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:
The feedback for A is taken from the output of the current dumper. It is easy to see that for the feedback signal, A, R1 and C1 form an inverting integrator. As usual, the integrator adds a phase shift of 90° (on top of inverting) and amplification, with gain falling by 20dB/decade as frequency increases, reaching unity at
F0 = 1/(2π×R1×C1)
With the values shown in the schematic above, F0 ≈ 4.82MHz. The gain of this integrator at any other frequency is
G = F0 / Fx
For example, at 20kHz the gain is 4,820 / 20 = 241, or 47.6 dB with the values shown. As in any other negative feedback amplifier, the integrator's gain works to reduce the open-loop distortion of the current dumper by a factor of (1+G).
The residual distortion from the output of the current dumper is fed to to the load via L1, which adds 90° of phase lag. Since L1's impedance increases with frequency, the distortion level at the load falls with frequency at 20dB/decade. The same residual distortion is amplified by the integrator and appears at its output (and, via R2, at the load). Here, the level also falls with frequency at 20dB/decade, but the phase shift is 270° (180° to account for inverting and 90° for integrating), that is, 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 cancelled.
According to Walker, perfect cancellation occurs when L1=R1×R2×C1. In the language of the AES paper, "For the linearity of Trl and Tr2 to be immaterial then L must equal RRC". This conclusion, however, is based on a number of assumptions, among which are that A has large gain at all frequencies, that the amplifier has closed-loop unity gain and that L1 has zero DC resistance. The cancellation condition can be generalized for a more realistic setup, but the details will have to wait for another post.
References:
- P. J. Walker and M. P. Albinson, "Current Dumping Audio Amplifier," presented at the 50th AES convention, March 1975.
- P. J. Walker “Current Dumping Audio Amplifier,” Wireless World, vol. 81, pp. 560-562, Dec. 1975.
