MTM Heresy

[cfelliot]

The past few weeks I have been exploring the idea of

building a speaker that is smaller than a Cornscala, but still provides that

big midrange horn sound.

I currently run Heresy IIIs with dual subs crossed at 85Hz.

The bass is much better than my former Cornwalls, but the midrange could be

better.

As a second objective, I’d like to raise the efficiency a

bit.

My first though was the Eliptrac horn with 2 Heresy woofers.

The issue here is the 20” width.

Then I found the Faital Pro HF140/ LTH142 Tractrix combo.

This and the 2 Heresy woofers will fit on a 16” by 38.75” motorboard and, a enclosure

of 17.5 x 40.25 and a depth of about 12”. This gives 4 ft^3 volume, sealed.

Here is my concept drawing:

On Monday I approached a board member about building the

motorboard for me. Well not only did he give me information on doing so, he

built a MDF prototype, crossover(500Hz), and did some initial testing. I am

some impressed!

Here is a picture of the prototype:

The first graph shows the relative output of the low and

high ends which is in favor of the horn, as you would expect, by 6dB. Given the

109dD rating of the horn, this says the system would be 103dB.

This graph shows the output with the prototype 500Hz

crossover.

This looks great!

This will be an expensive speaker which is still limited in

the low end compared to a Cornscala, but to me I don’t really care. My subs

give me the bass I want.

I would really like a critique of this system before I

commit to this build.

[Daddy Dee]

I'd love to hear that.

[cfelliot]

I'd love to hear that.

Well you're only about 140 miles away from the prototype which is a lot closer than me! []

[MikeFord]

Looks interesting, but shouldn't the crossover be higher the spec on the 140 is Minimum Crossover Frequency* 0.9 kHz with the * meaning 12db/oct.

My own experience is that "voicing" a speaker, working out the details of best crossover and placement has more than a few trial and error steps, so not sure I would want to live with a speculative design without listening to it and some options.

[djk]

An MTM works best with either 6dB or 18dB, 12dB is not a preferred choice.

I would try 900hz at 6dB.

[cfelliot]

The prototype crossover is a 2^nd order at this
point.

Word I get from the builder is that higher crossover points
don’t look as good as 500Hz.

I also hear the both that 140 and its 2” big brother the 200
both are good far below their recommended crossover points. This seems right in
that they are both being used in Cornscala projects.

I have no doubt that the crossover design is going to be
critical with this speaker. Various
articles say a 2^nd or 4^th order Linkwitz-Riley (6dB down
at CP) is critical for the improved polar response. I find myself thinking that
the cost of a 4^th order might make an active crossover an attractive
alternative for this design.

I don’t pretend to know as much as some of you and that why
I welcome these comments.

[djk]

"Word I get from the builder is that higher crossover points
don’t look as good as 500Hz."

He may not know what to try.

For the MTM a B18dB crossover works the best., for a non-MTM design I would consider a LR24. When one speaks of any crossover being ..........., he is speaking of the transfer function, not the response of the filter. An example might be an MTM design I did awhile back that had a 12dB HP that combined with the tweeter horn acoustic response had an 18dB transfer function. In a non-MTM design with the same tweeter horn I used a 12dB LP with an 18dB HP and achieved an acoustic LR24 transfer function.

"I also hear the both that 140 and its 2” big brother the 200
both are good far below their recommended crossover points."

90% of the response of the horn/driver combo is the horn, not the driver. The WE555 is a 2" aluminum diaphram compression driver (the K55Vcopied the basic diaphragm and phase-plug geometry in phenolic) that could be used down to 100hz on the appropriate (huge) horn. Olson showed a horn that responded to 40hz~13Khz with the WE594, a 4" aluminum diaphragm compression driver.

Mechanical stress and power handling determine the practical limits otherwise. Diaphragm motion increases by 4x every time you go down an octave, a driver that might handle 160W crossed at 1Khz might only handle 40W crossed at 500hz (JBL 375/2440), and only 10W at 125hz. The WE555 was rated for 5W, but will handle far more when crossed higher. A K55V will handle 40W when crossed at 6dB above the cut-off of the horn. In practice this is far more than what the woofer can handle in the Klipschorn.

"I have no doubt that the crossover design is going to be
critical with this speaker."

A good crossover design can make inexpensive parts sound exceptional, a mediocre crossover design can make exceptional parts sound quite poor.

"Various
articles say a 2^nd or 4^th order Linkwitz-Riley (6dB down
at CP) is critical for the improved polar response."

Not for the MTM, it's in a class of its own. The original articles in Speaker Builder
discussed this in great detail. In building many such MTM designs over
the last 30 years or so, I find the conclusions in the original to be
correct.

In general I do not like the MTM, and avoid it if possible. I have built the TMT with horns (top horn tilted down, bottom horn tilted up) for specific purposes (a church) with good results, these used the LR24.

[cfelliot]

DJK, thanks for the input.

My experience has been limited to minor mods to Heritage models. That being caps and replacement drivers etc. and is the reason for my seeking additional advice for this system.

I have left the builder's name out of this to leave it to his discression whether to join the conversation.

I believe I understand the concept of a system's transfer function as being the combination driver/horn/enclosure physical characteristics and the electrical characteristics of the crossover.

I also understand a horn/driver make up a system that must be treated as a whole and should not have stated it as just the driver when referencing its use in Cornscala builds.

Could you give me issue #s in SB so I can read the articles?

MTM operation isn't the only consideration I have for this system. Primarily it's the 3dB jump in efficiency.

What would you suggest as a crossover? (Edit: Ignore, I missed your 1st commant above)

Regards,

Chuck

[cfelliot]

Got the motorboards today. Even though I don't have the full cabinets I just had to put the mobo in place to see how it looks.

Also working on the cabinet plans:

[beeker]

props to you. nice to see your idea coming to life. interesting

[beeker]

Its a similar design to the epic cf4's

[cfelliot]

I started playing around with the crossover design over the
weekend. I wrote my own network analysis program in C#. I took DJK’s advice and
went to an 18db 3^rd order network. This is only the low-pass
section crossed at 500Hz. The woofers are Bob Crites CW1228 Hersey replacements.

I designed the network with a Zobel impedance compensation
section for fun as I had heard that it really wasn’t necessary, but I came up
with some interesting results. The effects of Le seem to really screw up the
response curve without the Zobel.

This is turning into quite a fun leaning experience.

Note mH mislabeled as uH below.

[cfelliot]

Just ordered the cabinet panels today from a local supplier. Should start the build in a week or two.

[cfelliot]

Well, it's been a lot longer than I thought! I just got the cabinets from the cabinet maker.

I have some questions about finishing. Anybody knowledgable?

[mustang guy]

It looks as though you have the WAF covered. She's grinning from ear to ear.

The builder did a very nice job on those boxes. Using the t-nuts for the back is better than wood screws. I like it! I'll bet those beasts are going to sound great.

Can't help you with the finish thing unless you want it painted like a car with racing stripes. []

[cfelliot]

[Daddy Dee]

looking good!

[cfelliot]

Well it took a while but this project is finally complete.
As you can see from the picture below the new speakers are sitting on top of my
1973 Cornwalls.

The final design took a lot of twists and turns, but the end
result was worth the effort on my part and many others.

The final design uses K28 woofers, the Faital HF140 w/Horn
and a custom extreme slope crossover (800Hz).

They are being driven by a solid line of Emotiva electronics
XDA-2 -> XSP-1 -> XPA-1L(pair). Source is TOSLink from HTPC.

Many thanks to:

Bob Crites – Lots of advice on initial design and cutting
the motor-board.

Al Klappenberger – Crossover design, teaching me more about crossovers
than you can imagine and putting up with a lot of stupid questions.

Dave – For connecting me with Al.

Bob Small – Cabinet work.

Wife - Tolerance of my turning the living-room into a lab
for 6 months.

I am more than happy with the results. I finally have that
big open Klipsch mid-range sound the the Heresy and Cornwall just couldn’t
do!!!

Up next, subs for same!

Al's crossovers:

[ZombieWoof]

NICE XO's

they Look beautiful & Heavy !

lotta good copper there !

all & all what a GREAT JOB !

[cfelliot]

Yes, about 100lbs each speaker! The 15 ply euro-ply plywood is heavy!

Al, did great work on the crossovers!