SBAcoustics, 3-Way Classic
Copyright 2016 © Troels Gravesen

DRIVERS    CROSSOVER    CABINET    MEASUREMENTS    SPEAKER-KIT     CROSSOVER LAYOUT     SOUND

There's no point in denying! I love the concept of a 3-way classic from an 8" bass driver, a 4" midrange and a 1" tweeter. We can get an awful lot of good sound from a modest investment. Searching the web for suppliers I find all six SBA drivers for 328 EUR incl VAT. This may be realised from other  brands as well, but here we get proper voice coil formers and suspensions ensuring high mechanical Q and on top of this symmetric drive (SD), something we - to my knowledge - do not find from any other manufacturer at this price level. Furthermore very rigid baskets - even for the midrange - and proper ventilated magnet systems (view bass driver below), hard-pressed paper cones and a tweeter that may well compete with the very best. Hmm...if the sound is also good, we're in for a treat.

DRIVERS
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Click images to view large

Download specs: SB23NRXS45-8     SB12MNRX25-4     SB29RDC-C000-4

Basics:
3-way speaker system
Points of crossover: 600 and 3700 Hz
Crossover: 2nd order LR
Power handling: 60 watts (SBA rating of bass driver), read here: http://www.troelsgravesen.dk/power-handling.htm
Recommended amplifier: 20 wpc minimum
Impedance: 4-8 Ohms
System sensitivity: 88 dB, 1 meter, 2.8 volts input.


CROSSOVER
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Doing simulation takes a few precautions when it comes to the actual response of the bass driver. Usually we measure at 0.5-1 meter distance and in a normal living room we get readings valid down to around 250 Hz. This reading we may merge with a nearfield response and get the full response of the driver. However, this is quite similar to a 2pi environment, not the narrow baffle we intend to place our driver on. Thus, we have bafflestep loss and the bass may be some 6 dB down at the lower end. Next we have to assume a certain amount of room-gain, but this depends on the actual placement of the speaker in the room - and this can vary enormously! Assumed room-gain is something the designer has to do. Placing the speaker some 60 cm above floor level, 80 cm from front wall and 150-180 cm from side walls, we're in for some 2-3 dB room-gain. Above you can see two simulations where this is not taken into account (right) and one where 4 dB has been subtracted from the bass driver response (left) and mid-tweeter tuned accordingly. 91-92 dB system sensitivity would certainly be really nice, but this is not what we get when we place our driver on a 30 x 60 cm baffle and put it into a room at suggested distances to nearest boundaries. 88 dB is more like it, and this it what it will be.
When reading manufacturers recommendations on their speakers we rarely - if ever - read something like "this speaker should not be placed near corners" or "don't place this speaker too far out into the room". One manufacturer sends a qualified person to the customer to suggest proper placement for optimum performance - but the customer pays dearly for this service. It goes without saying that this is not possible for a 1000 USD pair of speakers, or the manufacturer would soon be bankrupt.


Crossover components' values come with the kit.

The SBA drivers used here perform really well (see measurements below) and we can get away with a very simple LR2 filter due to the stepped baffle.
The "problem" with these SBA drivers is that they can really cope with the very best of components. C1 and C2 takes some 40 uF in total and they better be highest quality to deliver all the goodies. Obviously there will be an all-STANDARD-Z Cap version with standard PP capacitors if it has to be as cheap as possible.


CABINET
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TS data, SB data compared to my own measurements.

Looking at SBAcoustics TS data on the 23NRXS-45-8 made me a bit uneasy as it might not fit in with my 2 cubic feet standard 3-way classic cabinets and I wanted to stick to the design. However, doing my own TS measurements produced a few differences from the SB data - and quite opposite to what I usually experience. I don't measure at a particular high voltage, usually delivering higher Qt and lower Vas. Here I get a lower Qt - and slightly lower Vas, but still better suited for a 40 liter vented cabinet. Fs and Mms are quite similar. I'm not claiming my date better than the SB data as TS data can take a lot of values depending on voltage applied. My drivers were broken in according to SB recommendations. So, nothing beats experience and I tested the driver in my 33.4 liter test cabinet with the port tuned to ~32 Hz and from measurements and listening, this performs perfectly well with no boominess or lack of precision.


Click image to view large (box volume an example)

To further investigate the bass driver I made some cross-calculation in LspCad to see what happens. My measured data fits quite well with data produced from doing cross-calculation. A critical parametre here is BL and it may just be a bit higher than suggested in the SBA file. The calculated efficiency also fits well with the actual measurements done by SBA. Now, no need for being nit-picking here, I just wanted to make sure this bass driver would do what I wanted from a 40 litre vented design.

 


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Construction pics

I strongly suggest taking a look at SEAS 3WC, ScanSpeak 3WC and AT-3WC construction pics as this will be nothing more than repeating what has been made 4 times before. Even the QUATTRO mkII has similar features.


Sometimes it may be needed to apply a force between two opposite corners to pull the cabinet into a perfect rectangular shape.
I attached a piece of MDF to one of the sides and inserted a wedge between the MDF and a bracing until aligned.

My cabinets here are experimental and will house a wide range of drivers, hence all the fillets for attaching the rear panel by screws and the front panels will be attached by screws from the inside. Now, if your 3-ways are one-timers, you can of coarse skip all the fillets and just glue front and rear in place. Much easier than what is shown here.


Making the front panels. Left: Inner panel. Right: Plus outer panel.

As always: It's important not to obstruct the rear airflow from the midrange driver, hence proper chamfering is needed.
The bass driver is so large that only modest chamfering is necessary. The tweeter obviously doesn't need chamfering not having any rear radiation (I have this question regularly).


Click images above to view large.
Mark the mid cab panels on the rear side of the inner front panel.
I made a frame for chamfering the inner front panel without going into the area of the mid cab panels.

 
Left: Bass driver chamfering, only inner panel. Right: Testing finished front panels with drivers and port in place.
Make sure the midrange is fully isolated from the bass driver! Stuff the port hard and press the bass cone.
The midrange cone must not move! When you push the midrange cone it should move out again slowly.


Damping material seen from rear.

Add felt sheets to all internal panels except the (1) small area for the terminals, (2) rear panel behind bass driver, (3) front panel and (4) outside of midcab.
Fold a 25 x 50 cm piece of acoustilux and place in upper chamber.
Add a 25 x 50 cm piece of acoustilux on top of crossover and up half side panel next to bassdriver. See images below.
Add a piece of 12 x 14 cm acoustilux behind middriver.


MEASUREMENTS
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Everything that can be counted does not necessarily count; everything that counts cannot necessarily be counted". Albert Einstein.

A few comments on MEASUREMENTS before you start interpreting all 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.

Above you see the same graph presented in different width. Now, we may think the graph to the right looks rather rugged, but if we stretch out the presentation (left) it suddenly look rather smooth. The left presentation is often used by Chinese manufacturers to make their frequency response graphs look better. No smoothing was used on the above measurements, but if we apply e.g. 1/6 or 1/3 octave smoothing things look even better. Just this to display how easily measurements can be manipulated to look nice.


Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas. 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 us diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz, nor do manufacturers for that matter.
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 drivers, because cones made from polyprop, alu, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of colouring the sound.
The choice of crossover topology has a huge impact on the sound we get. We may produce the same frequency response from 1st, 2nd or 4th order filters and they may be Butterworth, Linkwitz-Riley, Bessel and others and they all sound different, very different indeed, so take care!

 

SBA 3-Way Classic measurements

 
Left: SPL from tweeter equivalent to 2.8V, 1 meter. Right: Same for woofer.

 
Left: SPL from midrange @ 2.8V, 1 meter. Quite flat response with a minor dip around 1700 Hz.
Right: Free-air impedance of the 12MNRX25-4 driver show no sign of any disturbance in the all to common 800-1500 Hz area.


 
Left: Reverse tweeter polarity displaying point of crossover between mid and tweeter at some 3700 Hz.
Right: Horizontal dispersion at 0, 10, 20 and 30 deg. off axis. Within 40 deg. this speaker is remarkably uniform.


Left: Overall response from merging response @ 200 Hz.
Right: Final system impedance. Minimum is 4.5 Ohms from 100-400 Hz.


SPEAKER-KIT
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Crossover components' values to come with the kit.


You can buy the kit with or without the drivers - or some of the drivers if you already have some.
Please ask Jantzen Audio for options.

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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Bass crossover layout


Mid-tweeter crossover layout.

Kit Instruction following the kit contains all information on components' values.


Wiring


You can obviously make the speaker for bi-wiring by ordering a second pair of binding posts.


Click image to view large.
I place the MT section on rear panel behind bass driver.


Bass crossover section is placed on bottom panel.


Above mounting the mid-tweeter crossover on the rear panel behind bass driver.
Place a piece of 50 x 25 cm acoustilux on top of crossover and half up the side panels.
The bass crossover is mounted on the bottom panel next to the port.


SOUND
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These speakers will seriously challenge any similar equipped constructions! The 12MNRX midrange delivers detail in spades and is gently supported by a clean, neutral and extended treble. Bass is surprisingly deep and on par or better than any of my other 3-Way Classis. The rigid paper cone does its thing and provides an articulate bass.
Is it overall lush? No! Is it overall laid-back? No! Is it... I could go on. I can't find anything negative for the size and cost. On the contrary, these drivers deliver qualities we're used to pay seriously more money for.