Current-drive or transconductance amplifiers for consumer audio applications are very rare. One example prior to Nelson Pass' current FirstWatt F-1 model was a design by Be Yamamura intended to be used with his single-driver full-range loudspeaker, both of which at the time were imported into the US by Joe Cohen of now Prana Cable.

The new FirstWatt marquee is a "kitchen table" effort by Nelson Pass who, as is well known, enjoys an enviable reputation as one of America's most influential amplifier designers and commercial success with his Pass Labs brand, catering to users of conventional multi-way/crossover loudspeakers and their needs for common voltage-drive amplification. FirstWatt was launched deliberately to funnel some of Nelson's most intriguing but highly unconventional amplifier circuits into short-run productions to becomes a creative outlet for his many ideas which are more far-reaching and diverse than could possibly be captured by his Pass Labs designs. At the same time, FirstWatt is envisioned as an amplifier resource for music lovers and experimenters who are faced with very specific special-application needs.

Nelson plans on limiting each FirstWatt model to 100 production units. This should allow him to tailor each model to a very particular and -- hence by necessity -- very limited usage scenario and thereafter make room for another unique application condition and an amplifier design that will optimally accommodate it and nothing else. Before I report on the F-1 and its intended interface with crossover-less single-driver loudspeaker designs, I want to direct our technically curious readers to an article by P.G.L. Mills and M.O.J. Hawksford that Nelson forwarded to me on the subject of "Distortion Reduction in Moving-Coil Loudpseaker Systems using Current-Drive Technology" as published in the J. Audio Eng. Soc., Vol. 37, No.3, March 1989. Those interested in the whole article and its graphs can download the PDF here. The following selective recap is intended for those more interested in the general gist of this research paper.

Moving-coil drive units make up the majority of what shows up in conventional loudspeakers. Simply think cones and domes rather than planars, electrostats or esoteric tweeters. Now precede such a cone'n'dome box with a conventional power amplifier's output voltage. Its current encounters the series elements of voice-coil inductance, voice-coil resistance and interconnect and amplifier output impedance. What happens? "A force related to the current in the system then acts on the drive-unit moving element as a result of the motor principle, and once motion occurs, an electromotive force is induced in the coil to oppose the applied signal voltage, thus constraining the magnitude of current flow." That means that how accurately the driver can translate the input signal into its motion equivalent depends on the series elements in the circuit. Any signal-induced changes in their values will result in distortion.

One of these distortion mechanisms is called thermal distortion. Thermal distortion occurs when the voice coil heats up often in excess of 200° C to change its resistance which alters the driver's operative sensitivity especially in the high frequencies and leads to a lack of damping and crossover misalignment. Another is called displacement distortion that occurs as a function of dynamic changes in the voice coil inductance and is further compounded by "eddy current coupling to the pole pieces in the magnetic circuit".

The researchers propose that to overcome these limitations, a loudspeaker drive unit should be coupled to an amplifier with current rather than voltage output and a high output impedance. Their lengthy paper then makes available computer simulations and objective measurement graphs to support their findings. They also complement those with "the application of both motional feedback and noninteractive frequency response shaping as a means of aligning the drive unit Q to the required value". The drive unit in their tests was a Celestion SL600 135-mm mid/bass unit and one series of graphs relates to measuring the accuracy of the transfer function between a voltage-source amplifier's output and cone velocity, with the former assumed to have zero impedance. Comparing results between voice coil temperatures at 20° C [left] and 200° C [right] in a passive two-way with a 2nd-order crossover at 3.4kHz shows pretty significant errors.

After conducting and documenting their tests, the researcher conclude that "the principal advantage of current drive are an elimination of performance dependence on voice-coil resistance (which is thermally modulated) and also coil-inductive effects, which give rise to high-frequency distortion along with nonlinear electromagnetic damping due to Bl variations." They go on to observe that transconductance drive is similarly insensitive to the series elements of the loudspeaker cable but recommend the lowering of the system Q in response to the loss of amplifier-generated damping. This recommendation is directly addressed by Nelson's previously published paper on compensation network optimization of loudspeaker frequency-response performance.

Before we investigate the subjective performance of the FirstWatt F-1 in the upcoming review, the reader needs to be reminded that the nature of current vs. voltage drive isn't unlike the old "you say potato, I say potaito; you say tomato, I say tomaito" bit in that a loudspeaker's resistive and inductive crossover elements will react in exactly the opposite fashion when driven by either amplifier output. Because conventional multi-way loudspeakers are optimized for conventional voltage-drive amplification, the F-1 will introduce "crossover inversion" effects and, while being perfectly safe to use, then cause response aberrations. It is thus not intended to be used with such loads. While the transconductance amplifier by Yamamura caught fire in Joe Cohen's home those many years back because it lacked protection circuitry, the FirstWatt F-1 does sport multi-tier protection and is perfectly safe to use. However, it is a specialty product intended for speciality applications, namely single-driver crossover-less speakers whose usually bandwidth-limited frequency response and associated response nonlinearities are said to benefit audibly from current rather than voltage drive. Nelson has also since published a paper on Crossovers for Current-Source Amplifiers to expand usage of the F-1 to crossover-type speakers whose networks have first been adapted to the special requirements of current-drive amps.

To test performance claims, I have finally taken receipt of Terry Cain's standard Abby (shipper and my remote address conspired to lengthy delays), Louis Chochos' Omega Loudspeaker System's 2 x 4" Fostex TS33 vented box and a Nelson Pass testing sample of a prototype Lowther DX-55 sealed box. In addition, I will use my AKG K-1000 dynamic headphones which conform to the single-driver crossover-less requirement and sport speaker-level connection. Comparator amps will be the Eastern Electric MiniMax and Audiopax Model 88 tube amplifiers which are examples of SETs and push-pull valves customarily fancied by single-driver loudspeaker owners. Preamps will be the Eastern Electric MiniMax, the Wyetech Labs Pearl and the Bel Canto Design PRe2.

Terry Cain of Cain & Cain had visited Nelson in his California digs to drop off the review pair of Abbys, hear the F-1 into this familiar load himself and allow Nelson to determine the precise values for his external compensation networks shown above and since in my possession. Nelson ended up purchasing the review pair and Terry since is in possession of an F-1. Unlike Dick Olsher of EnjoyTheMusic.Com and yours truly who will take the F-1 through its formal paces, Terry and his colleague at The Horne Shoppe feel under no obligations to go public with their findings - but I can tell you informally that both have reported stunning results on their speakers and already endorse the F-1 based on their personal experiences as a very viable and high-performance alternative to common SETs into such loads. Having allowed the "defense" to make its case with today's reference to an augmentative scientific research paper on this subject of transconductance amplifiers plus Nelson's prior White Paper, it now is our turn of the "prosecution" to compare those claims to subjective facts. Stay tuned as we shall soon report our findings and verdict. (Needless to say, the judicial posturing is in harmless jest - this is far too enjoyable and interesting an assignment to involve the law!)
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