response deviations. Fig. 5 gives an example of such a case, where the aberration is relatively minor; however, Fig. 6 shows the effects with one particular loudspeaker (which really exists) where the deviation is dramatic.

Just as the cables have an inductive element, so do the amplifiers which drive them. In tube amplifiers the output transformers provide the inductance whereas in most solid state designs the designer has deliberately provided inductance in the form of a coil for added circuit frequency stability. The reason that damping factor (8ohm/output Z) has been traditionally quoted at low frequencies is not only because much of our interest in damping centers on the woofer, but also because this coil destroys the damping factor at high frequencies. In fact, examples exist of solid state amplifiers which quote damping factors of 500 or greater at 20Hz but which have damping factors on the order of 15 at 20kHz. More recently a few designs (Threshold, Audio Research, Yamaha) have dispensed with output coils, giving them more constant damping factors across the audio band. The new IHF test standards call for measurement of damping factor at all audio frequencies.

Again, this parameter must be evaluated in the context of the system. For example, most loudspeakers have a considerable inductive component of their own, which may easily provide the desired case of amp Z + Cable Z < speaker Z. Conversely, the source and cable inductances become more important with loudspeakers whose load impedance is resistive or capacitive, especially with some electrostatic designs. Our initial premise does not always hold either, for I have seen "poor quality" cables used to isolate an amplifier from a reactive loudspeaker and thus improve the performance. In general, however, we are looking for amplifiers with low output impedance and cables having low series impedance.

Z01 a value expressed in ohms and determined by the inductance and capacitance:

When a wave travelling down a length of cable reaches the end of the cable, it will do one of three things depending on the impedance of the load. If there is a high impedance load, so that ZL, > Zc, the load will reflect energy positively back down the cable to reappear at the source (Fig. 9). If the load impedance is less than the characteristic impedance of the cable, the wave is reflected back negatively; and if ZL = Zo, then the wave is fully absorbed and none is reflected.

This mismatch of load impedance to cable impedance causes the resonance observed in Fig. 8, which we see diminish in Polk and Mogami cables when they are loaded with 8 ohms, a value near their characteristic impedance. By contrast, twin lead conductors have a higher characteristic impedance and perform better at multimegaHertz frequencies with about so ohms load impedance. The effect of twin lead cable on a 5uS pulse with an 8 ohm load shows the effect of a load impedance lower than Zo, where the cable inductance cable rolls off the edges of the pulse, but where the 8 ohm resistance is sufficient to damp the ringing which occurs with Z. = 0 or ZL = infinity . Not so for capacitive loads as shown in the fourth column where another resonance altogether has developed due to the inductance of the cable and the capacitance of the load where:

Note that these effects exist with all cables. The fact that only the newer, low inductance cables appear to affect amplifier stability brings us to a point which justifies our examination of a cable's performance in regions which are simply not audible. The lower resonant frequencies of the cables having low Zo enter into the output bandwidth of the amplifier as it approaches its unity loop gain, and by altering its phase response cause oscillation at the resonant frequency.

Earlier amplifiers as well as some currently available, having slower output stages (less than 1MHz), did not interact with these resonances because they occurred above the cutoff frequency of the active devices by an order of magnitude. However, as the newer cables decreased impedances and as amplifier output stages increased in bandwidth beyond 5MHz, the two effects met and resulted in various forms of sonic problems, fuse blowing, and worse. At Threshold we first ran across the problem with the mating of our 400A and Polk sound cables which caused fuse blowing (due to oscillatory cross-current conduction) with great regularity.

After a period of confusion, Matt Polk and I realized independently that the lack of a characteristic termination was causing the problem. Polk developed and patented a "damper" consisting of a .047uF capacitor and 6 ohm resistor in series placed across the loudspeaker, while I used the same network but with .1uF and 5 ohms. The results of this network are seen in Fig. 8 where the resonance in the pulsed waveform is damped out, restoring stability to an otherwise oscillating amplifier. Since Polk's commercial introduction of the damper circuit we have

Frankly, I found it difficult to assess the results except at the extremes of performance. For 10 foot lengths with properly terminated cables and speakers with inductive high frequency characteristics, the differences between low inductance cable and twin conductor are extremely subtle and subject to question. With a low output inductance amplifier and a Heil tweeter (whose impedance is a nearly perfect 6ohm resistive) the difference was discernible as a slightly but not unpleasant softening of the highest frequencies. Fulton or Monster cables were a clear improvement over 24 or even 18 gauge, though a little less subtle than I would have expected, leading me to believe that the effort associated with heavier cables pays off in bass response and in apparent midrange definition, especially at crossover frequencies. The worst case load, the modified Dayton Wright electrostatics, presented some interesting paradoxes: the extremely low impedance involved showed the greatest differences between all the types of cables. However, the best sound cables were not necessarily electrically the best because several amplifiers preferred the highest resistance cable. In one case, I had to use 24 gauge cable to prevent tripping the amplifier's protection circuitry.

CONCLUSIONS

Who am I to dispute the feelings of audiophiles who, evaluating any cable in the context of program source, amplifier, speaker, and listening room, decide they can hear the difference? A few guidelines have emerged here, but the final judgment belongs to the user. All the special cables mentioned worked well on the test bench and, given the assumption that series impedance should be minimized, all of them work better than 16 gauge wire. If, like many audiophiles, you have spent a small (or large) fortune on your hi-fi system, money spent for high quality cables and connectors is a reasonable investment.

4. Klipsch, Paul, "Speaker Wire," Dope From Hope, Sept. 13, 1978.

5. Pass, Nelson, "Loudspeaker Damping," Audio Magazine,

6. Gailus, Mark, "Speaker Wire: What Size is Sufficient?", Audio Forum, Aug.-Sept.,

7. Greinier, R. A., "Another Look At Speaker Cables," Speaker, The Boston Audio Society, Dec. 1978.

8. Gross, T. A. G., "Multistrand Litz Wire Adds "Skin" To Cut AC Losses in Switching Power Supplies," Electronic Design, Feb. 1, 1979.

Subscription: Speaker Builder, PO Box 494, Peterborough, NH 03458