Ensuring audio quality with post filter feedback

Originally, post filter feedback helped Axign’s Audio Amplifier Solutions to reach high fidelity levels of audio quality while using cheap components. For that reason, our credo was based on hi-fi quality for consumer prices. With years of development in progress, we realized that the feedback technology could be used for lower power consumption too. In this blog, we talk about post filter feedback. How does it work? In what way is power consumption affected? And just as interesting: can we improve the technology?

Correcting non-idealities

The post filter feedback loop compares a measured output signal across the loudspeaker connectors with its ideal digital input signal and makes alterations in the digital domain accordingly. This means the digital control loop can correct non-idealities. The digital control loop operates at high speeds: Almost fifty millions of measurements per second. Extremely fast compared to the relatively ‘slow’ audio signals.

‘Post filter’ means that we take a feedback point behind the LC output filter, balanced across the loudspeaker connectors. We mainly correct non-idealities in power stages and output filters. Without the post filter feedback loop, the output filters are very load impedance dependent and influence the frequency response a lot.

A shift in technology

Our first mission was to offer audio amplifier solutions for applications with best audio quality. Along the road, it became clear that Axign’s post filter feedback technology offers other unique values. Lower power consumption, for example. By using modulation techniques that would usually degrade the audio quality severely, the Axign post filter feedback loop prevented the audio quality from being negatively affected.

Techniques for reducing power consumption like Zero Common mode modulation would ruin audio quality under normal circumstances. The post filter feedback loop constantly ensures the quality is excellent. It leads to what Axign is doing today.

The outside world

Advantages of the new generation on a shorter notice are the addition of a Phase-locked loop for self-tailored clock signals and an external boost converter. These hardware changes will immediately be visible when the next-gen chip arrives. The adaptive algorithm to compensate the output filter is a huge step forward compared to older generations.

Our technological advances also mean we regularly face challenges. For example: the feedback loop assumes the ‘outside world’ to behave in a certain way, based on calculations and measurements. If the outside world (particularly the output filter) deviates greatly from that prediction, the high-order feedback loop can become unstable. The aggressiveness and effectiveness in the designs of our feedback loops are limited by this principle. In the far future, we will be measuring back the ‘outside world’ to adjust the loop.

Innovation page

We keep innovating to ensure the lowest power losses, at competitive cost, with best audio quality. Look at our other updates on our innovation page here!