Distortion in Contact Resistances
On top of all EMF-realted effects, loosened or oxidized contacts in the cables are also able to introduce modulation distortion in a voltage-driven speaker. While this may not form an issue for those who recognize the meaning of proper contacts, for the common consumer with aged wiring it can introduce a source of extra distortion that adds to those arising from the voltage-operated speaker itself.
The photo aside shows the simple test contact arrangement, that was used in series with a 4-Ω dummy load driven by a voltage amplifier. A slightly darkened, stranded copper wire on the right-hand side is only inserted in a clean, nickel-surfaced terminal block, but the screw is open. (The other side being normally tightened.)
The modulation distortion in the load current produced by the contact resistance is shown below, measured at 1 A level with a 200/2000 Hz (5:1) two-tone signal. The modulation components around the 2 kHz tone are about 50 dB below it. Both the harmonic and modulation distortion are mainly of 3rd order.
The results were quite independent of signal frequencies, but a clear level-dependence was present, higher currents yielding higher distortion. There is also not much difference whether the connector surface is nickel- or gold-plated.
If the wire is removed and placed again, sometimes the distortion doesn't appear and sometimes it is lower, but the result shown is quite typical and repeatable.
A more darkened speaker wire (photo aside) was also tried, and the result in the same conditions is shown below. The modulation products around the 2 kHz tone reach now considerably higher, and a host of other significant peaks are on the rise. The total MD relative to the 2 kHz tone amounts to more than 1%.
Typically, there are four contact points in a speaker's wiring; two of which are yet subject to mechanical vibration. One more is yet found in the speaker selection relay, that is usually of light construction. On current-drive, the condition of all these can be ignored up to the point where a total break takes place.