3-Way Classic Get 90 dB/2.8V and a valve friendly
Copyright 2010 © Troels Gravesen
After receiving the box of new SEAS drivers I wanted to quickly have an idea of the bass performance of the CA22RNX driver for the coming wide baffle set-up. And I wanted to hear what the new MCA12 middriver could do, so what was meant to be some quick experiments soon became more serious as this true classic set-up holds qualities that made me finish the design.
An 8" bass driver, a 4" midrange driver and a tweeter was very much what could have been on the agenda in the Seventies and except for the dome tweeter, these drivers could have been right out of the Seventies. Well, the spider chassis are new, but cones, surrounds and voice coils haven't changed much over the years. I do believe dome tweeters have improved significantly since then. The most common Audax and Peerless domes of the Seventies weren't the last things in this area.
Two 37 litre vented boxes from the Philips 9710 driver experiments were at hand and from the TS measurements a reasonably bass performance could be achieved. When ordering the drivers I included a ready-made enclosure for the small MCA12 drivers. I never used these plastic containers before, but it appears to do better than expected - and it saves an awful lot of time. The mid cab is lightly stuffed with MDM3 from Monacor. The SEAS 27TDC and 27TFFC were available and the 27TFFC appeared to have the smoothest sound so this one was chosen for the final design. These two tweeters were readily interchangeable without any modifications to the crossover.
These SEAS paper coned drivers takes some break-in. I've never experienced the magnesium drivers to change much over time, but in particular the CA22 bass drivers will take some heavy massage to come to life. The CA22 TS data suggest this driver to be close in performance to the EXCEL W22 driver, but here we have some 5-7 litres less volume and the CA22 does not reach quite as deep as the W22. We'll later see how the CA22 performs in the 45 litres wide baffle cabinets. My calculation predicts a F3 of 43 Hz. Not bad from a 37 litres cabinet and with a 90 dB sensitivity. The W22 has a little more cone weight, approx. 3 grams and the Qm, Qe and Qt are basically the same. Fs is lower for the W22 and Vas a little higher. So the CA22 is basically a lightweight paper cone and so it appears from the initial listening tests. Bass is speedy and transient but does not have the deep grip of the W22 or the ScanSpeak 18W/8531G00, BUT compared to the 18W/8531 it's around 5 dB more sensitive for the same voltage applied. And that makes a difference. Considering the cabinet size I'm sure the CA22 will perform very close to the W22 from a 45 litres cab.
The MCA12 is a darling. What you immediately recognise from this construction is the unusual wide dispersion of the midrange. If you're used to listening to 2-ways with a 6-7" midbass, the sound from the 3W Classic will be very much different. Besides the midrange dispersion, the MCA12 is fast as lightning and you will get a transients attack not possible from 15 grams 6" cones. The 4" middriver may be one of the clues to "an easy to listen to" speaker: http://www.troelsgravesen.dk/HES.htm.
The 27TFFC just does what it's supposed to do. No more, no less. This is a magnificent tweeter. No wonder it has found its way into a number of British speakers like the ProAc Studio 100 monitor. As we shall see later the impedance profile is very flat making the speaker an easy load on your amplifier.
I had expected the overall level of transparency to be compromised from the old time front panel layout, but this doesn't seem to be the case. And why should it? A lot of studio monitors are still built this way. Take a look at ATC monitors. The same configuration of drivers as seen here. Transparency is excellent - probably thanks to the MCA12. I would certainly place the vent on the rear panel to reduce any midrange escaping from here, but stuffing the vent doesn't seem to do much to the quality of the midrange.
the TJL3W aside, these speakers were an obvious
choice for comparison. Two quite different speakers - and
quite different prices too.
What is immediately clear from this comparison is the deeper bass from the TJL3W - and the overall tonal neutrality. These magnesium drivers' lack of colouration is hard to ignore. And I also suspect the 4th order filters of the TJL3W for being responsible for the lack of drivers overlapping and causing reduced distortion and coloration. But this is a controversial subject. I firmly believe there are benefits and trade-off from both approaches and I don't subscribe to any fixed belief of the "right" way of doing things. It all depends. Whether derived from the paper cones or low-order filters, the 3W-Classics have more "lushness" to the sound. They are more coloured, but in a sympathetic - and quite energetic way. It can be played loud, quite loud actually, but the more you push drivers driven by low-order filters the more driver interaction there will be and it starts smearing the sound.
A visitor was quite impressed with the sound but couldn't help noticing that these speakers certainly didn't look nice! Well, all depends on the eye that sees, but try imagine the speakers wrapped in exotic veneers - like some of British classics from Spendor or Harbeth - and they may start looking quite attractive. Maybe even a front grille might do here. The baffle is so wide that a grille frame might only do minor damage to the sound.
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Response of individual drivers. Mid and tweeter performs
very well and the bass (red) has trouble at 3, 4 and 5
kHz. The dust cap? Well, with a point of crossover
between bass and mid at 500 Hz we needn't worry too much
what the bass does above 3 kHz.
Left: Impedance of 27TFFC. Typical ferro-fluid profile. Right: Impedance and phase of individual drivers (here the 27TDC tweeter). With a port of 72 x 170 (WxL) we get a vent tuning of ~36 Hz. A slightly shorter vent may boost bass between 50 and 100 Hz but I didn't try this.
here we have the measured response (red) from the
speaker. System sensitivity is around 90 dB and the
speakers has a minor BBC dip towards higher frequencies
to gain a natural balance sound. Blue = minimum phase.
most difficult test: What happens if you invert polarity
of the middriver? What is seen are two deep suck-outs
around points of crossover suggesting excellent phase
tracking between drivers.
response of system. Note negative polarity of tweeter and
middriver and the smooth decay of the bass driver's
Only a few words on
cabinet construction. What's depicted above is my test
cabinet set-up and if you want to take advantage of the
crossover you have to use these front panel dimensions
and in particular the drivers' placement.
built by Egor