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18th November 2019, 04:23 PM  #51 
diyAudio Member
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.
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: As expected, the frequency response of the summed output is pretty much flat. Gain loss of 67dB is due to the resistors of course. What can I say, it works. :) 
19th November 2019, 04:23 PM  #52 
diyAudio Member
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: Could someone explain what these additional lines indicate? They look quite irregular which makes me wonder. Especially the loopback curve. 
19th November 2019, 05:51 PM  #53  
diyAudio Member
Join Date: Mar 2016

Quote:
Perhaps your 13.6 dB is specific to the specific circuit used to achieve the LinkwitzRiley response. The generic SallenKey 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 similarlooking topology, keeps the frequencydetermining C's equal to each other, but tweaks the voltage gain of the opamp 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 equalcapvalue implementation is much more practical in many cases. Since your LinkwitzRiley filter is two cascaded secondorder 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 lowQ 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 lowQ 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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