[Part 1 begins with an overview of amplifiers, and discusses basic requirements for current and voltage output, and transient response. Part 2 discusses nonlinear distortions in amplifiers, as well as amplifier classes and modes of operation, beginning with Class A and AB. Part 3 covers Class D and other amplifier classes, output transistor types (MOSFET or BJT), and choosing an amplifier. Part 4 begins a discussion about loudspeaker cables and their effect on performance.]
6.10 Some provable characteristics of cable performance
In the autumn of 2002, the authors, together with other collaborators, made a presentation to the Reproduced Sound conference of the Institute of Acoustics2. Figures 6.7, 6.8 and 6.9 were taken from that paper. They were from an experiment made at Czerwinski Laboratories in California, by two Russian engineers, Alexander Voishvillo and Alexander Terekhov, accompanied by Eugene Czerwinski, (the founder of Cerwin Vega). [In the 1950s Czerwinski had designed the first of its kind 10,000 watt amplifier using germanium transistors for sonar systems for the US Navy.]
They carried out tests on three, 6 metre lengths of different cables, firstly into a resistive 8 ohm load (Figure 6.7), then into a full-range loudspeaker system (Figure 6.8), and finally into a cabinet-mounted low frequency driver (Figure 6.9). The amplifier driving the cables was fed with a multitone test signal3,4, designed to show up non-linear distortions; in particular inter-modulation distortion.
In Figure 6.7 it can be seen that all six plots are more or less the same. The left hand plots were measured at the amplifier output terminals, whilst the right hand plots were measured at the other end of the 6 metre cables - at the resistive load. The three very different cables all appeared to perform equally. This was what the experimenters were expecting to occur, independently of the load. They were all very much sceptics with regard to significant loudspeaker cable differences - at least between adequately rated cables.
Figure 6.7 Three 6 m cables feeding an 8 ohm resistive load. The left-hand plots are from amplifier output terminals. The right-hand plots were taken from the load
Figure 6.8 Three 6 m cables feeding a full-range loudspeaker with a passive crossover. The left-hand plots are from the amplifier output terminals. The right-hand plots were measurements from the loudspeaker cabinet input terminals
Figure 6.9 Three 6 m cables feeding a sub-woofer box. The left-hand plots are from the amplifier output terminals. The right-hand plots were measurements from the loudspeaker cabinet input terminals
However, when they replaced the 8 ohm resistive load with a passively crossed-over loudspeaker system, they were surprised to see the results as shown in Figure 6.8. With the more complex load, all the plots had changed. After changing the load to the low frequency loudspeaker, all the plots changed again, as shown in Figure 6.9.
The distortion patterns were noticeably different, not only between the cables, but also between the input and output ends of each cable. The implication here is that the cables change the way that the complex load is seen by the amplifier. Voishvillo reported that upon seeing this, Czerwinski exclaimed "But they're only short cables!"