Hi Bill,
I had a great time yesterday auditioning my latest speaker creation on your source and amplification equipment. Clearly you have some of the best tube equipment in the world, and it is an honor to have my speakers represented with such top notch gear.
I also wanted to take the time to elaborate on why your impressions, after listening to the speaker with a variety of top notch recordings, mirror my findings with the measurements and design choices I made for this particular speaker.
Let me first comment on the cabinets pictured here:
The curved top is created with a stacked lamination technique. Each layer is cut on a CNC machine and glued together to create the curved "shell" for the top section. The lower section of the cabinet is a traditional rectangular enclosure rabbeted together with the top section. Everything is then heavily braced and the side walls are laminated with a sound deadening vinyl layer to absorb cabinet wall vibration.
You can see the bracing in this picture before the dampening fill and crossovers were installed:
All the walls are then lined with an additional layer of foam and fill to absorb standing waves in the cabinet (not shown). The result is a very smooth impedance response. The valley in between the twin peaks in the impedance shows the box tuning frequency (~ 38hz).
As you can see this speaker is a solid 4 ohm speaker, but the impedance stays within 3.2-4ohms for most of the frequency band, thus making it suitable for tube amplifiers with a rating of 50 watts or more into 4 ohm loads.
The bass extension is an F3 of 38-39hz as shown in this model verified with nearfield port and woofer data, as well as the impedance plot above. Since bass is so room dependent, it is safe to say with room loading, the bass will extend to the upper 30s.
Since this speaker uses dual Seas W18NX's, which have the more advanced motor and a rated 7mm xmax, expect these to play around 103-104db/1watt meter before running out of steam for most music excluding pipe organs of course! Don't play that loud or you will ruin your hearing anyway.
The on and off axis is very flat. The response favors a tweeter toed in such that the off axis is roughly 5-10degrees in the horizontal plane, but that is a personal preference. The vertical polar response favors the tweeter pointing directly at the ear level or slightly above ear level. This can be achieved with toe spikes to fire the tweeter at ear level. The power response and off axis response is clean very far off axis in the horizontal plane thanks to the ribbon's narrow element and the pistonic nature of a small midrange covering the frequencies that most MTMs start to beam like crazy. This is more than likely why the speaker imaged so well and created a large soundstage at Bill's. I also attribute the narrow baffle around the midrange and tweeter to that effect. It sure would have been easier and cheaper to build a rectangular box, but I wanted to take this one to the max.
I'm going to keep the crossover points and slopes a secret for now, but there was a lot of consideration placed on the Fcs (frequency cutoff) to minimize over excursion of the midrange, allow a smooth transition between the woofers, mid and tweeter, and do so with the best power response; given what is known about passive crossover design. Each driver was tested for overall harmonic distortion at realistic listening levels to insure they operate in the lowest distortion passbands. This also helped aid in the selection of the crossover points.
Phase alignment was also considered while designing this crossover, which was easier because the acoustic centers of the mid and tweeter are very close to being on the same plane. This allowed me to hit the desired transfer functions without worrying about the need for overly complex electrical circuits to hit the acoustic slopes or run into phasing problems. Therefore, the crossover design is elegant and uses a minimum amount of components to do the job the most effectively, but an impedance this flat does require an additional zobel circuit/s.
Don't worry though, the crossover uses nice components in the crossover in addition to the top notch drivers that were selected. Air Core inductors are used throughout, with ERSE high quality 14 gauge series inductors where lower DCR is needed. The caps are a mixture of Clarity Cap PX for values above 10mfd, Solen for values over 68mfd, and Clarity Cap ESA for the tweeter. Resistors are either of the non-inductive variety, or Mills where I felt they were most important. Absolutely no electrolytics are used in the circuit. I want these speakers to last a long time and the electrolytics I've measured in the past were either not within close enough tolerances, or they degraded over time. The wire I used was Supra wire from madisound.
So, these are some of the reasons I selected the parts that I did. The "sum" is a culmination of these design decisions. Pricing has yet to be finalized, as well as a name for the speaker, but expect that information soon and the pricing to be competitive to what is found here at audiocircle. I want to offer the best value possible, but these speakers are very time consuming to make! I hope you get a chance to listen to them at Response Audio soon.
Best Regards,
Jed
