High-pass Filter Cutoff Frequency for Buffer Frontend
Using a \(0.1 \mu\text{F}\) film capacitor with a \(1 \text{M}\Omega\) resistor is considered the gold standard in boutique pedal design; this exact pairing is found at the input of the Klon Centaur, for example.
The cutoff frequency is \(1.59 \text{Hz}\). Given the implications for phase distortion and preserving the exact time-alignment of transients, this modification is highly recommended.
Upgrading to a \(0.1 \mu\text{F}\) capacitor offers significant advantages for this specific DSP harmonizer, though it requires accepting two minor trade-offs.
The Win: Zero Phase Shift on the Low E String
As we established, a high-pass filter doesn't just cut volume; it bends time (phase shift) long before it starts cutting volume.
A filter's phase shift begins roughly one decade (10x) above its cutoff frequency.
- With the \(22\text{nF}\) cap (\(f_c = 7.2\text{Hz}\)), the phase begins shifting around \(72\text{Hz}\). Since the low E string of a guitar is \(82\text{Hz}\), the fundamental frequency of the lowest notes experiences a slight phase smear.
- With the \(0.1\mu\text{F}\) cap (\(f_c = 1.6\text{Hz}\)), the phase doesn't begin shifting until \(16\text{Hz}\). The entire audible spectrum of the guitar then operates in a zone of perfectly flat, linear phase, allowing low-end transients to hit the Wavelet engine with maximum impact.
The Trade-Offs
1. The Physical Size
A high-quality metallized film capacitor (like a WIMA MKP2 or Panasonic ECQ) at \(0.1\mu\text{F}\) is physically larger than a \(22\text{nF}\) version. However, unlike massive \(10\mu\text{F}\) film capacitors, a \(0.1\mu\text{F}\) film cap remains practical—typically a small component with 5mm or 7.5mm lead spacing that easily fits on a PCB.
2. Eating the ADC Headroom
Dropping the cutoff frequency to 1.6Hz allows sub-sonic frequencies to pass into the op-amp and reach the Daisy Seed's ADC.
Aggressive palm-muting or physical impacts on the instrument can create massive, inaudible 5Hz thumps that will be digitized. Because bass waves carry significant energy, these sub-sonic thumps consume 24-bit digital headroom, potentially leading to digital clipping even if the causal frequencies are inaudible.
The Verdict
The \(0.1\mu\text{F}\) film cap is the recommended choice. Because the DSP code separates the signal into three distinct frequency bands, any sub-sonic noise entering the ADC will fall into the lowest Approximation band and pass through without triggering the high-frequency Upward Compressor. This provides perfect phase alignment without a DSP penalty.