Friends,
I have considered the biwire question further and have identified a technical issue that may impart an effect on performance. A technical analysis by
J. Lesurf can be found at the link below.
http://www.audioholics.com/education/cables/bi-wiring-part-2-the-cable-conundrum/bi-wiring-from-amplifier-to-loudspeakerAnd follow up by
jneutron:
http://forums.audioholics.com/forums/showpost.php?p=248924&postcount=99The above articles represent a thorough analysis and shows that in the case of “perfect” (i.e., lossless) crossover components, there is no net difference in power delivered to the speakers, save that which results from the reduced losses that would result from the increase in total effective wire gauge. The same result could be had by simply increasing the gauge of a monowire, equivalent to the sum of the biwires.
The fact is though that the real world is a little more complex. As is pointed out, the analysis does not consider the back emf generated by the drivers themselves or that of the reactive components in the crossover. It is because of these that I suspect an improvement could be had. A deeper analysis of the real-world conditions requires a model as a starting point for further examination. The model I propose is as follows:
The driving amplifier is considered a “perfect” voltage source. In EE terms this is defined as a source of voltage that has an infinitely low output impedance. That means that the amplifier can continue to deliver more and more current as the load resistance approaches a dead short-circuit without the output voltage at the terminals collapsing. Of course, no such circuit exists as it would require infinite current from the wall outlet, on through the amplifier’s power supply and then through to the output devices.
Furthermore, the model requires that the amplifier employ negative feedback for correcting output errors/distortion. Without the inclusion of this condition the model and hence the analysis collapses. In fact, this may be the source of any significant effects that are manifest in the real-world scenario. As such, it represents the cornerstone of my further development.
Next, we are forced to consider that internally, from the amplifier’s output devices to its output terminals, there is some net finite resistance. After all, wiring and circuit traces must be employed and nobody is making claims of room temperature super conductors being used in their equipment.
As a side note, in the monowire case we could easily transfer all of the speaker cable resistance to the inside of the amplifier, between the output devices and the output terminals. Then we could (in theory) connect the loudspeaker directly to the output terminals and we would get the same result as if the wires were external to the chassis.
Regardless, there is (and for the time being) always will be some finite resistance between our perfect voltage source amplifier and the loudspeaker load. Since the amplifier “senses” or “picks off” its negative feedback somewhere in the chain between the output devices and the output terminals, the resistance of the speaker wire will have an effect upon the completed system.
If the speaker wire exhibits any net reactive term (i.e., capacitance or inductance) then it will impart some complex alteration to the corrective action of the feedback loop. If it were only a matter of pure resistance being exhibited by the wire, then simply using a larger gauge would be as effective as biwiring. The effect of cable resistance will be to “decouple” the errors (back emf) generated by the speaker load from the error correcting action of the negative feedback loop. To the degree this series resistance produces a voltage drop across it, proportionally the error correction at the speaker terminals is reduced.
Albeit it is argued the effect is small, it is often the basis upon which proponents of using large wire gauge cables make their argument. The effect higher resistance is to reduce amplifier Damping Factor and is most noticeable at low frequencies. Dividing the spectrum between 2 sets of wires would (theoretically) lower resistance as well, but many argue that simply using a larger gauge monowire arrangement will yield the same result. But…there is a “catch” to that argument.
In the real world, cables of sufficient reasonable gauge to offer the lowest possible resistance and hence, the highest Damping Factor, are seldom constructed such that they exhibit a net-zero reactance. Although the reactances they do tend to exhibit can be considered quite small, nevertheless it is unlikely to be zero. It can be argued that this reactance is so small that for all intents and purposes it can be ignored, and in a general sense this is true. Our systems still work pretty well using imperfect cables. Also, it is not impossible to construct such a large gauge cable that exhibits close to perfect resistance, but it’s somewhat difficult and seldom done.
Seeing that the question of audibility is rooted in our quest for ultimate fidelity, we are driven to consider these small reactive components. Any reactance in our cables will induce a frequency dependant phase shift of the error signal (back emf) generated by the speaker load, as seen by the amplifier’s feedback loop. At low frequencies it is justifiably arguable that these small reactive effects are inconsequential. The true question arises when we consider their effects at the highest frequencies of operation.
It is well known that amplifiers employing negative feedback will exhibit a tendency, due to circuit induced phase shifts, toward regenerative (positive feedback) oscillation at some high frequency. They are designed to position this frequency well above their operational pass-band in order to avoid ultimate self-destruction. In order to achieve this they are designed such that at the frequency of oscillation, the total loop gain is reduced to less than unity. Doing so limits the amplifier's ability to correct for errors incurred at or near their upper operational frequency limits. This is why we often see rising distortion figures as frequency is increased in amplifier test results.
Considering the above, it does not require a great leap of imagination to suggest that phase shifts resulting from cable reactances could negatively influence an amplifier’s feedback loop. In theory, the amplifiers ability to accurately correct for high frequency errors due to speaker load back emf could be reduced. In fact, it is altogether possible that such phase shifts would give rise to further errors being generated by the amplifier in its attempt to correct for these phase altered signals presented to its loop. This is the stuff of TIM and other Inter-modulation Distortion artifacts.
The upshot of all this is that it seems reasonable to suspect that a biwire approach, wherein each cable is optimized for the frequency band it carries, can be argued in favor of. In theory, this may not be an ultimate requirement as long as a monowire cable that exhibits zero reactance can be found and used. This represents a major difficulty in that such a cable will only manifest a zero reactance property into a single load impedance. it is common for loudspeakers to exhibit a changing impedance vs. frequency, often increasing to some degree as frequency is increased. In practice, it may simply be easier to construct a biwire arrangement that achieves the desired result.
As stated near the beginning, if negative feedback is removed from the equation – all bets are off. In fact, this whole scenario may be the very reason many advocate the use of amplifiers that make no use of negative feedback. In the case where the amplifier used does employ negative feedback, it would seem prudent to at least consider the reactive effects imparted by speaker cables…when making our selection thereof – biwiring or not.
-Bob
