Compact-Studio-Monitor

When the 18W/8545-00 and -K00 drivers came about some 15 years ago, everybody was impressed from the design, the "ugly" looking carbon-paperpulp membranes and the huge magnets. The rather slim chassis allows a 145 cm^membrane area from an overall 177 mm diameter, quite impressive. All 18W/85nn have copper plated pole piece, symmetric drive and vented centre pole piece. The huge magnets and low Qt suggests drivers suitable for 10-15 liter cabs and a few small monitor designs were launched immediately after the release of the drivers. Since then numerous designs have been produced and often we see the classic 18W/8545 driver used in up to 24 liter vented or transmission line constructions despite really suited for smaller cabs.
The 18W/8545-00 has an aluminium voice coil former, where the 18W/8545-K00 used here has a kapton voice coil former, the latter giving higher mechanical Q.

Why build small speakers when we can have bigger sound for the same money from a bigger cabinet? Well, the answer is simple: Because sometimes they just must to be small due to how they will fit in with a given idea about how our living rooms are going to look. WAF is high from small speakers. Even if a slim floorstander doesn't not take up more floor space than a mini on a stand, the WAF is higher for the latter. Not much we can do about it. Thus this Compact Studio Monitor. "Studio" because it started with the 8542 driver for studio use, by request from a Norwegian studio engineer.

How small is small? Well, the W15CY001/OW1 is small, the Ellam a little bigger (10 litres) and delivers a bass performance leaving the W15 behind. When we add 2-3 litres we may have room for a 6-6½" driver, a significant increase in membrane area and we can get a decent bass response down to 50 Hz. Comparing 18W/8545-00 to 15W/8530-G00 in 13 litres and 10 litre respectively, we get this picture:

The 15W/8530K00 may produce a slightly more extended bass response, but at the expense of ~2 dB efficiency. Listening to the bass response from W15W/10 litres and the 18W/13 litres - side by side - leaves no doubt which driver is most comfortable in this area. I'm not talking about midrange quality here, only bass.

The drivers: Download specs here:

ScanSpeak R2604/832000

ScanSpeak 18W/8545K00

CSM Crossover
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Crossover design, combination of 2nd/3rd order and 24dB on both slopes acoustically.

SPEAKER KIT, PARTS LIST
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Kit with Superior Z-caps

Components will differ from this test set-up made from leftovers.

The CSM cabinet
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Cabinet Construction
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Cabinet is simplicity itself. I suggest adding a horizontal brace between bass and tweeter.

If you want deeper bass, increase volume to 18-22 litre and tune port accordingly. 12.7 litre only if it has to be really small.
For larger volumes, extend cabinet below bass driver. Keep drivers' placement with regard to top.

Tweeter placement
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Selling kit#4 I had the question whether the kit would fit into an existing cabinet with tweeter placed center. Standard is what's seen on the drawing above. So, I made a test set-up from a 205 mm wide cabinet, tweeter placed center and I added fillets to make 75, 95 and 105 mm to top. Usually we have to avoid having the exact same distance to all three edges, so 105 mm is not recommended. What center placement does is rasing the response - a little - around 4 kHz. I still recommend 75 mm to top, but 95 mm will go too. Based on simulation, I don't think this is audible at all, so take your pick. The R2604/832000 is an easy driver.

Tweeter test set-up with a 15W filling the hole.

Cabinet Damping
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10 mm felt material is used as basic damping. Make sure the damping sheets are firmly glued to the panels. Part of rear panel is left open for mounting the crossover around terminals. Even i a 13 litre cabinet we have to carefullt consider size and placement of crossover.

No damping material on front panel. Above the vent a piece of MDM3/Acoustilux is placed as shown on photo. Place damping material on top of crossover to dampen reflections from the board and finally a piece of MDM3 at the bottom of cabinet as shown.

MEASUREMENTS
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Left: Left and right speaker, SPL @ 1 meter/2.8 volts, merged at 350 Hz with woofer nearfiled response.
Right: System impedance, min. 4.5 ohms at 500 Hz.

Left: individual response of drivers from x-over and summed response (red).
Right: Response with inverted tweeter polarity (blue) showing excellent phase tracking at point of crossover. Point of crossover around 2600 Hz.

Response from builders
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Hi Troels
Attached is a photo of my system. The silver amp was home-made almost 2 years ago (The stand was also home made). As for the speakers, amazing may be the right word, great sound with a deeper bass that I expected from what I've read about this driver and considering that the cabs are not tuned like they should and the driver is new. The R2604/832000 really is a great driver, especially at this price. As I've told you I'm going to build new cabs, but that may take a while. I'll send photos when they're done. I'm going to share with you my idea for new cabs: Imagine something like the Guarneri where the stand is hollow. The look of a monitor with the volume of a floorstander. Something around 24 liters (like the Amish) may give a bass extension not expected from something that looks small. And I think the 8545 "ugly" look will go great with a leather front panel. What do you think? Now, if I can do it, is another story. Many thanks for everything. Best regards, Adelino

Hi Adelino. Thanks for taking time to respond to the CSM. I'm pleased it seems to work out as intended and thanks asking about tweeter placement. It made me do the tests on tweeter placement - and the kits generally more versatile. The R2604/832000 is an easy driver. The "ugly" cones should look really excellent with leather as they did on the 1st generation Amati from Sonus Faber.

CMS Built by Michaël/France

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