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SBA-861-PFCR
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There are customers who don't want more for less. Loudspeakers are not only for music, rather often a statement of quality conscience. Speakers must look expensive and for sure we often get very good looking drivers when price in not an issue.
However, as a designer, there is always the challenge of making more for less - and had all speakers front grill, I guess there would be sold more speakers with not so expensive drivers. Looks can be deceiving and something that looks good just must be good. A lot of people listen too much with their eyes rather than ears. I've had visitors distracted by The Loudspeaker-3 due to its front grill. It may leave us confused as we have to rely solely on our ears. It's like my QUADs as they're really just a panel of fabric. Vision rules so much of what we make of the world.
Anyway, the issue here is not the front grill, rather cost-performance. What I would like to do is a speaker capable of quite some sound stage - for less than 400 EUR/pair + cabinet materials. Actually the kit price landed at 280 EUR excl. VAT. 387 EUR + VAT when you want better caps for the tweeter.
The role model for this is my Discovery-861. To my ears it offers significant value for less than 800 EUR + cabinet materials. But this also includes high-quality crossover capacitors, which makes up much of the kit price. There is a significant gap between standard PP capacitors and super-caps. Larger gauge wire coils also adds to overall cost. To hit a <400 EUR target, we just must use standard PP capacitors. Would the construction here do better with super caps? Indeed so - and the crossover layout will be prepared for doing so, because the drivers more than deliver from such measures.

Searching for suitable drivers took quite some time. Monacor and Dayton are typical brands for low cost drivers. I actually bought a pair of 2" mid-domes from Dayton for another project only to find out their specified frequency response graphs were pure phantasy. On paper it looked like a dream - reality not so much.
I've been aware of the low-cost drivers from SBAcoustics for a long time, but never really studied the data sheets properly. Usually what is shown on data sheets is fact when it comes to SBAcoustics, and all three drivers here were no exception. Thus, 8" and a 6" drivers were requested from SBAcoustics and I also picked the SB29RDAC-C000-4 tweeter as I've used it before and it offers phenomenal performance for less than 50 EUR. And since last time it has gotten an aluminum face plate. Nice! The 8" and 6" drivers actually look really nice and only thing that tell us this is a low-cost driver is the use of plastic chassis. What pleased me the most was the 6" driver having a ruler flat response up to some 4.5 kHz. No sign of any rubber resonances in the 800-1600 Hz area. The all too common problem for many drivers with rubber surrounds. See measurements below. This 6" will carry most of the ever so important midrange.
What we also usually don't get from low-cost drivers is excessive power handling. The 6" has a 1" voice coil and handles 40 watts - but run in parallel with the 8" driver, this speaker can play loud - and I mean really loud - without distortion.
After measuring frequency response and impedance on a mockup cabinet, many, many simulations were done. The 6" causing the most trouble due to its fairly small magnet (relatively high Qt) and ruler flat response. We may think this is a good thing, but having a very small series coil only rises impedance slightly and we need some rise in impedance in shaping the upper roll-off. Eventually I found a way helped by the fact that the 5 kHz peak is good natured and being some 12 dB below average amplitude.

The cabinet is exactly the same as for Discovery-861 and by using a slotted port we save money on plastic ports. Actually a long slotted port is better and it also provides structural benefits to the cabinets. Cabinets can be made from 16-19 mm MDF or similar.


Click images to view large.

Firing up the speakers for the first time was - to be honest - a bit of a shock. Well behaved drivers having an overall smooth midrange and only modest break-up in the high-frequency range can do stuff. The cones of these drivers are fairly rigid, hard pressed paper, and they deliver a level of transparency as good as any driver you can find. More expensive drivers may have more dynamic headroom, but here in this 2½-way the sheer size of the combined membrane area makes up for this. I'd say the only true bottleneck in this construction is C1. That capacitor defines the level of refinement. Using a standard PP capacitors the treble becomes noticeable rough and it will colour the timbre of the clarinet and obo. It's not sounding bad, only not right. We have had kit buyers missing the distortion of their Bennic capacitors, saying super-caps took away the "transient edge" of high-frequency material. Well, it may take time to get used to undistorted treble! For all of the other capacitors I have tried it all, from electrolytic caps to standard PP and super caps. It just did not make any significant difference. Maybe a little for C4 when replaced by an electrolytic cap, but it was really, really hard to tell. And as always, the problem is when we know things. We're all masters of fooling ourselves.
Also noticeable was the more than average high sensitivity of the speakers. These run loud from an only 30 wpc PP tube amp. And the SB20PFCR goes deep in its ~45 litre cabinet (adding the volume given by the damping materials).
I honestly don't think you'll find a kit giving more value for money.

Above most recent vinyl harvest. A friend pointed me to a South Korean singer, Youn Sun Nah. Never heard of her, but he played a tune from the Same Girl LP and well, I went home and ordered two albums. Very, very good recordings that for sure will tell you if your speakers - and cartridge - will handle demanding female vocals. The SBA-861PFCR does it. My wife came in and asked: "What is this? This kind of music?". Hmm.. I couldn't tell. Well, just music. I mean, with a repertoire from "My Favorite Things" to "Breakfast in Baghdad"! Hello! I was brought up on Rodgers & Hammerstein! One of my parents' less than a handful of LPs.
BTW: When my friend played the record he handed me the sleeve and reading Ulf Wakenius and Leif Danielsen, I was sold. I love these guys.

Whatever material I played on the SBA-861PFCR it presented a dynamic and not least ultra-transparent soundstage. This from the SUPERIOR version. Having a standard-PP capacitors to the tweeter made the treble slightly more rough, not a lot, but noticeable. The bass goes deep and does the Tin Pan Ally track very well.
So, what do the drivers of the SBA-861PFCR not do that more costly drivers will do? Well, that was a question I tried to be prepared for even before I started the project. The 8+6" drivers have a combined membrane area of no less than 340 cm^2. This is more than many 10" drivers and given the speakers 91+ dB sensitivity, they don't move much. Limited discursion = low distortion + minimum break-up. One thing you want from expensive drivers is dynamic headroom. This means the driver can be run really hard before we measure serious distortion. This problem can be overcome by membrane area - if we have the space for it! If we made a line source from say eight SB16PFCR drivers they would hardly move at all - even for thunderous SPL levels. Here we have 340 cm^2 - and I'm sure it will do for most.     


Basics:
3-driver speaker.
Dimensions: 25 x 28 x 105 cm, WxDxH (H+30 mm for feet to allow free ventilation from port).
System sensitivity: 91-92 dB/2.8V/1 meter.
Impedance: 4-8 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
http://www.troelsgravesen.dk/Inverted-Polarity.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 specified.
http://www.troelsgravesen.dk/crossovers.htm
http://www.troelsgravesen.dk/choices.htm

 


DRIVERS
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Download specs here:  SB29RDAC-C000-4    SB16PFCR25-8    SB20PFCR30-8


CROSSOVER
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CABINET
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I built my cabinets from 20 mm Baltic birch - which cost-wise is overkill, but this is what I have at hand. I suggest 19 mm MDF. If you use 16 mm MDF, add a brace below tweeter and below 8" driver and maybe also bitumen pads. Up to you.
Add felt damping to all internal panels except behind 8" driver on rear panel. This is where the crossover will be.
Fold 3 pcs of 20 x 50 cm acoustilux and place behind upper woofer.
Fold 4 pcs of 20 x 50 cm acoustilux and place 2 pcs in front section next to the port and 2 pcs in rear section next to port.
Cut 2 pcs 22 x 50 cm acoustilux to cover crossover/rear panel behind 8" driver.


Placement of acoustilux.


Workshop pics
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Cabinets were made from overkill 20 mm Baltic birch - but this was what I had at hand. I suggest 16-19 mm MDF unless you want to treat your drivers really well. Click image above to see all parts cut to +5 mm on all dimensions and added one coat of lacquer. This prevents edge ripping and glue penetrating the veneer and making light spots of final coats.

The making of the cabinet is similar to Discovery-861. Look for more pics and suggestions here.


It doesn't get much simpler than this!


Left: Gluing the terminal panel support. Right: Don't forget to chamfer driver holes for 8" and 6". Route 45 deg. some 10 mm depth.


Testing driver rebates. For the 6" driver I made a 4 mm rebate, for the 8" driver ~4.8 mm.


Left: Routing the tweeter rebate and slots for terminals.
Right: Before gluing the front panel I added two coats of lacquer to the tweeter panel. Easier to do on a stepped baffle than after gluing the front panel. Add a strip of tape to leave some gluing area for the front panel.


All felt damping in place.


Left: Acoustilux, 2 layers, above crossover. Right: The acoustilux next to the port.
DO NOT forget to seal the wire hole with Superfix. The upper compartment must be absolutely air-tight.


Left: Acoustilux in upper midbass compartment. Right: Finally the drivers is place.

 


Workshop setup during crossover development.

 


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.


Frequency response of 16PFCR25-8 (green) and 20PFCR30-8 on test baffle, 30 x 120 cm, mounted 30 cm from one end.
What pleased me the most was an exceptional smooth response of the 16PFCR25-8 all the way to above 4 kHz. No sign of the all too familiar rubber resonances around 1 kHz. A lucky combination of mass and elasticity/resilience of suspensions.
SB16- merged with nearfield response at 200 Hz. SB20- merged with nearfield response at 300 Hz.

 


CSD of 16PFCR25-8.
We notice of peak a 5 kHz, but it dies out quickly. Being accustomed to listening to the MLS signal, you become aware of anything distressful to the ear. No such thing occurred during measurements of the 16PFCR25-8. Smooth on the ear, which suggests that we don't have to pay particularly attention of this peak. 

 


CSD of 20PFCR30-8. Some ripples from 2.5 - 5 kHz, but again, they die out quickly and was not distressful to the ear.


Impedance, free air, of bass (red)  and mid-bass (green) drivers.


Response of MT section at 1 meter distance.


Final system impedance. An overall very easy load.

 

 

 


SPEAKER-KIT
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What's not included in kit is the feet.
You can make them from wood and rubber pads - or buy some spikes from Jantzen Audio.
See "optional accessories" in the Jantzen link.
Remember 30 mm clearance between cabinet and floor. 

 

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: 

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 these files:
http://www.troelsgravesen.dk/tips.htm#Glue
http://www.troelsgravesen.dk/tips.htm#CONSTRUCTION_OF_CROSSOVERS
http://www.troelsgravesen.dk/LCR-RC.htm

 


Crossover layout for the Superior-Z version.
This version has CrossCap for the two midbass drivers and Superior-Z for the tweeter.

 


Crossover layout for the STANDARD version.
This has slightly smaller gauge wire air core coils and a mix of CrossCap and Jantzen EleCap.

 


Should you at a later stage want to upgrade, the red marking above tell the size of C1 and C6.
L3 in the Superior version has thicker wire lowering the DCR, giving a fraction of a dB more response, but hardly worthwhile.

 


Layout for SUPERIOR  version.
Disregard the long leads (loop) of the large Superior-Z capacitor.
The capacitor here was fastened lightly to allow other caps to be tried, e.g. the CrossCap in the STANDARD version. Follow the layout drawing.


Pay notice to the wire going from A to B for connecting the tweeter circuit.


Place crossover on rear panel behind the 8" driver.