Meier Audio “FF” Frequency Adaptive Feedback



Musical Hearing



Analogy: Dolby B


Amplifier Feedback

Solid state amplifiers have a negative feedback loop that reduces distortion and increases stability. Essentially, a portion of the output signal is inverted, attenuated, and fed back into the input.

Why do this? Because amplifiers are non-linear: their output contains frequencies that aren’t in the input. Harmonic distortion (HD) is frequencies that are multiples of input frequencies. Intermodulation distortion (IMD) is frequencies that are the differences between frequencies. These distortions apply cumulatively to both input and distortion tones, so it all adds up quickly.

Imagine what happens if you invert the output having these extra frequencies and feed it back into the input. Because it’s inverted, each spurious frequency becomes its mirror-image opposite. As this passes through the amplifier, it perfectly opposes the spurious frequency that the amp produces as distortion. This weakens or attenuates the distortion frequencies.

Frequency Adaptive Feedback


    • Alternately, attenuate frequencies above & below this range. This can be a better approach since attenuation means no chance of clipping.
    • Apply as the first thing you do when the signal enters the amp.
  • Step 3: attenuate the critical frequency range
    • Do the reverse of what you did in step 1.
    • Apply as the last thing you do before the signal leaves the amp.


In step 3, when you attenuate the critical frequency range back to its original level, this has the side effect of attenuating any residual distortion in that range. Thus the ratio of signal to distortion is improved in this frequency range.


Incidentally, the Redbook CD specification has something called “emphasis”, which boosts high frequencies (flat at 1 kHz, increasing to +10 dB @ 20 kHz). CD players are expected to attenuate those frequencies on playback. Similar to FF, this reduces high frequency noise.


Here we’ll play some devil’s advocate.


FF actually increases distortion outside the critical frequency range! With FF you will have higher distortion at the extreme low and high frequencies (because FF attenuates them in the feedback loop). But you’ll have lower distortion in the midrange and treble. FF shapes distortion to match the sensitivity of our hearing: less distortion where our hearing is most sensitive, at the cost of higher distortion at frequencies where we can’t hear it.