dB of gain.

I wouldn't worry at all about 60 Hz vs 61 Hz corner frequencies - really a meaningless difference in such a low-Q filter type, where the "corner" is very imprecise and extends over quite a range of frequencies. A bit like looking for the edge of the rainbow.


-Gnobuddy
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