The SubStation Axis provides a more economical third option with the considerable benefits of DC blocking, but without multistage filtering. Where the mains waveform is not perfectly symmetrical in energy content a DC component will be present, this partially saturates the power supply’s mains transformer. This can lead to acoustic hum and reduced performance from connected equipment. A high current version of the acclaimed ISOL-8 Axis circuit blocks any DC on the incoming mains. Four outlets supply a continuous current capability of 16A across the unit with a suitable mains connection. The SubStation Axis is eminently suitable for lifting the performance of all high end system components.
Noise can enter any current loop in a number of ways, but can be largely categorised into two types or modes.
Firstly we’ll look at Differential Mode Noise. This occurs when only one conductor of a current loop encounters a noise source. The noise therefore circulates within the loop. For example this occurs when conductors are not closely coupled; such as in equipment internal wiring or when the noise is generated from circuit elements within the load or source itself.
Secondly we’ll look at Common Mode Noise. This occurs in both conductors of a current loop; equally and in phase with respect to ground. An instance being closely coupled live and neutral conductors encountering an electromagnetic field that immerses both conductors equally. For example; a cable picking up transmitted radio energy, from broadcast or wi-fi transmitters for example. The noise has its own current loop and returns through the safety earth conductor via the stray capacitance present in all distributed electrical systems.
Both these types of noise are commonly found in the real world. Now let’s look at how to reduce the unwanted noise entering the load. With Common Mode Noise a filter can be readily designed to attenuate the energy down to a given frequency using a common mode inductor. This uses phase cancellation to attenuate unwanted noise. The physical geometry of the inductor produces a relatively large value of inductance, given the core size and number of wire turns. It therefore produces effective filtering but for common noise only. Differential mode noise is able to pass through the filter readily.
A different topology is needed to attenuate Differential Mode noise. In this case only separate series inductors in the send (live) and return (neutral) in combination with a shunt capacitor in the current loop can attenuate the noise. Unfortunately the favourable geometry of the common mode inductor is not effective in this case. The series inductors used in the filter must use larger cores and more turns of wire to provide effective attenuation for differential mode noise compared with common mode.
Inferior mains conditioners may claim they filter differential mode noise but in fact use shunt elements without series inductors to divert the noise to earth (known as a Delta configuration). This then pollutes the earth instead, not the optimum solution for sensitive audio equipment as noise is still present on the earth line.
ISOL-8 Transmodal Filter: Warning! All mains conditioning is not created equal. Most manufacturers of mains conditioning equipment completely fail to effectively filter differential mode noise as it adds both complexity and cost. ISOL-8 Transmodal filters combat both common and differential modes of transmitted noise whilst keeping the earth clean for maximum filter performance at all times.
Cross Contamination Explained
The problem of noise cross contamination through power supply is often overlooked. Reducing its effect can result in significant performance gains for any system.
The key point is to understand is that each system component is not blameless and generates significant mains noise itself. In terms of power supply, it is also usually in close proximity to other components in the system, compounding the problem.
If you have system that contains a single component, one filter will protect it from mains borne noise:
Despite the presence of the filter, adding other components still allows noise to circulate in the system. Any noise generated within the system passes from one system component to another as they are all connected to each other in parallel after the filter. The presence of the filter will have an effect on this transmission, but it is far from the optimum solution.
A multiple filter configuration, as pioneered by ISOL-8’s designer Nic Poulson, is the optimum solution. External noise is blocked as before. But significantly, each component is also prevented from influencing the performance of another, releasing maximum performance.