SBAcoustics 61-MFC
Copyright 2016 © Troels Gravesen

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DRIVERS   CROSSOVER    CABINET    MEASUREMENTS    SPEAKER-KIT     CROSSOVER LAYOUT

"MFC" is polypropylene, here mineral filled polypropylene as is for my reference ATS4 midrange drivers. Having full symmetric drive and a proper ventilated magnet system, this driver provides a sonic signature very much like the AT 18H52 driver. I like it. I've gotten used to it over the last 3 years. Good PP cones fitted into a proper magnet system and basket usually deliver some extra dynamic headroom compared to hard-cone drivers. PP break up in an ear-friendly way. I honestly don't think PP cones offer less resolution compared to hard-cones, but they do it in another way which is as hard to describe like the taste of wine.
These SBA 6" drivers are what I call small 6" drivers, having a membrane area of ~120 cm2. A SEAS 6" is 128 cm^2 and a ScanSpeak Classic is 145 cm^2, a Revelator 150 cm^2 and Illuminator 154 cm^2, making the latter more like a 6½ inc. No international standard exist for this and loudspeaker manufacturers gladly claim a small 6" a full-blown 7".

The construction here differs significantly from the -61AL version due to a low-order LR2 filter. The smooth response and roll-off characteristic of the PP cone allows a very simple crossover and the alu tweeter has no trouble following the trend.

I'm sure I'll have the question which I prefer, the AL or the PP. Please spare the mail because I like them both for what they do their way. And their way is not only due to cone material but also crossover topology. Only thing I'll say is that if you think alu sucks, built it and tell me if you still think so. You may be pleasantly surprised. If you wear belt and suspenders build the PP.


Basics:
2-way 16 liter speaker from 6" mid-bass and 1" dome tweeter.
System sensitivity: 85 dB.
Impedance: 8 Ohms, 6 Ohm minimum.
Power handling: 60 watts, and please read here: http://www.troelsgravesen.dk/power-handling.htm. Any burned driver is a misused driver!
Point of crossover: 2600 Hz, 2nd order LR2 topology.


DRIVERS
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Click images to view large

Download data sheets here: SB17MFC35-8   SB26ADC-C000-4


CROSSOVER
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Above the LR2 crossover - almost as simple as it gets. The PP driver can almost do with a 1st order filter having a little help from the LCR circuit removing some energy around 5 kHz.
To the right simulated response from drivers and summed response. Point of crossover ~2500 Hz.

Crossover components' values come with Kit Instruction.



CABINET
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My TS data compared to SBA. Guess my drivers need some break-in.
Whatever the truth is we get F3 = 48 Hz and lower Fs and higher Vas will increase response ~1 dB around 80 Hz.


Net volume is 16 litres from 20 mm panel thickness.
Keep external dimensions regardless panel thickness.
16 mm MDF can be used for bracing.

Go to SBA-61-NAC for cabinet details and construction.


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 with the same 50 dB scaling, but at different width and height. Now, we may think the graph to the right looks rather rugged, but if we stretch out the presentation (left) it suddenly looks rather smooth. The left presentation is often used by  driver manufacturers to make their frequency response graphs look better. 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.
Some further info here on commercial speakers: http://www.troelsgravesen.dk/Stereophile.htm. If I ever made a speaker displaying such un-linear response, no one would probably ever build it.
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 61MFC MEASUREMENTS

 
Final response of speaker. Sensitivity around 85-86 dB/2.8V, 1 meter.


Response of individual drivers driven from crossover and summed response (red). Point of crossover around 2600 Hz.


Right: Final system impedance for full length port (145 mm).

 
Horizontal dispersion at 0, 10, 20, 30 and 40 deg. off-axis.

 


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 section layout, level 1


Tweeter section layout, level 1

 

For level 2, the Superior-Z caps (C1 and C3) are replaced with STANDARD-Z caps. No other changes.


WIRING 


Speaker wiring. Tweeter is connected with negative polarity.

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