DIY Phase Plug for Lowther PX3 in Oris 250 Horns

Previously, I attempted to solve the Lowther DX3’s nasty treble by lining a paper towel over the driver like this.

Paper towel over the whizzer on the Lowther DX3

The results were mixed. While this approach smoothed out the frequency response at 11.25 degrees off-axis, it didn’t do much for the on-axis response. We can see this in the plot below. While paper towel strip did slightly even out the dip at 7kHz, it didn’t do jack to the upper mid and treble peaks at all. Given how the paper towel pushed down the treble (at 11.25 degrees off-axis), the polar response was even less uniform than before! (As an aside, any kind of EQ would be pointless unless the polar response can be made more uniform).

Green = Stock on-axis
Yellow = Paper towel strip on-axis

I knew that a better solution would be required. I searched the Internet and discovered possible solutions such as the DX4’s pepper pot phase plugs and bulb / mushroom type phase plugs. I ruled out the DX4 phase plug based on these measurements which seemed show no better on-axis performance. A few subjective impressions indicated that the pepper pot plug actually made the Lowther drivers even more directional, which is not what I wanted. Thus I opted down the path of bulb / mushroom type plugs. Unfortunately, I couldn’t find any on the Internet for sale; and I didn’t want to spend time on a wood lathe to construct one (another issue would be finding a mounting screw of the right size and thread pitch, which no one on the Internet seems to know).

So being the cheap bastard that I am, I opted for this: a folded paper dodecahedron from http://www.korthalsaltes.com/

I made the version with an extraneous trianglular fold that I could tape on to the existing phase plug. Here it is fit onto the driver in the horn. LOL! I do think it looks much better than the paper towel strips!

Lowther DX3 in Oris 250 with folded paper dodecahedron phase plug

Let’s take a look at some on-axis measurements with the folded paper dodecahedron phase plug in place. Note that I took these measurements inside the house, so the baseline measurements with the stock phase plug might be slightly different from prior posts where I took the measurements outdoors.

Yellow = Dodecahedronon plug on-axis
Gray = Stock phase plug on-axis

Lowther DX 3 Oris 250 Horn with Dodecahedron Phase Plug On-Axis

Holy crap, this actually works! The dodecahedronon plug doesn’t do much to the 8.5kHz peak, but it eliminates that big honking peak at 4.5kHz (which seems to be a huge contributor to the horn coloration and treble nastiness). We end up with narrow dip at 5kHz, but I’ll take this dip over that peak any day. Subjectively, there is quite a bit of difference while sitting directly in front of the horn, placing and removing the dodecahedronon plug.

Now here’s the 15 degrees off-axis frequency response with the folded paper dodecahedron phase. (15 degrees happens to be the incident angle at the listening position):

Yellow = Dodecahedronon plug off-axis 15 degrees (approximate)
Gray = Stock phase plug off-axis 15 degrees  (approximate)

Lowther DX 3 Oris 250 Horn with Dodecahedron Phase Plug 15 Degrees Off-Axis

Compared to the stock plug, the dodecahedronon plug measures with slightly more emphasis between 5-6kHz (not necessarily bad because this region is now more consistent with the on-axis response), but lowers the problematic 7kHz to 10kHz area.

Subjectively, the dodecahedronon plug offers a good improvement over the stock plug and paper towel strip approaches. The more uniform polar response is definitely noticeable – the sound is more consistent when I move my head forward, back, left, or right a few inches while sitting on the couch. There’s almost a mellow quality to the DX3s now, to such an extent that that I needed to toe out the horns a bit from 15 degrees off-axis to 10.

One thought on “DIY Phase Plug for Lowther PX3 in Oris 250 Horns”

  1. Hey Marvey, welcome to the Oris club!

    Well done on finding some horns in the USA, and thanks for the shout out on mine on that SBAF thread.

    It looks like you’re starting to get some nice results there with the DX3’s in the Oris 250. That paper phase plus is a clever idea, and seems to work well. You could now go down the road of buying some ultra expensive Japanese paper and selling a ton of them made out of that. I’m sure they’d transform the treble quality no end….

    FWIW a few things I’ve found, and commenting on a couple of things on your set up:

    1) For the missing highs, I’m using a Beyma CP-21/F compression tweeter, rolled in with just a cap and padded down just slightly. If you look at the pics of mine and you’ll see the tweeter hanging down on the scaffolding under the horn. I’m sure the Fostex horns are better than the Beyma, but in Europe they are an expensive add-on.

    2) Rolling the horn off lower is definitely better than at 500hz. My current set-up uses a 1st Order PLLXO on the 2A3 SET driving the AER’s at 300hz. Though I’ve just bought a First Watt B4 so am experimenting with higher orders and slightly different slopes too. A 2nd order PLLXO would be fine too assuming you’re running amps with a 100khz load.

    3) Having the horns so that you can look level into the phase plug seemed the best height to use the Oris I found. I can’t see properly but yours seem quite low? If you add a tweeter underneath that would be slower still. I did experiment once with an actively powered ribbon hung inside the horn.

    4) Active bass / biamping is a definite improvement. I’m currently using a Behringer Class D amp with DSP crossover. I’m using 4th order at 300hz to roll in.

    5) An upgrade to AER’s, AER cones on the DX3 magnet or even a Voxative is the best upgrade and renders tweeters and fancy phase plugs less important. I know know, it then starts to get expensive! Or actually a DX4 if you can find one cheap is a worthwhile improvement from the DX3’s too. The DX3 and PM6’s have the worst “shout” of them all I think.

    6) I’d be interested on where you think the colourations are. I don’t get a lot on mine (maybe the AER’s also help here)? I did also experiment with the back chamber loading. Does yours have a reflex hole or is closed? Altering this does definitely alter the impedance plot, its worth looking at that.

    All good fun though I’m sure for you. Your black horns have inspired me though to change the colour of mine. Having the Oris 200’s sprayed in the colour on of my expensive curtains wasn’t the smartest move…

    I’ll keep up with this thread now I’ve found it. Good luck with future changes!

    Kind regards,

    Jonathan

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