Current Dumping Revisited

This post is a part of the series on audio amplifier feedback. The contents of the series can be found here.

Current dumping is a way of constructing a power amplifier where a low-power, low-distortion amplifier is used to correct the distortion of a higher-power, but less linear, amplifier ("current dumper"). The underlying assumption is that it is easier to construct a low power, low distortion amplifier than a high power, low distortion amplifier.

Current dumping was introduced by quintessential English audio company Quad and was used in a series of Quad's power amplifiers starting with the Quad 405. Quad's founder, P. J. Walker, presented the concept at the 50th AES convention in 1975 [1].

There has been much interest and public discussion of current dumping in late 1970s and early 1980s. While most reviewers used more or less complicated math to explain why and how current dumping works, the basic implementation is easy to understand on an intuitive level.

The following schematic is from Walker's original AES paper:

Here, A is the low power, low distortion amplifier, and Tr1 and Tr2 form the "current dumper". The feedback for A is taken from the output of the current dumper. The load is connected to both A (via R2) and the current dumper (via L1). 

Any distortion appearing at the output of the current dumper Tr1 Tr2 (point D in the schematic) is fed to the load via two parallel branches:

• Via L1
• Via the integrator A R1 C1, followed by R2:

 

If these two branches feed the load with the distortion of equal amplitude and opposite in phase, the load will see zero distortion - this is the big promise of current dumping.

Intuitively, since L1's impedance increases with frequency at 20dB/decade, the distortion it feeds to the load falls with frequency at the same rate and lags in phase by 90°. The same distortion coming via the integrator also falls with frequency at 20dB/decade, has a 90° phase lag, and is inverted - that is, its the phase is opposite to the distortion coming via L1. Since the levels are proportional and phases are opposite, with the right choice of R2, the residual distortion from the current dumper can be nulled.

The circuit is particularly easy to analyze if the gain of A is assumed to be infinite. In this case, the inverting input of A (labeled F) is at the ground level (that is, zero distortion signal) due to the feedback via C1. If the distortion is nulled perfectly, the load (labeled L) is also at the ground level. The current via R1 is equal to that via C1, and the current via L1 is equal to that via R2. A little algebra quickly shows that this can only happen when the DC resistance of L1 is zero, and its inductance is L1=R1×R2×C1. In the language of the AES paper, "For the linearity of Tr1 and Tr2 to be immaterial then L must equal RRC".

It is worth nothing that the gain of the integrator A R1 C1 is the loop gain of the amplifier, and that R1×C1 is the time constant corresponding to the frequency where the loop gain becomes unity (in magnitude; a 90° phase lag remains). For a perfect distortion cancellation, the impedances of L1 and R2 at that frequency must be equal.

Let's make a reality check and see if the ideal distortion cancellation can be implemented with reasonable parts. A typical Miller-compensated audio amplifier behaves like an integrator A R1 C1 above and reaches the unity loop gain at the frequency of about 1 MHz (see my previous post for an explanation). 1MHz corresponds to R1×C1 of about 0.16 μS. (Walker's own values, shown on the schematic above, give the unity-loop-gain frequency of 4.8MHz, which I believe is rather optimistic with the parts that were available in 1975.)

A typical air core inductor of the type commonly found at the output of such an amplifier will have an inductance in the low single μH range, so let's use Walker's 3.3μH. With R1×C1=0.16 μS, the cancellation condition above gives us 21 ohm for the value of R2. If we want the whole current dumping amplifier deliver, say, 100W peak power (50W RMS on a sinewave) into a 8 ohm load, our low-power, low-distortion amplifier A would only need to provide about 140mW into R2 at 20kHz, and less at lower frequencies. That looks rather realistic.

What is unrealistic are the assumptions of infinite gain for A and a zero DC resistance for the inductor. Still, the cancellation condition can be generalized for a more realistic setup, but the details will have to wait for another post.

References:

1. P. J. Walker and M. P. Albinson, "Current Dumping Audio Amplifier," presented at the 50th AES convention, March 1975.
2. P. J. Walker “Current Dumping Audio Amplifier,” Wireless World, vol. 81, pp. 560-562, Dec. 1975.