Publisher's preface
In preparation for my upcoming review of Nelson Pass' new F-1 First Watt current-source amplifier, Nelson asked what speakers I would be using. His whole optimization scheme for the single-driver speaker/amp interface hinges on external compensation networks which are specific to each loudspeaker. Indeed, what speaker would I be using? Terry Cain of Cain & Cain and Louis Chochos of Omega Loudspeakers to the rescue. To enable Nelson to include optimized networks with the review amp, Terry Cain personally delivered a pair of Abbys to Nelson's California digs while Louis shipped his pair instead. Using on-site acoustical measurements of either speaker's frequency response allowed Nelson to tailor the compensation networks in conjunction with his F-1 amplifier and the actual speakers. What I'm expecting shortly are two pairs of single-driver loudspeakers, one amplifier and two pairs of optimized networks.


Coincident with the launch of his new First Watt brand and site, Nelson has penned a project overview paper. I asked for permission to republish it. This will serve as an introduction to my review as well as become general interest information for those interested in the high-efficiency single-driver speaker phenomenon. Said Terry Cain upon his return from Nelson's facility: "Nelson probably has a good idea every couple of minutes and a great one once a day. He probably has built 1% of these. Our HiFi world is so lucky." Indeed. When someone of Admiral Nelson's credentials investigates a subject, audiophiles worldwide benefit. The reason for the canny First Watt moniker should be self-explanatory by now. If the first watts sucks, why would you be interested in another 299 of the same? This is no flippant marketing slogan. When you run 100dB-efficient loudspeakers, you never get out of first watt. This requires a very different kind of amplifier whose 'torque' curve is in full swing while other amplifiers are still happily asleep. Needless to say, Nelson's new First Watt solid-state F-1 is such a rare beast and thus specifically and unapologetically engineered for a niche group within a niche group of audio users: 100 high-efficiency single-driver speaker owners.


Current Source Amplifiers and Sensitive/Full-Range Drivers by Nelson Pass
Introduction
Conventional wisdom holds that a pure voltage-source amplifier is ideal for audio applications and designers of loudspeakers generally work to that assumption. This belief has been particularly dominant since the development of high power solid-state amplifiers as begun in the 1960s. A small minority of audiophiles thinks otherwise. These are often people who use low-wattage tube amplifiers with unusual looking speakers. Well, of course entertainment is full of fringe elements.


A couple of years ago, Kent English and I were playing around with various ribbon tweeters. We noted how the ribbons themselves seemed perfectly happy being driven by a current source and without their usual matching transformer. We built an audio current source that delivered 1 amp of AC current per input volt to a maximum of 10 amps. This kind of amplifier is known as a power transconductance amplifier. Such circuits are fairly common in small chips which don't generate the high current necessary to drive a loudspeaker. Our amplifier had a high output impedance and thus no damping factor. Still, we found that the ribbon in the tweeter didn't seem to need much damping factor, measuring and sounding a bit better without the matching transformer. We decided that a current source was likely delivering more accurate force/acceleration to the ribbon than a voltage source. After playing with it for a couple of days, we put it away and went on to other projects.

During about this same time, Kent and I began playing with full-range drivers, those loudspeakers which deliver bass, midrange and treble from one single cone. In a number of ways, they don't measure as well as multiple specialized drivers particularly at the bottom and top of the audible range but there is something aesthetically appealing about the simplicity of the idea - and on many occasions, they manage to sound very good especially driven by tube amplifiers.


Most interesting are the full-range high-efficiency drivers that deliver the goods with only a watt or so. It's a big design challenge to produce a good sounding full-range acoustic transducer with 100dB/watt efficiency. When it is properly achieved, you get a wealth of detail, exceptional dynamic range and a sense of musical 'aliveness' that you don't often hear elsewhere.


Tube amplifiers seem to bring out the best from such drivers. They have more bottom end, a warmer mellower mid- and upper mid-range and often more top octave. By comparison, the 'best' solid-state amplifiers make them sound more like transistor radios - less bottom end and an occasionally strident upper midrange. If you are a solid-state kind of guy like me, you start wondering how that could be. If you are a tube aficionado, you smirk and say, "I told you so." The solid-state guy probably starts fixing the response with a parametric equalizer and the tube guy enjoys his music with a nice glass of wine.


Critical damping -- that resistive combination of electrical source impedance, suspension friction and acoustic load -- occurs when you apply a step pulse to the voice coil and the cone's motion doesn't overshoot. Under-damping results in bass notes that hang around a little longer than the amplifier intended. Over-damping has good transient bass control but also suffers a significant loss of bottom end response. Generally, we want something in-between, something closer to critical damping. Whether we slightly over-damp or under-damp seems to be a matter of taste.


The need for electrical damping is different for each type of loudspeaker and acoustic environment. High-efficiency full-range drivers are more easily damped than other types due to their powerful efficient motors and light cones. Looking at their bass response curves, we conclude that they are easily over-damped, resulting in excessive loss of bottom end. This partially explains the preference for tube amps with such loudspeakers.

Anyway, this assortment of observations arrived at a particular confluence when I hooked up a Son of Zen amplifier (Audio Electronics 1997 #2) to a pair of Fostex 208Es in sealed enclosures. The Son of Zen operates without feedback and has an output impedance of about 16 ohms. This nets a damping factor of 0.5, miniscule compared to the 100 to 1000 you can achieve with regular solid-state amplifiers.


With the low damping factor, the Fostex became a totally different speaker. It suddenly had bottom end response and a better top end. It still had the same annoying upper midrange that had Dick Olsher devise his passive equalization network. The low damping factor didn't cure the upper midrange faults but it seemed to work improvements everywhere else. A year later, Kent had acquired about 20 different full-range mid to high-efficiency drivers for us to play with (that's part of his job description) and we spent as much time exploring them as we reasonably could, trying different things to coax the best sound out of them with a current-source amplifier and various passive parallel networks.

What is a High-Efficiency/Full-Range Loudspeaker?
To arrive at high efficiency and wide bandwidth, we fundamentally need two things - a great motor and a great radiating surface. A great motor means getting the most force/acceleration for the least amount of electricity. It means a big magnet with lots of magnetic density in a precisely machined gap where a very light voice coil sits perfectly aligned. This voice coil is wound in a cylindrical assembly that maximizes the current exposed to the magnetic field and generates the highest amount of force (acceleration) for a given amount of electrical current. In other words, we need an expensive motor.


The other half of the equation is the cone assembly attached to the voice coil. In contrast to the fine motor engineering, here we start seeing more of the black art involved in designing such a drive unit. The radiating surface for such a loudspeaker must be very light to maximize the acceleration from the voice coil. At the same time, the cone needs to be stiff and inflexible so that this acceleration can be accurately transmitted to the entire radiating surface at once. When this fails at high frequencies (which it will), the cone needs to decouple the force gracefully to a smaller and smaller surface so that it effectively shrinks at the highest frequencies. In practice, it's a lot of art coupled to even more trial and error. Thomas Edison could have made a fantastic version of such a speaker and for his time, I believe he did.


Besides obviously requiring low power and only one driver per channel, the advantages for a full-range efficient loudspeaker are found in three areas. First, the speaker requires no crossover network to apportion frequency bands to different drivers and thus does not suffer the phase shifts that come with such filters. Second, the sound radiates from one point source - diffraction effects between multiple drivers are removed. Third, the electrical and mechanical qualities required by such drivers give rise to subjectively good dynamic range and detail, assuming we deal with an ideal example of a full-range efficient driver to begin with.


Naturally, the reality can fall well short of the promise. Real drivers tend to be too limited to satisfactorily deliver both the treble and bass octaves of audio. The decoupling of high frequencies on the cone is often not smooth, and with that come response peaks and dips in the upper midrange often followed by a high frequency roll-off. On the bottom end, over-damping of the very light cone contributes to a roll-off starting an octave or more above resonance so that many examples of such drivers fall short below 100Hz. Driven from a voltage source in a closed box, the response of such light-weight cones can be down by as much as 15dB at resonance. Some of these drivers are better than others, most are delicate and some of them are very expensive.