SBA-741
Copyright 2019 © Troels Gravesen

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DRIVERS   CROSSOVER    CABINET    MEASUREMENTS    SPEAKER-KIT     CROSSOVER LAYOUT

This speaker is the second design of two open constructions to be published at SBA website for the DIY community. The first one was the SBA-761 to be found here. The speaker here has been a long time in the making. Test cabinets were made to make the best acoustic environment for the drivers giving as smooth response as possible, and more than ten crossovers have been wired up for testing before the final one was picked for publication. Looking below you find a crossover fairly simple featuring a mix of 2nd and 1st order topology. Having a huge overlap between drivers calls for units that blend well to produce a homogenous sound. The significant part of the treble here is produced as much by the midrange driver as the tweeter, recalling the treble range being 1260-10200 Hz. 
The bass driver - like in any system - defines the obtainable system sensitivity. I here picked the 8 Ohm version of the MW19P driver making it a suitable speaker for even modest powered tube amps, although I won't recommend 4-8 watts SET amps unless you never play very loud. The 4 Ohm MW19P would play louder for the same volume setting, but only because it draws more current from your amplifier, not because it is more efficient. The 8 Ohm version here runs smoothly from my 30 watt tube amp (EAR-861), so it don't consider it a tough load.

The virtue of this kind of speaker is obviously the small midrange driver, something that was taken for granted in many of the 70'ies speakers. Why? Because it's a natural link between a 6-7" mid-bass and the tweeter as discussed in the intro to my Illuminator-5 speaker. Please go to page for further reading. On paper, dispersion measurements of upper-mid/lower-treble may look the same from a 6+1 compared to a 6+4+1, but it just does not sound the same, hence top brand names like Gryphon, Wilson Audio, Vandersteen and others use a small 4" midrange driver to bridge the gap between a more dedicated midrange driver and tweeter in their top speakers. The reason for so many 6+1 speakers is cost. For a manufacturer of speakers, adding an extra driver and expanding the crossover is a no-go. Too expensive. For DIY'ers, not so much, as we don't have to ensure a viable profit of our creations.

Having the 19MW driver fully run in I find the speaker delivering a surprisingly deep bass for the size, giving a forceful presentation of lower octaves. The 1st order low-pass filter of the midrange and tweeter high-pass section pays off in overall fidelity of musical instruments and vocals. Generally these SBA drivers can deliver the level of detail and transparency we have come to expect for state of the art drivers. Our only challenge is to make the best possible crossover - and make sure the crossover parts do not distort the signal fed the drivers.
Give the speakers stands that place your listening height somewhere between tweeter and midrange and you get the flattest frequency response.


 

Basics:
3-way stand-mount speaker.
Dimensions: 27 x 36 x 55 cm, WxDxH.
System sensitivity: 86-87 dB/2.8V/1 meter.
Impedance: 4-8 Ohms, minimum 3.6 Ohms at 125 Hz.
Point of crossover: 600 and ~4000 Hz.
Power requirement: 20+ watts/channel.
Power handling: 70 watts. Please also read: http://www.troelsgravesen.dk/power-handling.htm, and remember any burned driver is a misused driver.

Useful links:
http://www.troelsgravesen.dk/tips.htm
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/LCR-RC.htm

FAQ:
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-mailing:
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/choices.htm
 

DRIVERS
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Click images to view large

Download specs:  TW29RN-B-4   MR13P-8    MW19P-8


CROSSOVER
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The crossover features a mix of LR2 for bass-mid and and 1st order for mid low-pass and tweeter high-pass, hence reverse polarity to midrange for proper summation. Using 1st order filters requires drivers that work well above and below points of crossover and the only driver here that takes special precaution is the MW19P-8 requiring some equalisation above 3.5 kHz, hence R8+C5 to smooth the roll-off profile. It all works very well. Basically I used some of the topology from the SBA-941 speaker.


CABINET
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Cabinets are made from 20 mm Baltic birch and front panel from 25 mm HDF. Follow images below.
Internal panels are added 4 mm bitumen pads and 8 mm felt.
Midrange cabinet is filled with 65 grams wool.
The cavity behind the midrange driver is filled with 50 grams wool.
A piece of 30 x 50 cm acoustilux is placed at bass cabinet bottom and up the sides. See image below.

As always: Any change to front panel dimensions or drivers' placement and you need a new crossover - and I can't help.

 


Any new construction starts with a MDF mockup to optimise design and try out preliminary crossovers to see if this has any future at all.


Making the final cabinets from 20 mm Baltic birch.


Adding 15 mm fillets to fasten rear panel. I suggest 10 mm fillets giving a little more room for the crossover mounted on rear panel.
I strongly suggest making the rear panel detachable as we need access to the crossover for tweaking.


Adding bitumen pads to internal panels. Go to Tips.


Adding felt to all internal panels except cavity behind mid/tweeter drivers and rear panel.
Add 65 grams wool to mid-tweeter cavity and 50 grams for cavity behind mid-tweeter.


Trimming front panels and chamfering bass driver hole some 15 mm depth.
My front panels made from 20+8 mm HDF laminate.


Make sure chamfering mid driver hole, some 15-17 mm.


Testing all driver rebates and chamfering front panels.
Table saw at 30º and side table guide set to 2º. Same as Ellipticor-3.
Chamfering can be done by hand, check here: http://www.troelsgravesen.dk/tips.htm#Faceting


Front panels ready for sanding.


Preparing boards for crossovers and front panel paint.

 


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.


SPL normalised for 2.8V/1 meter. Green = minimum phase.




Final system impedance (red) and electrical phase (green).
Minimum impedance = 3.6 Ohms @ ~125 Hz.


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

Download Complete Kit Sale Presentations (pdf file):

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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Please read here.


Tweeter section layout
To be placed on upper rear panel.


Layout midrange section
To be placed on lower rear panel. Make holes for port and terminals.


Layout bass section.
Place at bottom of cabinet.

Speaker wiring:

Crossover pics

Click some of the images to view large


Left: Bass section. Right: Midrange section.
The Alumen-Z caps you'll be getting are bigger than those shown here, due to now being in alu tubes. Performance is obviously the same.


Tweeter section

 


Midrange section, close ups.


Bass section, close up and mounting on bottom panel.

some images showing the mid and tweeter section on rear panel.


50 x 50 cm acoustilux covering bottom/crossovers.

BASS MODULE FOR SBA-741
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