Transmission of an electrical signal from one end of a cable to another is fast but not instantaneous. Unless certain strict conditions are met, signal reflections occur at the cable ends, so that a delayed and attenuated version of the signal bounces back and forth within the cable until it is finally dissipated. At radio frequencies, where the transmission delay is comparable to the reciprocal of frequency (1/f), such reflections can completely disrupt the accurate transfer of information.
To prevent this, electrical connections are impedance matched. That is, the characteristic impedance of the cable is the same as the input impedance of the signal receiver. When this is the case, signal transmission is reflection-free. Examples of impedance matched connections familiar to audiophiles are the aerial input to an FM tuner and the digital interfaces between equipment such as CD players and stand-alone digital-to-analogue converters.
According to conventional wisdom, impedance matching at audio frequencies is unnecessary because the transmission delay is very short for the lengths of cable used in a typical hi-fi system, and the signal frequencies are comparatively low. Some have taken this to mean that cables do not act as transmission lines at audio frequencies but that is not the case.
The issue is not whether cables act as transmission lines, (they do, whatever the frequency), but whether their transmission line behavior is significant in an audio context. At Townshend Audio, we are convinced that it is. Our design aim is simple: to ensure that the signal waveform undergoes the smallest possible change between one end of the cable and the other. Impedance matching helps achieve this.
As the longest cables in most hi-fi systems are the loud-speaker cables these are most in need of impedance matching. But typical loudspeaker cables have a much higher characteristic impedance than the loudspeakers to which they are connected. Whereas most hi-fi loudspeakers have a nominal input impedance of 8 ohms or less, most loudspeaker cables have a characteristic impedance of 8.0 ohms or greater. As a result, the audio signal is not faithfully conveyed.
Critics of impedance-matched loudspeaker cables point out that true impedance matching is impossible to achieve between a conventional amplifier and loudspeakers. Loudspeakers usually have an input impedance that varies substantially with frequency, cable characteristic impedance also varies with frequency over the audible range, and the output impedance of a typical power amplifier is a fraction of an ohm.
All these factors make a genuine impedance matched connection unfeasible. It’s quite true, the theoretical ideal cannot be achieved. But by lowering the characteristic impedance of the cable to 8-ohms you still achieve more accurate signal transmission than with a conventional loudspeaker cable. To achieve such a low characteristic impedance in a loudspeaker cable requires a special form of construction, with the conductors placed as close together as possible to minimize cable inductance and maximize capacitance.
The Townshend lsolda speaker cables achieve this by using two thin strip conductors laid face-to-face, with only a very thin layer of insulation material separating them. Because some power amplifiers rely on cable inductance to ensure their feedback stability, a low inductance loudspeaker cable can cause certain amplifiers to oscillate and eventually self-destruct.
To prevent this occurring with lsolda loudspeaker cables, series inductors are incorporated within the elongated terminating cylinder at the source end of the cable. The value of these inductors (1.5µH per conductor) has been carefully calculated to ensure that amplifier instability will not occur. Because the strip conductors in lsolda loudspeaker cable are so closely spaced, any Radio Frequency Interference (RFI) pickup, from mobile phones or the myriad other VHF and UHF transmitters in use today is substantially common-mode (i.e., equal in both conductors). The series inductors at the ‘send’ end of the cable perform an important secondary function in preventing RFI entering the amplifier via its output terminals.
To back up our claim that an impedance matched loudspeaker cable gives superior performance, the accompanying oscillographs show the outcome of a revealing experiment. In each case the cable was connected between a conventional audio power amplifier and loudspeaker, so the results are representative of normal use. Four lsolda cables were used, two of them 1 metre long and the other two 6 meters long. The difference between the cables of equivalent length was simply that one was constructed as normal, with the copper strip conductors closely spaced, whereas in the other two the conductors were separated. In the first case, the characteristic impedance of the cable is 8ohms; in the second instance, the characteristic impedance rises to about 180ohms. Cable resistance, of course, remains identical. Figs 1 and 2 show the input (top) and output traces for the 1 m and 6m length 5-ohm cables on a square wave input.
To emphasise that the traces are almost identical, Fig 3 shows the input and output traces for the 6m cable slightly offset for easier comparison. Figs 5 and 6 show similarly offset traces for the 1m and 6m 180ohm cables. Here the differences between the input and output traces are clearly larger, and they increase in severity with cable length. These disparities are much greater than those introduced by the DC resistance of the cable, which is 0.0034ohm per metre per conductor. In the case of the 6m cable this corresponds to a total (loop) resistance of 0.041 ohm. If we assume a speaker whose impedance varies between 6 and 30ohms across the audible range, the variation in frequency response introduced by this resistance is a negligible 0.047dB. With the 1m cable it would be just 0.0079dB.
These results prove that lsolda impedance matched loud-speaker cable maintains signal waveform with a fidelity that no higher impedance cable can match. They also prove that lsolda cable, again unlike conventional speakers cables, can be used in unequal lengths on either channel without audible effect.
