Zahra
A friend of mine came up with this project and had decided on the well-known Scan-Speak 18W/8543-00 drivers for bass. These drivers have been seen in ProAc speakers and deliver a solid bass and due to the polypropylene membranes also know to deliver a smooth sound without being edgy at all (i.e. no serious cone break-ups). The midrange driver was new to me and one of the reasons for choosing this one was because it looks good! Hmm...this is probably not a good reason for picking a driver but after looking at the data from the dst website I had no arguments for not choosing this driver. I should later learn that it also sounds very good. The 9800 is the best metal dome tweeter I know of. So, I made the cabinet drawings and driver layout, my friend routed the front panels and the cabinetmaker did the raw cabinets and I had to assemble the whole thing and - not least - produce the crossovers. After having finished the speakers - and being very happy about the result - I asked my friend if I could publish the construction. After all, he paid for all the hours going into the project. He said yes....if I would promise that all "cloners" would make their cabinets from thin balsa wood. So, there you go..... View the
Zahra, built by Mats/Sweden This project did fit in very well with a lot of things I have been targeting lately: High sensitivity and the use of smaller upper midrange drivers. The 18W/8543-00 midbass has a claimed 88.5 dB sensitivity (large magnet and low membrane weight) and take a look at the blue curve below and you will see the FR from the 8543 driver at 2.8 V applied at a measuring distance of 1 metre. A quite sensitive driver. Two of these in parallel should produce a sensitivity of 94 dB/2.8V (not to be confused with efficiency), but the lower bass is not going to handle the same frequency range as the upper bass, hence an overall sensitivity around 91-92 dB/2.8V. This relatively high sensitivity is an immediate positive quality you realise when you connect to the amplifier. With a 100 wpc (8 ohms) amplifier this speaker can rip off your ears without serious distortion and looking at the impedance curves, it's an easy load on the amplifier too. Due to the midrange it can play low too without loosing grip of program material and it can do well for a Saturday party set-up. Quite versatile I should say. My 50 wpc valve amp likes it too, but due to the pp cone midbass drivers and the kevlar midrange, this speaker is quite tolerant on programme material and I like the solid state amp the best here. With the valve amp it became a little too smooth and nice. Transient response is excellent and the overall level of transparency is far better than anticipated despite the soft cones. Maybe the valve amp wasn't too happy about a 4 ohms impedance. It did well, but the solid state was slightly better. Making a 3½-way crossover does - as always - takes quite some time and without the LspCAD it would have taken ages. The good thing about these drivers is that they can handle frequency ranges far below and above of their target area:
As can be seen from the frequency
response graphs the bass driver handles well op to
4-5 kHz without any serious break-ups. The 13M is indeed
a very broad-banded driver, doing fine all the way up to
10 kHz and the 9800 tweeter is flat down to below 1000
Hz. So, major surplus on all drivers as the points of
crossover is going to be around 700-800 Hz between bass
and mid and around 4 kHz between mid and treble. Pics from cabinet construction
Cabinet construction: Details on cabinet construction, use of damping materials, etc. can be found in SP38, TJL and other files and only basic dimensions and in particular driver placement will be given here. It is important that drivers are placed exactly as shown on the drawing, otherwise the crossover will not work as intended.
Due to the thickness of the front
panel it is very important to chamfer the
driver holes, in particular for the midrange. If you
don't, the midrange will sound congested due to poor rear
ventilation. I dare say this does more to the sound than
the quality of the crossover components. Damping of the
mid enclosure has quite some impact on sound as well.
Don't forget to experiement here. Too much damping makes
a dry, forward sound.
The finished Zahras
The Crossover
Due to the smooth rolloff of the drivers, the crossover can be made quite simple. Basically 2nd order filters are used except for the tweeter that needed a 3rd order filter to match the mid-driver and to produce a good phase profile. As can be seen from the lower graph above, the phase tracking between drivers is unusually nice. As shall be shown later, this was followed by actual measurements, where inverting polarity of the mid-driver produces broad and deep suck-outs in the FR response. Due to the 2nd order topology, the mid is connected with negative polarity and the tweeter has same polarity (negative) as the middriver.
Crossover components
Measurements:
Left: Impedance of all drivers in cabinet.
Purple/black = 8543 in parallel. Green/yellow =
middriver. Red/blue = tweeter. The impedance for the bass
drivers are before the vent tuning was changed to 33 Hz.
Left: Red = mid and tweeter with opposite
polarity. Blue = mid + tweeter with same polarity. Right:
Significance of mid attenuation. 0R, 1R0, 1R5, 2R2. 1R5
is preferred. The sonic impact of this is hard to ignore
and don't miss the opportunity to hear what happens to
the overall sound from such a small changes in driver
attenuation. Looking at the frequency response graph, you may wonder if this speakers isn't balanced to have an overall dark sound. It isn't. Despite the significant tilt downwards - a little more than the BBC dip - the speakers have a slightly bright and forward sound. In an absolutely non-aggressive way I rush to say. Due to the dispersion characteristics of the individual drivers, the on-axis response may look different from speaker to speaker despite my attemt to tonally balance the speakers in a similar way. The 13M driver has a much better sound dispersion in the 1-3 kHz region compared to a 6" going up to e.g. 3 kHz, thus needs a little further attenuation to produce an overall balanced sound. The tweeter, as always, has an attenuation until it only "dissapears", i.e. if you can still spot the tweeter in the overall sound, be sure it's playing too loud. The 9800 is very sensitive to this and changing the tweeter attenuation resistor from 2R2 to 1R8 is clearly audible.
Left: Step response from finished speaker. As can be seen: tweeter and mid with negative polarity. Bass with positive polarity. Right: Impedance of finished speaker. Vent tuning still 33 Hz here. 33-34 Hz in final construction. This is a 4 ohms speaker but an overall very easy load on the amplifier. |
