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Monsieur Rankin's Euro Selection
For a DIY project I recommend making speakers that are foolproof. Keep it simple, like a two-way with an 8" and a 1" dome tweeter. There are a few interesting Cabasse drivers that look good on paper, specifically the DOM4 tweeter at 96dB/W/m and the 21M18 Woofer at 93dB/W/m sensitivity. The woofer is good out to 5 or 6 kHz. Zalytron offers some kits using these drivers. Unfortunately the crossovers are hogs, but you can buy the drive units and cabinets and work on the x-over yourself. I personally don't like the sound of D'Appolito configurations or multiple driver arrays. Like I said, keep it simple.
Aside from the obvious requirement for reasonable sensitivity, speakers that work well with low power triodes usually have a well tempered (untemperamental) impedance curve. It really helps if the x-over uses a simple first order network so that power is going to drive the speaker not the network.
Perhaps it is because most American designers opt for higher order networks that I find I usually prefer some European models which present an easier load to a tube amplifier. On a few occasions I worked with series x-over networks and I found that they worked better than the standard parallel networks, though they were harder to develop.
My current favorite commercial speakers are the Swiss-made Reference 3A Royal Master Control (US importer Fanfare Int'l 212-734-1041). They are two way systems featuring an 8" driver and a partially horn loaded tweeter. The 3A uses no crossover on the bass/mid driver and two resistors and one Hovland cap on the tweeter. Wiring is Siltech silver 22/2 (approximately 15 gauge). They are rated at 93dB/W/m with an impedance that doesn't dip below 7.8 Ohms and they sound great with my 300B amps. Aside from the Reference 3As, I suggest that owners of my 300B amps be sure to have a listen to Spendor 71 and QUAD ESL II (both from QS&D 800-659-3711) and also the ProAc Response I & 2 (US importer Richard Getberg 410-486-5975).
The only vintage speaker I have had good luck with is the WE/Altec 755A, maybe because this full range cone driver requires less tweaking than multi-way systems. I used a 1.8 cu ft cabinet with the 755A with two Scan Speak Variovents to extend the LF a touch. Although these ancient, impossible-to-find speakers are great in the mids, they lack bass and high end. Other vintage components may have potential but I get frustrated with all the work necessary to get them right.
LS 0/65 Single Driver Single Ended Partner
The Triangle drivers are now distributed in the US market by ORCA, which is a good thing because they can provide the basis for a reasonably priced, non-time consuming, near-foolproof efficient speaker project.
I built speakers for years back in the early 80s when I had access to great test equipment in college. I even designed a test fixture for the DEC VAX 11/760 for my senior project. After school, I got bummed out with speaker projects because testing can take forever. Luckily there are some great PC and Mac packages to help steer us modern speaker craftsmen through our explorations. I used the new Audiosuite software from Liberty Instruments, mainly because it was cheap, it does the job, and Bill Waslo lives here in town.
I also downloaded some programs from the Madisound BBS to calculate the cabinet sizes. The Ti 7 FLV has a resonance of 49 Hz, nominal impedance of 12 Ohms, and it is 94dB/W/m efficient. With a Qt of 0.41, the volume of a ported enclosure needed to be 1.8 cu. ft. I used the Woodstyle WS123 cabinets for my 755A experiments and I remembered that these were just the size I needed. For this project, I used the new slim line (I2" W x 13.75" D x 24" H o.d.) version.
With a 1.8 Cu. ft. Cabinet and a box resonance of 52 Hz (-3 dB point), I came up with a port size of 4" round and 4.5" long. Using the Audiosuite setup, I was able to adjust the port equal to the resonance frequency empirically. The optimal port size determined by measurement wound up very close to the calculated results at 4.375". There was a slight rise in the output of the driver at 2 kHz that was up around 5 dB higher than the mean of the response curve. I wrote a program in "C" on my PC to determine the appropriate notch filter for that frequency. I came up with 20 uF in parallel with a 6 ohm resistor and a 0.5 mH inductor.
Using the frequency response plotter in Audiosuite, I tuned the filter by varying the resistance and capacitance. The EQ sounded and measured best with a 5 Ohm instead of 6 Ohm resistor and with a paralleled 0.22 uF Hovland across the SCR 20 uF capacitor. As you can see from the impedance plot, the notch filter centered around 1.75kHz instead of 2kHz. I think it sounds better with the filter because the main irregularities in the response shift down to the less offensive <2kHz region, aside from having a flatter response overall. The minimum impedance of the system is 11.9 Ohms at 240 Hz with a mean sensitivity of 94dB/W/m. Perfect for your SE amplifiers.
This speaker plays realistic bass down to about -3 dB @ 50 Hz. The highs are a bit attenuated but what's there sounds very good. Overall, the sound quality of the LS 0/65 in on a level with many of the high quality commercial speakers I have heard. The nicest thing about the speaker is the midband speed of the crossoverless driver. Also, this design avoids x-over distortion problems that usually accentuate driver mismatch in the upper midrange.
SCR 20 uF / 630 Vcaps
Cut and route the driver holes and the duct. Install Deflex pads directly behind the driver and at the top of the cabinet above the driver. This will help clear up any cabinet resonances. Install the driver 8" from the top of the cabinet to driver center and install the port 8" from the bottom to center.
I think you will be very pleased with the performance of this simple, low resource drain project. After listening to these one-ways, I have a better appreciation of why audio purists have always searched for the perfect single driver loudspeaker. A single cone and a nice triode amp can yield some fine musical enjoyment.
Zalytron Industries Corp.
ORCA Design and Manufacturing Corp.