PLLXO - tractrix, semi open baffle & sealed bass

[hasselbaink]

PLLXO - tractrix, semi open baffle & sealed bass

This thread is related to the topic discussed here:
Passive line level x-over - CAN IT BE DONE?

And yes, I can answer in the affirmative. I believe PLLXO, which is an offspring of the active topology, has some properties that are superior to the traditional passive loudspeaker crossover. For someone like me who seeks high efficiency playback of 100 dB sensitivity or higher it is the way to go unless one can accommodate huge bass line arrays using multiple large woofers. Another consideration prior to any design decision was that the bass channel should gently transfer into the lower midrange in order to capitalize on the ATC drivers superior abilities in this region.
The improvement in sound using PLLXO is readily audible with large orchestral pieces and complex music in general. The sound and the imaging never falls apart or becomes muddled even during multiple crescendos. Working with the speakers time alignment I have managed to get a piano to sound like a piano.
There are of course other design choices than PLLXO that contribute to the fidelity of this system which means that PLLXO is not a magic bullet. You need to do a lot of work on the line level design (although not as much as you'd do on a fully passive speaker level design), and you need to consider the behaviour of each speaker section in order to arrive at the right high pass and low pass network. This means doing a lot of measurements as mandated.

One of the advantages of this approach is that you take much of the strain away from the tube amp such as most of the load in the bass register, which is good news for a 20 watt PP amplifier like mine. Imagine the benefits a single ended triode design would have under these conditions!! Well, nothing new under the sun, some might say but I have found out that a surprising number of audiophiles not only do not know about the benefits of active speakers, some are not even aware of the possibility or of the sonic benefits of this method. Therefore I feel comfortable reiterating these points.

The title of this thread refers to parts of a playback system consisting of a 1. order 2-way passive line level cross over working in the 100-150 Hz range. The pllxo combines the bass channel with the semi OB midrange driver.
The bass channel is a 200 liter sealed enclosure with a pair of ATC SB100-375 15 inch drivers (see picture). The midrange channel is an 18Sound 12" driver in a semi OB construction. The baffle has very very graduate round overs (see pic) and the open backed box is tightly stuffed with acoustic wool and open cell foam. This driver combined with the horn section has a sensitivity of 100 dB. The bass section is driven by a Technics SE 9060 while a Radford STA25 handles the midrange and horn section. The tractrix horn is mounted on a Coral M-100 compression driver which is the only "vintage" piece in the speaker system apart from the Radford. The whole concept should be fairly easy to implement with current production drivers and amps should one wish to do so. This is not a project for the novice, but if you have a couple of speakers under your belt and you done a few cross overs it should be doable.

This is the midrange/horn section:


The bass channel:


Amps + pllxo box:


Pllxo:


There are some older measurements of the midrange/horn section in this thread:
Horn & mixed-order XOver. Please criticize!
These are not really up to date but give an impression of what is possible from upper bass and upwards.

I am quite satisfied with the sound in a way I haven't been with my other speakers including non-diy types. There are things I would like to try differently such as a all tube amp set up. I really think many solid state amps have improper bass presentation. They lack roundness and the tone is curtailed in an over-dampened fashion. The bass channel is 91 dB sensitive and the impedance is very benign so I think a tube amplifier would work well.

You are all welcome to criticize, ask questions or make suggestions.

Best
Kris

[Hylle]

Hello Kris, Very interesting project you have there. I have thought about getting these stereolab horns, and combine it with a 2" compressiondriver. Can you comment on the sound? I am a bit affraight of the nasal honky sound from some horns. It is a lot of money to spend if it is not for ones tast. I plan on using a tweeter on top of the tractrix horn. I think you setup is really nice!
Best, Allan

[hasselbaink]

Velkommen Allan!

Regarding the Stereolab horn, I think this represents a good value for money solution to horn loading. You can buy a nicer looking wood horn for a lot higher price but I doubt that it will actually sound better.
The tractrix contour has no horn honk if used correctly. This means not crossing the driver too close to the horn cut-off frequency. I use a 400Hz horn crossed at ~800 Hz so that the compression driver is loaded all the way down. Another thing to consider is that most horn types sound best over a limited bandwith. So a 2" driver could be crossed at 500 Hz - 2kHz or maybe 4kHz. You should decide for yourself where the driver sounds best. I feel that a tweeter on top is the right way to go and this is also something I will pursue in the near future. Suggestions are welcome as long as they are accompanied by an explanation for why this particular tweeter is recommended (How does it sound?).

The sound of the Stereolab horn with the Coral m-100 is very civilized and pleasant. I think it would be even better if I used in in a narrower band. It handles dynamic swings very well almost gracefully. A 2" driver would be even better in this regard. The sound is just a tiny bit dry but I believe this is caused by the push-pull tube amp and possibly the high frequency roll off that is characteristic of the Coral driver.
The 12" speaker is crossed to the horn passively and these two elements are carefully time-aligned which is one of the better design decisions I have made. I will state that the imaging has lost sharpness but gained definition.

I have to advice you that the sweetspot of round tractrix horns is narrow unless you are more than 3 meters/ 10 feet away from the speaker. Some might think that is unacceptable, others not. The coverage of the horn is 60 degrees so the bass-mid should match this dispersion.

Best
Kris

[Hylle]

hej kris, I guess a narrow dispersion could be good, then there will be less refection from the room.
I am suppriced you belive that the horn should be used over such a small frequency range. I have hoped to use it from 1000hz to 6-7000Hz.

[hasselbaink]

Originally Posted by Hylle

hej kris, I guess a narrow dispersion could be good, then there will be less refection from the room.
I am suppriced you belive that the horn should be used over such a small frequency range. I have hoped to use it from 1000hz to 6-7000Hz.

This is actually less than three octaves and a good range to cover with a single driver if it is well behaved in this range. I am not an expert on 2" compression drivers but I'd try to get it down to 500 or lower (for home use not pro applications).

If it sounds great up to 7kHz - great stuff, otherwise roll in the tweeter lower.
Might I ask what driver you will be using?

[Hylle]

For a start, I plan on using an older Electro Voice T25A, that I can borrow from a friend. Later I am thinking to use something without metal cone, like BMS, B&C or Obertone, a brand I feel looks very good at least on datasheets, and they are suppose to be cheap.

[hasselbaink]

Originally Posted by Hylle

For a start, I plan on using an older Electro Voice T25A, that I can borrow from a friend. Later I am thinking to use something without metal cone, like BMS, B&C or Obertone, a brand I feel looks very good at least on datasheets, and they are suppose to be cheap.

I just googled the EV driver and it seems to be a non standard size driver. The exit is 7/8 inch which means it might need an adapter to fit a 1" horn. Perhaps a gasket would help make the transition to the 1 inch horn throat...

[Hylle]

Yes you are correct, That is also why I am hesitating to use this driver, but my friend recomends it highly. But I would like it to have a bigger voicecoil/cone.