Peter Baxandall's 1952 tone control is still the go-to circuit in guitar amps, mixing consoles, and hi-fi preamps. Unlike a passive James/RC tone stack that only attenuates, the Baxandall network wraps around an op-amp so you get true boost and cut around a center flat position — typically ±15 dB at 100 Hz and 10 kHz — with low output impedance and no insertion loss at the flat setting.

How it works. The circuit is a shunt-feedback inverting amplifier where the input and feedback impedances are frequency-dependent. Bass and treble each get their own pot:

• Bass pot sits between an input resistor and a feedback resistor, with a shunt capacitor to ground (Cb ~22–47 nF). At low frequencies Cb is open, so the wiper position sets the ratio of input to feedback resistance — sliding the wiper toward the input side boosts bass, toward the output side cuts it. At high frequencies Cb shorts, forcing unity gain and taking the bass pot out of play.
• Treble pot is AC-coupled through small caps (~2.2–4.7 nF) on both input and feedback sides. At low frequencies those caps are open (no effect); at high frequencies they conduct and the wiper directly steers treble boost/cut.

Design rule of thumb. Pick the bass corner around 100–300 Hz and treble corner around 3–10 kHz. For a resistor R around the pot, choose:

• Bass cap: Cb ≈ 1 / (2π · R · f_bass)
• Treble cap: Ct ≈ 1 / (2π · 10R · f_treble) (the ×10 keeps treble action out of the midrange)

Example: R = 100 kΩ pot, f_bass = 160 Hz → Cb = 10 nF. f_treble = 3.2 kHz with 10R = 1 MΩ → Ct = 50 pF (round to 47 pF). Use 1% film caps if you care about channel matching in stereo.

Real-world example. The Fender Blackface preamp, JBL/UREI mixing consoles, and countless Behringer-era mixers all use variants of this network. In a Marshall JCM800 modification, swapping the passive tone stack for a Baxandall around a TL072 gives you clean ±12 dB shelving without loading down the preceding gain stage — useful when you want tone shaping after the distortion instead of before.

Watch out for: log-taper pots ruin the symmetry (use linear); large bass caps with electrolytic construction inject distortion (use film); and the op-amp needs bandwidth of at least 1 MHz for clean treble response.