Technical Features of the Wells Audio Looking Glass II, Level II Power Conditioner

Design Principles

The Looking Glass II Level II power conditioner uses traditional LCR filters combined in a novel way to produce maximum synergy among the filter sections. The circuit employs a 3-stage fully symmetrical frequency-staggered noise filter to optimize the suppression of AC line harmonics across a broad spectrum of frequencies. The guiding principles behind the filter design are: 1) the use of state-of the-art audio grade parts; 2) the use of passive filters optimized for low, mid, and high-frequency noise suppression; and 3) the use of resistive damping to control transient overshoots and ringing.

The AC Harmonics Problem

Passively filtered power conditioners use chokes and capacitors to suppress and bypass noise on the AC line, leaving a more or less pure 60Hz voltage at the output. Unfortunately, high-frequency line harmonics can exploit leakage paths in the filter components and pass through to the conditioner’s outputs in a typical passively filtered power conditioner. These unfiltered harmonics are then passed along to whatever equipment is plugged into the conditioner. To one degree or another, all chokes and caps contain leakage paths that allow unwanted noise harmonics to pass through them.

The Looking Glass Solution

To minimize this effect, the three filters inside the Level II Looking Glass are each optimized for a different frequency range: low, mid, and high. The three filters are lined up in series from low to high, so harmonics that pass through the leakage paths of the low-frequency filters are captured and suppressed by the high-frequency filters. The three filter stages in the series combine their effects to create a wideband wall of harmonic suppression.

Transient Overshoots & Ringing

Transient noise pulses, which occur when an appliance is turned on or off or when a motor is revved up or down, produce a broad spectrum of high-frequency harmonics on the AC line. This results in a fast change of state inside the chokes and caps of the filters. When that happens, undamped chokes and caps can overshoot and ring. In the worst case, they can oscillate uncontrollably.

The Looking Glass Solution

Resistors are the forgotten component in passively filtered power conditioners. Resistors are energy dissipators that can be used to dampen uncontrolled resonances in electrical circuits. The resonant frequency of each LC filter in the Level II Looking Glass is resistively damped to suppress overshoots and ringing when the conditioner is struck by high-energy transient pulses. In an undamped filter, the effect of such pulses is prolonged and amplified. Resistive damping allows the Level II Looking Glass to control overshoots like a shock absorber controls bumps in the road—by dissipating their energy.