Discovery 12W
Copyright 2015 © Troels Gravesen

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DRIVERS   CROSSOVER    CABINET    MEASUREMENTS    SPEAKER-KIT     CROSSOVER-LAYOUT    D3004/604010

Making small minis is fun! I made a small mini for ScanSpeak's 2014 Munich Highend show, and I'm still waiting for the new 12WU (see June 2014 post) driver to be launched. Until then we might take a look at the smallest midbass driver in the Discovery range, the 12W/8524G00. It goes without saying this provides a significant reduction in building costs and adding to this is the use of the R3004/602010 in replacement of the Be dome used for the Munich show. The R3004/602010 is not exactly cheap at 145 EUR, but the small size comes handy and basically offers the same quality in sound compared to standard face-plate ring radiators in the Illuminator range of tweeters, like R3004/662000. Using the R2604/832000 would mean we have to start cutting the face-plate and I don't want to do this. In total the drivers alone are 409 EUR. We can get a more speaker for the same money, but not this small. And this speaker must be small to suit desk top application and small sound systems. And why not make your kids get used to good sound from an e.g. NAD 3020D amp, being fed from a laptop or other digital sources? The DISC-12W may be a way to better sound culture.

Before you start building such small speakers, be sure you realise what a modest 59 cm^2 membrane area can do. From the cabinet size we have F3 = ~70 Hz. Don't expect gut shaking, earthquake impact on your listening room. There are limits to how much air such small membranes can move. But the midrange...many of these small 4" drivers deliver a midrange so smooth and dynamic. The 602010 tweeter is costly compared to the 12W, but it's just sooo good - and it's small.
Don't place the Discovery 12Ws too far apart, 1.5-2 meters is ideal from a similar listening distance. You can place the speakers close to the wall and get some more bass impact although this - as always - reduce the sense of soundstage depth. Hope you enjoy!

Basics:
System sensitivity: 85 dB/2.8V, 1 meter.
Impedance: 8 Ohms.
Amplifier requirement: Minimum 5-10 watts.
Power handling: 40 watts (ScanSpeak specifications).

DRIVERS
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Download specs here: 12W/8524G00   R3004/602010


CROSSOVER
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Above the crossover simulation targeting a point of crossover around 2.2 kHz. The common notch at 800-900 Hz is taken care of by a modest working LCR circuit. Usually this is done by increasing the value of L2011, but this leaves a dip in the ever so important midrange. Without the LCR circuit the upper mid becomes too forward.
R2012+C2011 takes care of a minor peak of the 12W driver at 6.5 kHz otherwise impacting tweeter performance.
For the tweeter, R1041+C1041+L1041 flattens the impedance peak allowing a nice LR2 roll-off.
Generally the drivers take very little equalisation to perform to target transfer.
From the stepped baffle we can accomplish a true time-aligned Linkwitz-Riley 2nd order crossover topology, to my ears the best sounding compromise, providing good power handling and crossover simplicity. An LR4 crossover would provide better vertical dispersion but I prefer the LR2 sound to the dry and forward/reduced-sense-of-depth sound of the LR4. Some do, and I don't. Every crossover is a compromise.

THE FINAL CROSSOVER


Crossover components' values come with the kit.

Fine-tuning the crossover always require minor changes to components' values, but the final set-up didn't change from the simulation. LCR2031 just removes a tiny bit too much energy around 1 kHz and LCR 1041 handles tweeter impedance bump and smooths roll-off towards lower frequences, thus also improves power handling. R2012+C2011 suppress a minor 12W peak in the 7-8 kHz area, which would otherwise mingle with treble performance. This is a small speaker with an overall very neutral and balanced presentation.


Based on the drivers' effective radiating area and placement (only 75 mm apart) this is the predicted
horizontal and vertical dispersion primarily in the crossover region.
Having a small tweeter close the the midrange driver pays off when using shallow-slope crossovers.


CABINET
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Above box simulation. As can be seen, 4 liter closed makes Qtc = 0.63, but here we want a little bass boost and use a vented design.
If you have to place the speaker close to the wall, you may try stuffing the vents with some acoustilux.
If you stuff the port lightly, you'll have an aperiodic tuning. Stuff hard and we have a closed box.
I finally decided on Fb = 65 Hz, port length 10 cm. Cut supplied port to 10 cm length.


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The cabinets were designed for 20 mm black MDF, actually HDF, high-density fiber board. Alternatively use 19 mm standard MDF. The W12 baffle can be from 18-20 mm thickness. Do remember chamfering the 12W driver hole as shown on drawing. Due to the combined thickness of the front panel, the small driver needs room to breathe.
I used 8 mm felt on sides, top and bottom of cabinet. On top of the crossover I placed at 100 x 150 mm acoustilux damping material (30 mm thick). That's all the damping needed.
Rear panel is fastened with 6 long wood screws as it also serves as the crossover board, thus easily accessible for tweaking and tweeter level adjustment.


Except for the rear panel, all is cut 45 deg.


Test assembly of front panel.


Routing for the tweeter takes special care. It's really tiny compared to standard face plates.
The rear hole for the 12W is 127 mm diameter.


Chamfer rear bass driver hole as seen on photo. 45 deg. some half way through.
Chamfer 12W front panel 45 deg to 5 mm from driver rebate.


Left: This is how the it should look to provide proper ventilation for the midbass driver.

 
These cabs were glued using tape only. Apply a not too thick layer of glue and pull the tape hard just before it brakes.
Using tape only requires all panels are cut fairly accurate.
Left: Finding nice black screws for the tweeter is kind of a problem! Screw heads need to be some 5.8 mm diameter. It would be a nice feature from ScanSpeak to provide 6 suitable screws with the kit.


Left: You need to expand tweeter hole a little as seen on photo.
Tweeter housing needs a little more space. Use chisel or plunge a router bit three times into the panel as I did here.
Right: Cabs sanded and time to glue outer 12W panel in place.


Cabs sanded and ready for terminals and port.


Routing for port and holes for terminals made and marking space for crossover.


There's very little space for the screws. I used 6 screws slightly tilted to enter the middle of the side panels, thus does not fracture the MDF.


Damping is easy. Cut a 10 cm strip of 8 mm felt. Make cut-out for L2011 and the tweeter and glue the felt to the front end of the cabinet.
When all is mounted and wires are connected to drivers, cut a 10 x 15 cm piece of 30 mm acoustilux to cover the crossover. That's all.


The final speakers were given a mineral wax treatment. I like the raw, black MDF appearance and wanted to keep it that way.

MEASUREMENTS
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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, very much wrong indeed. 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. 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. 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.

 

Above left: The 12W response without filter (red) and with filter (green) but with no midrange LCR correction. In the beginning it looked like we could omit the LCR circuit, but one the tweeter was in place, things changed and we have a small notch around 1 kHz. Taming this area, brought back the overall balanced presentation. Actually you can place a switch on your rear panel to put on/off the LCR circuit if you want a more forward sound. Also tweeter level could be adjusted by a switch, making R1011 either 0R, 0R47 or 1R0. 0R is bright, 0R47 is balanced, 1R0 is slightly laid-back.
Right: Tweeter response w/wo impedance correcting circuit (LCR 1041).

Above left: Response from finished system w/wo midrange notch filter. Right: Final system response from my preferred tuning, w/ notch filter and R1011 = 0R47. My default for R1011 is 1R0.


Left: Horizontal dispersion at 0, 10, 20 and 30 deg (red/green/yellow/blue).
Right: Vertical dispersion starting at point between drivers. Red = 0, green = +5, orange: +10, blue = -5, yellow: -10.


Final system impedance and phase (green).

 


SPEAKER-KIT
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Crossover components' values to come with the kit.
Kit can be ordered plus/minus drivers.


Kit Instruction following the kit.

All questions regarding purchase of kits, please mail Jantzen Audio at contact@jantzen-audio.com

All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.
Download Prestige Version Sales Presentations here.
 
All technical questions to troels.gravesen@hotmail.com

 

CROSSOVER-LAYOUT
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Crossover layout on rear panel.


Pay notice tweeter is connected with inverted polarity.

Due to limited space the two foil coils are supported by crates made from 20 mm MDF.
All components are glued with Superfix.

 


Left: Make sure not shorting any wires from using the foil coil terminations.


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Crossover mounted on rear panel seen from behind.

 

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Wanna make the ultimate Discovery W12 version? The new beryllium dome, D3004/604010, obviously fits the routing for the R3004/602010 ring radiator.
The crossover, however, has new values for all components. Should you want to make the ultimate super-mini, I'll ask Jantzen Audio to make a kit calculation.

Given the excellent mid and upper mid of the 12W midbass, the Be dome even further enhance transparency and micro detail resolution.

 

 


Crossover components' values come with the kit.
Crossover layout is the same as for R3004/602010.


Point of crossover is ~2340 Hz. Orange graph is with inverted polarity.