xrk971 Got Foam? diyAudio Member



Join Date: Oct 2012 Location: Metro DC (Virginia)

S. Harsch XO







I first started with the B&O XO which requires a wide bandwidth mid that is band pass filtered with 1st order Butterworth and straddled on either side with a LR2 filter for the woofers and tweeter. I was then turned onto the

1. Set the low pass filter for the woofer as a 4th order Butterworth at central frequency, fc for the XO centerpoint.

2. Set the high pass filter for the tweeter as a 2nd order Bessel at fc.

3. Set the delay of the tweeter equal to 1/2 of the period of one cycle at fc.

4. Use all positive phase on woofer and tweeter.



Here is an example of the XO plot from the Harsch paper:





Here is the step function from the above XO as predicted by Harsch:





So I tried it out on my 2-way





For the 10F/RS225 FAST, this was the corresponding measured Step Response:





For the 10F/RS225 FAST, here was the measured phase (notice the little bump at the XO frequency as predicted - but no wrap):





For the 10F/RS2225 FAST, here was the measured group delay, not over 5ms above 40Hz:





What the Harsch XO does nicely is that it allows one to use a steep XO to block out the nasty cone breakup on a woofer with a high order filter. Yet still not have a large phase wrap in order to maintain a time aligned transient perfect response.



So I got to thinking if this can be applied to a 3-way by mirroring the XO topology. That is Butterworth 4th order for the low pass on the woofer, Bessel 2nd order for the mid high pass and Bessel 2nd order for the mid low pass (or Bessel 2nd order bandpass for mid), and a Butterworth 4th order for the tweeter high pass. So what this means is that pretty generic components can be used in that a special wide band mid is not required anymore as the filter is already a fairly steep 2nd order. And for the woofer and tweeter, we can use 4th order filters which are great for controlling distortion, and preventing cone breakup leak-thru from the woofer.



As I had a 3-way XO test bed already handy, it was just a matter of putting the XO settings into miniDSP. So here is the electrical XO I implemented in miniDSP:







And here is the resulting acoustic XO that was measured (yes the SPL levels are fairly low so that I did not wake up the family during sweeps). As you can see, the curves are fairly common and innocuous. The 1st order textbook curves of the B&O were especially hard to achieve in practice. With the Harsch topology 2nd order and 4th order are much easier to get. The tricky thing is what to do for the time delays? Well, it is exactly the same delay assuming driver acoustic centers are already time aligned. The mid needs to be delayed from the woofer by 0.5/fc_low and the tweeter needs to be delayed from the mid by 0.5/fc_high. For my case, I used a 500Hz and 3500Hz XO points:







Here is the resulting Impulse Response:







And here are the resulting Step Responses - I know there is a little pre-rise but overall it is much better than a jumbled up-down-up-down mess you would see with an all LR2 or LR4 XO:







This is still a work in progress. Does anyone see any problems with it? I think this works. I am still having a problem getting the phase to be wrap free - it seems to wrap at the centeral frequency and not at the low and high XO frequencies. Anyhow, still debugging it. This XO may be useful for folks looking for an easy to implement quasi-transient perfect XO using IIR filters in DSP. How does it sound? Fantastic! My speakers are essentially 98dB sensitive and super loud and clean with this XO. I am really enjoying the sound and how the percussive sounds like guitar and drums really have a "live" feel to them.



Attached are some sound clips (stereo recording using Zoom H4 of the mono- Right channel playing). Change file extension from .asc to .mp3 in order to listen. This work started in the Filler Driver thread . I was studying transient XO's and built a test bed speaker to quickly test out new XO ideas with miniDSP. Here is a photo of my test bed. It is a basic open baffle for ease of construction and has a pair of Eminence Beta 12cx (no CD, with coax horn sealed up), a pair of PRV 5MR450NDY mids, and a PRV D280Ti with a PRV 45x45 deg waveguide. These are all fairly efficient drivers and are what I had on hand. The amp is provided by TDA7498 100w class D amps powered by 19v (mid and tweet) and 24v (woofers) SMPS bricks.I first started with the B&O XO which requires a wide bandwidth mid that is band pass filtered with 1st order Butterworth and straddled on either side with a LR2 filter for the woofers and tweeter. I was then turned onto the XO topology proposed by Samuel Harsch by member Phase_Accurate. The paper is in French but I figured out that the basics are:1. Set the low pass filter for the woofer as a 4th order Butterworth at central frequency, fc for the XO centerpoint.2. Set the high pass filter for the tweeter as a 2nd order Bessel at fc.3. Set the delay of the tweeter equal to 1/2 of the period of one cycle at fc.4. Use all positive phase on woofer and tweeter.Here is an example of the XO plot from the Harsch paper:Here is the step function from the above XO as predicted by Harsch:So I tried it out on my 2-way 10F/8424 & RS225-8 FAST Ref Monitors and got this for the XO plot:For the 10F/RS225 FAST, this was the corresponding measured Step Response:For the 10F/RS225 FAST, here was the measured phase (notice the little bump at the XO frequency as predicted - but no wrap):For the 10F/RS2225 FAST, here was the measured group delay, not over 5ms above 40Hz:What the Harsch XO does nicely is that it allows one to use a steep XO to block out the nasty cone breakup on a woofer with a high order filter. Yet still not have a large phase wrap in order to maintain a time aligned transient perfect response.So I got to thinking if this can be applied to a 3-way by mirroring the XO topology. That is Butterworth 4th order for the low pass on the woofer, Bessel 2nd order for the mid high pass and Bessel 2nd order for the mid low pass (or Bessel 2nd order bandpass for mid), and a Butterworth 4th order for the tweeter high pass. So what this means is that pretty generic components can be used in that a special wide band mid is not required anymore as the filter is already a fairly steep 2nd order. And for the woofer and tweeter, we can use 4th order filters which are great for controlling distortion, and preventing cone breakup leak-thru from the woofer.As I had a 3-way XO test bed already handy, it was just a matter of putting the XO settings into miniDSP. So here is the electrical XO I implemented in miniDSP:And here is the resulting acoustic XO that was measured (yes the SPL levels are fairly low so that I did not wake up the family during sweeps). As you can see, the curves are fairly common and innocuous. The 1st order textbook curves of the B&O were especially hard to achieve in practice. With the Harsch topology 2nd order and 4th order are much easier to get. The tricky thing is what to do for the time delays? Well, it is exactly the same delay assuming driver acoustic centers are already time aligned. The mid needs to be delayed from the woofer by 0.5/fc_low and the tweeter needs to be delayed from the mid by 0.5/fc_high. For my case, I used a 500Hz and 3500Hz XO points:Here is the resulting Impulse Response:And here are the resulting Step Responses - I know there is a little pre-rise but overall it is much better than a jumbled up-down-up-down mess you would see with an all LR2 or LR4 XO:This is still a work in progress. Does anyone see any problems with it? I think this works. I am still having a problem getting the phase to be wrap free - it seems to wrap at the centeral frequency and not at the low and high XO frequencies. Anyhow, still debugging it. This XO may be useful for folks looking for an easy to implement quasi-transient perfect XO using IIR filters in DSP. How does it sound? Fantastic! My speakers are essentially 98dB sensitive and super loud and clean with this XO. I am really enjoying the sound and how the percussive sounds like guitar and drums really have a "live" feel to them. Attached Images harsch-3-way-miniDSP-xo.png (104.6 KB, 7844 views) harsch-3-way-xo.png (86.6 KB, 9010 views) harsch-3-way-ir.png (40.1 KB, 7626 views) harsch-3-way-sr.png (72.5 KB, 7994 views) Attached Files harsch-3-way-clip-1.asc (1.73 MB, 87 views) harsch-3-way-clip-2.asc (1.72 MB, 35 views) Last edited by xrk971; 27th July 2015 at 06:11 AM .