A major challenge of committing to single ended triode sound is finding a speaker that works well with a SET amp in a home environment. As a manufacturer of SET amps for over 20 years we at Bottlehead have tried so many different speakers it makes my head spin recalling them. The traditional approach of using huge, expensive, super efficient horn speakers looks good on paper. But in practice we found that the lack of deep bass, sometimes “down the well” sound and the large inter-driver distances make this kind of speaker more suitable for large rooms like theaters than the typical 12’x18′ living room. Another option is to set up a near field setup with inefficient small speakers. This can work, but typically requires the added complexity of a subwoofer to achieve a satisfying tonal balance. And it won’t play very loud without the amp clipping.

In 2008 I needed a system that I could use to evaluate the quality of recordings being released on a new record label I had founded with partners Paul Stubblebine and Michael Romanowski, called the Tape Project. The reel to reel tape recordings made from original masters were being touted as the best sounding format available. From years of working with studios and high end speaker manufacturers who asked us to demo our tapes on their speakers at trade shows, I drew ideas from what I experienced and settled upon a unique solution for my own system. The result was a system that had lovely even response from the low 30s to well above 20kHz, capable of 105dB peaks. But that system required four woofers, a power hungry midrange driver, an expensive ribbon tweeter, an active tube crossover spread across three chassis and up to nine power amplifiers to operate it. It was huge, expensive and required constant maintenance. All that was justifiable for a system being used for studio work, but it had little to do with the real word systems we were selling our kits for.

After spending the past few years fleshing out our product line to include three levels of great price/performance amp and preamp kits, it was time to address this speaker issue. We needed a real world speaker that we could use to demonstrate any of our amps, one that would be suitable for a home listening environment. This wasn’t our first rodeo. I had put out a modestly successful speaker kit in 2001 called the Straight 8, and we had developed a couple of other designs in later years that, for lack of consistent supply, never made into production. So we had a pretty good database of ideas to draw from, even if some hadn’t panned out.

We knew we needed decent speaker sensitivity. Everyone dreams of 100dB+ sensitive speakers. The laws of physics tell us these will need to be huge if they must deliver decent deep bass. Very few people’s listening setups are really constrained to 100dB sensitive speakers. If we use the rule of thumb that a speaker needs to be able to hit 102dB peaks to be able to play any kind of music at a volume above which one will damage one’s hearing, heck, even for that you only need a couple of watts. As the sensitivity requirement decreases we can reduce the cabinet size to a more room friendly one. With 94dB speaker one can achieve clean peaks of 103dB with 8 watts (think 300B). And one can still get quite loud with even a couple of watts. And so we settled on 94dB @2.83V/1M for our sensitivity goal.

Most of this issue of sensitivity vs. size is relative to bass performance. Tube amps typically have a relatively low damping factor, particularly amps with low or zero global negative feedback. This freedom from feedback is part of what we feel makes the SET sound so eerily real. A few years back Paul Joppa realized that we could adjust the Q of a vented box to take advantage of the low damping factor, and in the process gain about half an octave of deep bass over a box designed with a more traditional Q value. And so we have employed this approach and the Jäger gets down nicely into the mid 30s. To keep those woofers working in the most optimal fashion we have given each one a separate vented chamber.

Another important goal was good imaging. The upper woofer crosses over to the tweeter at 2800Hz, while the lower woofer has a lower crossover point. This shading of the lower woofer helps this by reducing the potential for cancellation effects near the crossover frequency. The relatively narrow vertical dispersion of the tweeter horn and the rather complex felt diffraction panels also help to reduce phase interactions that might spoil the sense of soundstage.

The cabinet material is Baltic birch plywood, with birch edge banding. A lot of theory is bandied about regarding cabinet materials (and just about every other aspect of speaker design). I said the heck with relying upon theory alone, and we built test cabinets in both Baltic birch ply and veneered MDF. The birch speaker was more dynamic, and sounded cleaner, and the added cost was considered worthwhile. The panels are precision CNC’d and assemble with camlock fasteners, just like Ikea furniture but with the addition of wood glue in each seam to make the box air tight. The camlocks assure easy alignment of the panels and good clamping force as the glue dries and a cabinet assembles in just a few minutes. We have had a few requests to sell this kit without cabinets. That’s not going to be any more possible than buying a new BMW without the suspension. The panels are a fundamental part of the sonic character and thus there is no way we could guarantee the sonic quality of this speaker if we didn’t have supervision over the making of the cabinet. But you do get to choose your favorite way of finishing them. The prototype in the picture was sanded with 220 grit paper, given two coats of Seafin teak oil, sanded with 400 grit paper and buffed to a satiny sheen. It took about 20 minutes and looks and feels great.

The drivers are given extra consideration as well. The tweeter horn is covered with a damping material and the mid-woofers are iso-mounted from the cabinet with rubber. Impedance compensation has been developed for each driver and it lives on a board inside the cabinet. This allows the crossover to be made in a very straight forward manner, the passive one supplied can be updated with an active tube crossover we hope to release in the future. Both impedance and crossover boards are engraved and cut from plywood with our in house laser cutter, and components are attached in the traditional method, soldered to metal eyelets – no PC traces! The passive crossover can be used with a single amp via the use of jumpers, and with the jumpers removed the speaker may be biamped. It fits in a small cubby hole under the speaker, so that it is easily removed for modification or replacement with an active unit. Wiring is all Teflon coated OFC – better known as plenum grade CAT5 cable. Crossover components include air core inductors, polypropylene film capacitors and low inductance resistors.