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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
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Old 20th September 2020, 01:30 PM   #1
xrk971 is online now xrk971  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
Default Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide

I have been using the RS225-8 for years in a 2-way FAST with a full range for a tweeter. Recently, I have been using an RS28F in a Monacor WG300 waveguide with a Purifi PTT6.5 TL speaker with great success. Since I already have FRD and ZMA files handy for these drivers I wondered how well the combo of the RS225-8 and the RS28F in a WG300 would work out together. A very quick simulation in Xsim seemed to yield pretty good results. Although the RS28F is no longer available, I have tested the RST28F in the same waveguide and it measures very close - allowing for similar performance. The crossover is based on the assumption of a certain acoustical delay which I have not been able to measure yet, but estimated based on the positions of the voice coils on a baffle and the depth of the waveguide. The crossover I came up with is strange in that it has a deep notch if invert the polarity of the tweeter and sums up fine if left normal. Not sure why this is.

Schematic for crossover with 1.5kHz crossover frequency:
Click the image to open in full size.

Predicted frequency response (if we mount this in a TL, the bass below 100Hz will extend to 30Hz):
Click the image to open in full size.

Predicted impedance curve (min ~6ohms):
Click the image to open in full size.

Predicted step response showing great time alignment:
Click the image to open in full size.

Next step is to make a test baffle and measure in-situ. Is this worth a try and what do you guys think about the polar integration between an 8in driver and 7in waveguide if XO is at 1.5kHz?

One other nice thing about this XO design is that it would fit perfectly into my existing passive XO PCB which has been used for both of the speakers mentioned above, and would look something like this:
Click the image to open in full size.
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Last edited by xrk971; 20th September 2020 at 02:10 PM.
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Old 20th September 2020, 01:46 PM   #2
boswald is offline boswald  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
Why do you think that the notch on polarity flip is strange? The better aligned the waves, the deeper the notch(more perfectly out of phase) on reversal of one of the drivers.
An old check on phase in the xo region.
It means you guessed the offset well.


With that and similar areas coupling to the room with 8" & 7" your only concern might be running out of tweeter excursion at higher volume.


Should be fine with the TL version where the RS runs out of xmax a few db earlier, 'cause going lower.

Last edited by boswald; 20th September 2020 at 01:48 PM.
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Old 20th September 2020, 02:00 PM   #3
xrk971 is online now xrk971  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
I guess I have always heard that second order crossovers need inverted tweeters to not have the cancellation notch at the crossover frequency. Perhaps it’s because they are on flat baffle with no waveguide? The waveguide reduces cone excursion on the tweeter so should help it achieve the SPL of the RS225.
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Old 20th September 2020, 02:56 PM   #4
twinter is offline twinter  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
"
Quote:
I guess I have always heard that second order crossovers need inverted tweeters to not have the cancellation notch at the crossover frequency.
That statement is true for straight filter theory, not taking into account the time/phase alignment of the two driver acoustical centers.

1) straight crossover electrical filter
2) crossover electrical filter + drivers actual impedance = total system electrical response
3) crossover electrical filter + drivers actual impedance + driver acoustical response + time alignment relationship of drivers = total acoustical response at microphone


A crossover filter shifts 45 degrees per order at the crossover frequency. A crossover filter has separate high pass and a low pass filters.
For a 2nd order hi/low pass crossover, that's 2 x 45 degrees for the tweeter crossover in one phase direction, and 2 x 45 degrees for the woofer in the other phase direction, for a total of 180 degrees between the two outputs.

The difference in travel time from each driver to the measuring microphone need to be addressed.
An estimate is from the front of the drivers voice coil to the microphone, but acoustical measurements are best.
At a temperature of 72F degrees, the speed of sound is 13560 in/sec (1130 ft/sec).
At a crossover frequency of 1.5 kHz, some phase and distance relationships are listed:
1 wave length Phase 360 degrees = 9.04 inches
1/2 wave length Phase 180 degrees = 4.52 inches
1/4 wave length Phase 90 degrees = 2.26 inches
1/8 wave length Phase 45 degrees = 1.13 inches

Or every 1/4" = 10 degrees phase at 1.5 kHz

Last edited by twinter; 20th September 2020 at 03:22 PM.
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Old 20th September 2020, 02:59 PM   #5
mark100 is offline mark100  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
Quote:
Originally Posted by xrk971 View Post
I guess I have always heard that second order crossovers need inverted tweeters to not have the cancellation notch at the crossover frequency. Perhaps it’s because they are on flat baffle with no waveguide? The waveguide reduces cone excursion on the tweeter so should help it achieve the SPL of the RS225.
Hi xrk971, nice!
And I think it's strange too.
Yep, a two-way 2nd order should need an inverted high.

After verifying driver polarities with a battery, I always suspect a time-misalignment of 1/2 period at xover frequency..... which produces some ripple, but gets rid of the notch.
Below is the electrical summation from 2nd order, 1500Hz 1/2 period off.
As you can see, entirely possible it might get lost in acoustical measurements.
But 1/2 period is 0.33ms, or about 4.5"...which is alot of distance .....
So I dunno, but something does seem off.....

I give thumbs up to the 8" driver with 7" waveguide at 1.5kHz. Looks like about ka=2.5 for the driver., which i personally like.
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File Type: jpg ripple due to half period.JPG (35.7 KB, 42 views)
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Old 20th September 2020, 03:42 PM   #6
xrk971 is online now xrk971  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
Thanks for that analysis. 4.5in does seem a bit much. I will probably need a stepped baffle to bring the woofer closer to front or 3D print a custom deeper waveguide. We will see once measurements are made in same baffle.
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Old 23rd October 2020, 04:28 PM   #7
oxoxbyx is offline oxoxbyx
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very exciting project to replace 10F/8424 with tweeter, hows it sound ?
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Old 23rd October 2020, 04:39 PM   #8
xrk971 is online now xrk971  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
It’s only theoretical at this point - the simulation was done using measured FRD files for each of the drivers in a 10in wide baffle, but has not actually been built together yet. I do have some older test cabinets made from XPS foam and with a thin plywood veneer. I would have to cut the baffle larger to fit the WG300 waveguide. Or, I have a spare 3/4in BB plywood baffle already cut for an RS225 and a smaller full range. I could get that properly cut for the waveguide rebated and all. Install it in my 24L sealed BB ply cabinet and see how it goes. But the power and clarity of the RS225 is really unleashed when it is mounted in a tapered PMC style TL.

This could sound very good I think as the WG will help you integrate the tweeter with the woofer’s polar response much better.
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Old 10th August 2021, 05:59 PM   #9
jimk04 is offline jimk04  United Kingdom
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I am considering having a go at something like this. I have built the RS225 WAW speaker . I used the PS95. I love them but just recently for no reason other than they are there I put back into my system a build I did with an SB17NBAC35-4 and a DA25BG08-06 tweeter. Had them in place for a week or so.
I think it has made me realise that to my ears, in my room that a tweeter performs better.
I have some RST28F and the waveguides so with a new baffle and XO I can try this design.
When I get the time I shall try this.

Thanks for rhe design X.
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Old 10th August 2021, 07:29 PM   #10
xrk971 is online now xrk971  United States
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Passive XO for Time-Aligned RS225-8 and RS28F in Waveguide
Hi Jimk04,
Great - thanks for giving this a try! I just have been too busy to give it a go. Good luck!
X
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