A Null Test is a definitive way of confirming whether a difference exists between two copies of the same audio sample. Two time-aligned copies of the same audio sample are played with one copy phase inverted. If the copies are identical, they will cancel, leaving only residual random noise.
The DUT (Device Under Test) is a Maselec MTC-1X Stereo Mastering Console. No inserts are engaged, just pure transfer between the input and output of the console. Only the power cord was changed. The power cords evaluated included the 18-gauge power cord that was supplied as original equipment, a 14-gauge power cord and MusicCord-PRO.
Multiple audio samples were produced with each power cord and null tested to establish “baselines”. Then, samples made with different power cords were null tested. The results of the baseline tests for both power cords and of the comparison are plotted together.
Each set of “baseline” and “comparative” null test curves shows significant differences over the audible frequency range. The null test differences range from 10dB to 20dB! These results confirm that each power cord has a distinct and undeniable impact upon the performance of the connected component.
Electric power travels from the power station through high-voltage transmission lines. The idea behind high-voltage transmission is minimal energy loss and current flow. A single transmission line typically carries enough power for multiple homes, and can be thought of as an "energy resevoir". Outside our homes and businesses a "step down" transformer converts the energy conduction to low-voltage, high-current wiring, which is relatively "lossy" or inefficient. Low resistance materials like copper wire are used to minimize energy loss.
Electrical codes in North America define 14-gauge copper as the smallest wire size used for ac mains service. So, why do most audio components come with an 18-gauge power cord? The "stock" power cord creates a "bottle neck" restricting current flow.
Audio components have complex power supplies that perform several functions including rectifying "ac" to "dc", stepping down the voltage, and storing energy for use as demanded.
For most of the carrier frequency cycle no current actually flows to the component, as it is powered by the stored energy in the power supply. Current flow is "switched" on/off, flowing in "pulses" every half-cycle of the ac mains carrier frequency. Power supply inductance causes a delay or negative phase shift between voltage and current. Current draw is mainly to refill the suply capacitors. If the current pulse were a continuous sine wave, its frequency would be significantly higher than the ac mains carrier frequency.
Current pulse amplitude increases with increasing demand. The oscilloscope screen image shows 4-seconds of music being amplified. What is revealing is how closely this modulation follows the musical signal being processed and how dynamic are the instantaneous current changes. To the extent that current flow modulations are restricted or delayed, the power supply cannot meet demand to accurately process the musical signal. Therefore, not only is flow capacity important, "response time" and "bandwidth" of the power cord matters!
This distortion is not comprehended by current performance metrics. However, it is clearly audible and is characterized by dynamic compression, harsh midrange and treble transients, and lack of bass extension.