Current Dumping Revisited

It's probably about ten years ago that I read a letter in Wireless World magazine from the late, and great, Scroggie deploring the term "current dumping" to describe the amplifier topology invented and exploited by the Acoustical Manufacturing Company in the Quad 405. Scroggie acknowledged that the term pertained to the relatively straightforward task undertaken by the output transistors in such an amplifier. However, he argued, since the current the output transistors passed was that used directly to energise the loudspeaker and thus produce sound, the act these transistors played could hardly be considered "dumping" suggesting - as the word does - that they were involved in disposing of a useless commodity. Times change and words with it. Now dumping is an everyday word for storing something precious - computer data. Perhaps it is only the ever-Victorian, scatological British who could regard responsible "dumping" as the hallmark of a accountable professional life. Woe will surely befall a man who fails to dump his data in the appropriate way and at the appropriate time! Not that Scroggie was criticising the development of the Quad 405. I would say that the belief is generally held that Quad's current-dumping idea is a clever conception. I've owned a Quad 405 as a studio amplifier for many years and it's a wonderful product. I choose to ignore the engineers and academics who consign it to the waste-paper basket of history, they - blinded by mathematics - prefer their own (faulty) reasoning to the evidence of their own senses. Technically current-dumping does work - believe me I've measured it. It works well and it goes on working, year after year. But how exactly does it work?

In order to illustrate this, I've drawn Fig. 1; a simple power amplifier with a voltage amplification stage (A1) and a complementary bipolar transistor output-stage. In many ways this looks like any other complementary power amplifier except that there is no static bias applied to the output transistors. Negative feedback encloses the whole and attempts to maintain linearity despite the large transfer-characteristic discontinuity introduced by the unbiased output stage. If you breadboard something like my Fig. 1, I guarantee you'll be surprised. It doesn't sound that bad. Provided the gain/bandwidth of A1 is large enough, the output signal is remarkably undistorted, especially at low frequencies. Essentially this arrangement would be entirely acceptable were it possible to construct the amplifier A1 perfectly, so that it "slewed" infinitely quickly across the crossover "deadband". I've tried to show the way A1 behaves by sketching the signal waveform at the bases of the two transistors. Of course, it isn't possible to construct a perfect amplifier for A1 and, in practice, as frequency increases, crossover distortion starts to make itself heard. The essence of the idea behind current-dumping is illustrated by the inclusion of Rd (shown with dotted connections in Fig. 1). Rd feeds current directly to the load during the proportion of the output cycle when both the output transistors are off. In effect, Rd reduces the "gap" A1 is required to slew across and permits a practical amplifier to be used instead of a mythological, perfect amplifier. Stripped of their duty during the essential and fragile crossover region, the output transistors are only called upon to deliver drive into the load at powers above about 100mW whilst A1 does all the clever bit in between - hence the term "current-dumping" to describe the uncomplicated job they perform. In Quad's final circuit, the resistor is augmented by a capacitor and inductor which act in a reactance-bridge arrangement to enhance the effect of Rd across the whole audio bandwidth.

Despite the undeniable engineering ingenuity displayed in the 405 amplifier, Quad never seem to have earned creme-de-la-creme, audiophile status for this product nor indeed its heirs. However, comments that amplifiers of this stable lack, "slam" or "punch", probably derive from Quad's prudent decision to employ extensive current-protection and sub-bass roll-off (features which have many times saved my 405's life during its spell in my studio). Importantly, it is almost certainly not the action of the amplifier topology itself which accounts for this apparent lack of subjective impact, but the effect of these other ancillary features and design decisions. Is it, perhaps, because of this poor subjective reaction to the Quad amplifiers that other manufacturers have not trodden the current-dumping route? Unlikely, more probable is that Quad's patents have prevented a flood of carbon-copy, current-dumping amplifiers.

That brings me to the nub of this month's column, does patenting an invention like current-dumping really do anybody any good in the long-run? Granted, Quad have kept the technology to themselves but have they ultimately profited from this? Have they sold more amplifiers by virtue of their innovation? I doubt it. Quad's undoubted reputation of reliability and solid engineering would hardly have fallen apart if they had (like everybody else) built upon their success with the 303 and continued with the development of the class A/B audio power amplifier. True the designers, freed of their usual commercially-rooted secrecy by the sanctions of the law, were able to write articles in learned periodicals explaining the technique in full and bask in the glory of their own cleverness. But did Quad really hope that other manufacturers, wowed by the brilliance of their idea, would opt to pay Quad a royalty for every power amplifier sold? Surely not. After all, putting the invention in perspective, what was done in the 405 was to engineer a power amplifier minus the usual requirement for a quiescent bias adjustment and that simply just does not confer a big enough commercial advantage for others to follow. It's not as if they invented an amplifier without the need for a mains transformer! Sadly, I have to conclude that protecting the current-dumping invention has done neither Quad, nor its inventors, nor- most importantly - the idea itself any favours. Interestingly, Quad's patents for current-dumping were published in 1975 which means the protection expires next year. So perhaps current dumping could yet see its full flowering and become household bywords for high-quality amplification. Sorry Mr. Scroggie.