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Discovery-MTM
Copyright 2021 © Troels Gravesen

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DRIVERS   CROSSOVER    CABINET    MEASUREMENTS    SPEAKER-KIT     CROSSOVER LAYOUT

MTMs were introduced back in the 60'ies and became popular after the concept was further developed by Joseph d'Appolito in the 80'ies (?) and have been used by many commercial manufacturers, e.g. Dali and Living Voice. Various crossovers have been used with MTMs, all with the aim of reducing crossover lobing.
Adding a second driver obviously doubles the radiating area, in this case to 274 cm^2, close to the size of a 10" bass driver, thus capable of quite some sound level. Another benefit of having two drivers in parallel is reduction in crossover complexity compared to e.g. 3-ways or 2½-ways, although the latter not so much if drivers are well behaved like seen in the Discovery 861.
MTMs can have a strong presence character. Having 274 cm^2 membrane area up to point of crossover, rather than a small 4" midrange driver with some 50-60 cm^2 area, has an impact on sound and the crossover must be fine-tuned accordingly. In this case the size of L2. In the final voicing of the speaker L2 coils with increments of 0.1 mH were tried until a proper upper-mid balance was found.
What this huge midrange driver area does is most noticeable on e.g. piano and vocals delivering effortless sound pressure for the size of the speaker. I also found it very good in terms of actual hearing the lyrics of a song. Listening mostly to English/American music (even Danish singers mostly sing in English in case somebody outside our borders should listen) I'm happy for singers being articulate in order to catch the lyrics. Not always that easy when English is not your mother tongue.
In our living room I have the speakers pretty much free of most room gain, yet found the bass powerful for the size. The 18W drivers are not subwoofers, so it you want the bottom octave (20-40 Hz), add a subwoofer.
I have a friend who is always the first visitor to evaluate my new constructions, and the first thing he noticed was that the midrange reminded him of the midrange of The Loudspeaker, which I really hadn't thought about. But he was right, having two 6" drivers in parallel doing all up almost the same point of crossover, makes about the same membrane area as the 10" midrange driver of The Loudspeaker. He liked what he heard - and so do I. It's got the crisp and strong presence of Ingram Washington's voice as I lastly enjoyed on my The Loudspeaker-III speakers.

Having done a lot of work on the ScanSpeak Discovery drivers lately I have become increasingly fond of their inherent smooth response and subsequent ease of crossover construction. The 12W, 15W, 18W and even the 22W drivers have some of the smoothest midrange I have ever measured, regardless of price. See below. The 18W/8434G00 driver used here displays none of the all too familiar rubber surround resonances, a clear sign of lack of linear distortion. And let's be honest, a lucky combination of all materials going into a dynamic driver with regard to stiffness, inherent damping, elasticity and resilience is to some extent gambling. You really never know what will happen when you put together all the components, but Birger Jorgensen of ScanSpeak did a good job when he made these drivers to replace the Vifa and Peerless range of drivers after the split.

For this speaker I picked the ScanSpeak D2905/990000 tweeter having a rather large face-plate, kind of a waveguide claimed to have "constant directivity", a term no longer in use, thus "large waveguide front" it is - and presumably meant to provide a more uniform coverage of treble over a given listening angle. Whatever, I didn't pick the 9900 for that reason, rather because I like what it does. I have used it before in my AudioTechnology PRELUDE speaker and wanted to use it again.
On paper this 9900 tweeter makes some 90 dB sensitivity. I've always wondered how people claiming e.g. 94 dB sensitivity for their system could use the 9900 in such a set-up. Here we reach some true 92 dB sensitivity and I can tell the system is not shy of treble. I guess the slightly focused treble due to the waveguide makes up for that, despite not being measureable. The benefit is that we don't need any attenuation.


Click images to view large.

 

I have thoroughly enjoyed some recent Herbie Hancock records, in particular the "River". Being a devoted Joni Michell fan, I was anxious to hear what Hancock could make of the Joni Michell's impressive song catalogue - and I'm wasn't disappointed. Mastered by Bernie Grundman a magnificent record, which cannot be said about the Montreux live recordings. Nothing to write home about, but it does feature Michael Breaker and John Scofield. Nothing on these records put the Discovery MTM to shame.

 

Basics:
3-driver, 2-way speaker
Dimensions: 21.5 x 29 x 102 cm (excl. feet), WxDxH.
System sensitivity: 92 dB/2.8V/1 meter.
Impedance: 4 Ohms.
Power requirement: 20+ watts/channel.
Power handling: 100 watts.
Please also read: http://www.troelsgravesen.dk/power-handling.htm, and remember, any burned driver is a misused driver.

Useful links (Please read before writing!):
http://www.troelsgravesen.dk/tips.htm
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/LCR-RC.htm

FAQ (Please read before writing!):
You cannot change cabinet front panel dimensions and drivers' placement without needing a new crossover - and I cannot help.
You cannot use any other drivers with the crossover shown here.
Please read these files before e-maling:
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/choices.htm

 


DRIVERS
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Click images to view large.

 

Download specs here: 18W/8434G00   D2905/990000


CROSSOVER
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CABINET
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Cabinets were made throughout with 20 mm Baltic birch. The bottom panel was made detachable as there is no way we can fasten the binding post screws through the lower midbass driver hole and through two braces. This also gives the opportunity to place the crossover here, although I prefer having it on the rear panel behind the lower midbass driver.
I decided to use the more expensive 70 mm port kit. I like the extensive flanged design. Total length is 130 mm. Cut two pieces of 65 mm tube and glue flanges with acoustilux.

All internal panels were added felt damping sheets and at the top two layers acoustilux. Behind midbass drivers, two layers of acoustilux. At bottom 6 layers of acoustilux to eliminate any standing waves between top and bottom - the critical part of slim and tall floorstanders.

 


Workshop pics

There won't be a whole lot of workshop images this time, as this is as simple as can be, a rectangular box with braces - and a flat front panel.


Braces shaping up.


Route easy and slow with Baltic birch not to cause edge ripping. Lower the router some 0.5 mm at a time in the beginning.
Both the 9900 tweeter and 18W midbass requires some 5.2 mm recess.
I used maple wood for side fillets. You can have the edges sharp or rounded, r=5 mm, or slightly chamfered, 5 mm. No big deal.


So, what's the point of three front panels? Because I'm an idiot! Forgot to mirror the front panels!! Costs some 2 hours extra work...
Chamfer midbass driver holes a bit, some 5-7 mm, 45 deg.


Testing driver rebates and cabs are ready for damping materials before gluing the front panels.


Add felt to all internal panels except rear panel behind lower midbass driver (crossover).
Add 2 sheets 17 x 40 cm acoustilux behind upper midbass driver covering top and rear panel.


The rear panel behind lower midbass:
Fold two pieces of 17 x 50 cm like seen on photo and place above crossover.

 


Port site. No acoustilux here. Add felt pad to front panel like seen on photo.
I hadn't received the ports at this moment and wanted to measure the hole diameter before routing.


Fasten the acoustilux to top panel with a pair of staples.

 


Left: Fold 3 pcs 17 x 50 cm and place at bottom. Right: Gluing front panel.
The front panel is made about 1 mm wider and sanded after gluing to cabinet width.


Applying the final fillet to the front panel.


Bottom panel in place and first lacquer coat.


Routing for the port, Ø117 mm.
The kit is supplied with two inner flanges, two outer flanges and one 235 mm tube. Cut 2 pcs 65 mm tubes and glue the flanges with Superfix. The port total length will be ~130 mm, 125-135 mm is fine. This will make a port tuning of around 38Hz.

 

After crossover development I - just for fun and plain craziness - tried replacing all caps with Amber-Z, which would probably cost 4 times more that all of the kit.
Does it matter? Not much compared to level 1 (left), but yes, significantly. Dammit! Which only shows that quality components come through even on modest priced speaker drivers. And capacitors are what matters. Coils are of less importance as long as being well designed and baked.


These slim floorstanders needed some feet/base-plate. I like oak and made the above with adjustable feet. Details here.


To balance the colour of the drivers the base plate was given black water based stain and a single clear coat of laquer.
The adjustable feet do well on a wooden floor.


Wire up the crossover. Fasten to rear panel behind lower midbass.


Cover crossover with acoustilux.


MEASUREMENTS
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A few comments on MEASUREMENTS before you start interpreting the readings below.
First of all, if we think measurements will tell us how a speaker sounds, we're wrong. The perception of sound is way too subjective to be reflected in any measurements we can perform. A loudspeaker system is meant to give us a satisfying idea of an acoustic event and for some people a pair of 5 USD ear-plugs are enough, others spend 200 kUSD on a truly full-range pair of speakers - and the latter may not be happier than the former.
Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas, although dispersion characteristics play a vital role here. A two-way 7+1 and a three-way 7+4+1 may display similar horizontal dispersion, yet sound very different. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this, ports may contribute to bass extension. Most of we diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz.  
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another. Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell much unless seriously bad, and most modern drivers display low distortion within their specified operating range. 
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are more to good sound than what can be extracted from a few graphs. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.
What measurements certainly do not tell is the sonic signature of the speaker, because speaker cones made from polypropylene, aluminum, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of adding spices to the stew. Nor do measurements tell what impact the quality of the crossover components add to the sound, from state of the art components to the cheapest of coils and caps, they all measure the same if values are correct, yet sound very different.



Above the dispersion of the D2905/990000 tweeter @ 0, 10, 20 and 30 deg. off-axis.

 


SPL of drivers on actual baffle. Green and orange, the 18W/8434G00 driver. What's not to like?
Ever soooo smooooth in all of the midrange and far beyond.
Green is upper bass and due to less baffle are around the driver a quicker roll-off towards lower registers. See below.
Red = tweeter response. 


Above the 18W drivers alone.


Final frequency response at 1 meter, 2.8V. Final system sensitivity ~92 dB.

 


Final system impedance. Basically a 4 Ohms speaker.


SPEAKER-KIT
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Above level 1 parts list.
You get CrossCoil and Alumen-Z for the tweeter and all Silver-Z for the midbass drivers.

 


Above parts list level 2.
You get wire coils and Superior-Z for the tweeter and STANDARD-Z for the midbass drivers.

All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.

Download Kit Sale Presentations:

All technical questions to troels.gravesen@hotmail.com

All questions regarding purchase of kits, please mail Jantzen Audio at contact@jantzen-audio.com


CROSSOVER-LAYOUT
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Connect A to B with wire.


Click image to view large.


Use e.g. a 10 mm dowel to secure the foil coils.
Make sure to solder leads properly!
Add a ring of Superfix around the dowel. Check here.

 

Speaker wiring:


Connect A to B with piece of wire.
Note tweeter with inverted polarity.
Connect on either side of R att depending on preferred tweeter level.