Meier Audio has a feature in their amps called “FF” or Frequency Adaptive Feedback. Jan Meier describes it here. His article is detailed yet long and can be hard to understand exactly what it does, and why. Here I give a simpler explanation. FF is based on 3 key concepts.
If my explanation here makes sense, go back and read Meier’s and you’ll get an even deeper understanding.
When it comes to human perception of sound and music, all frequencies are not created equal. The ear is most sensitive to frequencies from around 100 to 2000 Hz. And, most music (at least voices and acoustic music) is concentrated in this range.
Consequently, this is the most critical range for reducing distortion. You might not hear 1% distortion at 30 Hz, but you can definitely hear it at 1000 Hz.
Analogy: Dolby B
Readers with a few grey hairs remember cassette tapes and Dolby B noise reduction from the 1970s and 80s. Dolby B was brilliant in its simplicity. Tape hiss has a wide frequency spectrum but it’s most noticeable in the treble. If you cut the treble during playback, it reduces hiss but it also dulls the music. So when recording, boost the treble. Then during playback, cut the treble by the same amount you boosted it. You get the same hiss reduction without any reduction in treble, because you’re only cutting exactly what you boosted earlier. The music has flat frequency response with less hiss, sounds cleaner with higher S/N ratio.
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
Combine these 3 ideas and you have Meier Audio’s FF. Start with the musical signal.
- Step 1: boost the critical frequency range (100 Hz to 2000 Hz)
- 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 2: pass the signal through the normal amp / feedback stage
- The signal being amplified and in the feedback loop has the critical frequency range exaggerated.
- 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 2, because the critical frequency range is exaggerated, the feedback loop reduces distortion in this range more effectively.
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.
In summary, FF does to distortion what Dolby B does to tape hiss. It’s based on the same concept.
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.
If distortion is already below audibility, then FF is a solution looking for a problem – what is the point? In fact, the cure could be worse than the disease! FF requires filters on the input and output to shape the frequency response. These filters cause their own distortions (such as phase distortion from analog filters or minimum phase digital filters). The overall effect is a trade-off between the benefits of FF and the drawbacks of having this extra signal processing.
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.