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SBAcoustics 61-NRXC

Copyright 2016 © Troels Gravesen


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Here the final SBA-61 construction. The SBAcoustics SB17NRXC35-8 driver obviously fit right into the cabinet used for the two other constructions.
Hard pressed paper cones offer very much the same level of transparency as other hard cones. They also share the same cone break-up at higher frequences like aluminium, hence must be treated accordingly, i.e. 4th order filters. Only a few changes from the 61-NAC crossover had to be made. As can be seen below, a fairly straight forward LR4, 4th order filter.
The primary thing that characterise these three speakers is the crossover topology. Thoughts on the difference in sound from LR2 and LR4 filters can be found here. Secondly the cone material adds to the particular sound of a speaker and I won't even try to add words to what it means, it must be experienced. I've had quite a few visitors being very surprised by the smooth sound of the 61-NAC. Not what they expected. So, three different cone materials, take your pick!

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2-way 16 liter speaker from 6" mid-bass and 1" dome tweeter.
System sensitivity: 85 dB.
Impedance: 8 Ohms.
Point of crossover: 2000 Hz, LR4 (forth order) topology.
Power handling: 60 watts, and please read here: Any burned driver is a misused driver!


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Download data sheets here: SB17NRXC35-8   SB26ADC-C000-4


Above the LR4 crossover - almost as simple as it gets.
To the right simulated response from drivers and summed response. Point of crossover ~2000 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.


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: 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!



Left: Tweeter response in cabinet, no crossover. Right: Midbass response in cabinet, no crossover.

Left: Response of individual drivers driven from crossover and summed response (red). Point of crossover around 2000 Hz.
Right: Final system impedance. Minimum impedance = 5.7 Ohm. 


Suggested parts list

Here you can see the required DCR of coils. Can be +/- 0.1 Ohm.


Bass section layout, level 1

Tweeter section layout, level 1


For level 2, the Superior-Z caps (C3 and C8) are replaced with STANDARD-Z caps. No other changes.

For level 3, all caps are STANDARD-Z caps.


Speaker wiring. Both drivers connected with positive polarity.