Electronics Guide

Topology Considerations

Modern preamp designs frequently employ differential input stages using matched transistor pairs or dedicated integrated circuits. The SSM2019, THAT1510, and INA163 represent popular choices for low-noise differential amplification. Discrete designs often use paralleled input transistors to reduce noise by the square root of the number of devices, a technique exploited in premium studio equipment.

Gain control presents its own challenges. Switched resistor networks provide precise, repeatable gain settings, while continuous potentiometers offer smooth adjustment but may introduce noise and channel matching errors in stereo applications. Many professional preamps combine both approaches, using a stepped attenuator for coarse adjustment and a trim control for fine calibration.

Circuit Implementation

RIAA equalization can be implemented using passive or active networks. Passive networks placed between two gain stages offer simplicity but require additional amplification to compensate for insertion loss. Active implementations integrate the equalization into the feedback network of an operational amplifier, combining gain and frequency shaping in a single stage.

The choice of capacitors in RIAA circuits significantly impacts performance. Polypropylene and polystyrene types are preferred for their low dielectric absorption, which could otherwise cause subtle time-domain distortions. Component tolerances directly affect RIAA accuracy, with 1% resistors and 2% capacitors typically required for deviations below 0.5 dB from the ideal curve.

Moving-coil cartridges, with outputs often below 0.5 mV, require step-up transformers or active head amplifiers before the main RIAA stage. These ultra-low-noise front ends represent some of the most demanding analog design challenges in consumer audio.

Baxandall Tone Controls

The Baxandall circuit, introduced by Peter Baxandall in 1952, remains the most common tone control topology. This design provides symmetrical boost and cut characteristics around a flat position, with the shelving frequencies typically set around 100 Hz for bass and 10 kHz for treble. The elegant circuit achieves this using feedback networks around an inverting amplifier, with the potentiometers varying the amount of frequency-dependent feedback.

A key advantage of the Baxandall design is its flat response when both controls are centered, introducing no frequency coloration when tone adjustment is not desired. The circuit also exhibits relatively constant input and output impedances across the control range, simplifying integration with other stages.

Parametric Equalizers

Parametric equalizers provide more sophisticated frequency control, allowing adjustment of center frequency, bandwidth (Q factor), and boost/cut amount. These circuits typically employ state-variable filter topologies that simultaneously provide low-pass, band-pass, and high-pass outputs, enabling flexible frequency manipulation.

Graphic equalizers, with their multiple fixed-frequency bands, represent a simplified form of parametric control. Each band uses a bandpass filter with adjustable gain, and the individual band outputs are summed to produce the equalized signal. Professional graphic equalizers may provide 31 bands per channel, offering precise control at one-third octave intervals.

Class A Amplifiers

Class A amplifiers conduct current through their output devices continuously, regardless of signal level. This eliminates crossover distortion entirely but limits theoretical efficiency to 25% with resistive loads and 50% with transformer coupling. Despite their power inefficiency, Class A designs are prized in high-end audio for their sonic purity and freedom from switching artifacts.

Practical Class A designs often use single-ended configurations with one active device and a constant current source or resistive load, or push-pull arrangements where both devices conduct simultaneously. The constant power dissipation simplifies thermal design but necessitates substantial heatsinking.

Class AB Amplifiers

Class AB represents the dominant topology in audio amplification, combining reasonable efficiency with low distortion. The output stage uses complementary transistor pairs biased to conduct slightly more than half the signal cycle, eliminating the severe crossover distortion of Class B operation while avoiding the power waste of Class A.

Bias stability is crucial in Class AB designs. Thermal runaway, where increased temperature causes increased current flow leading to further heating, must be prevented through careful bias network design. Common techniques include using the bias-setting transistor as a thermal sensor mounted on the output stage heatsink, providing automatic compensation as temperature varies.

Class D Amplifiers

Class D, or switching amplifiers, achieve efficiencies exceeding 90% by operating their output devices as switches rather than linear amplifiers. A pulse-width modulated signal drives the output stage, with a low-pass filter recovering the audio signal while rejecting the switching frequency components.

Modern Class D designs have overcome early limitations in audio quality, achieving distortion and noise specifications rivaling the best linear amplifiers. Key challenges include minimizing dead-time distortion during output transitions, controlling electromagnetic interference from the switching waveforms, and ensuring stability with reactive speaker loads.

Portable Considerations

Battery-powered headphone amplifiers face additional constraints on power consumption. Class A designs, while sonically excellent, quickly deplete batteries. Modern portable amplifiers often use Class AB output stages with carefully controlled quiescent current, or Class G/H topologies that adjust supply rails based on signal level to improve efficiency without sacrificing linearity.

Compressor Operation

A compressor consists of a level detector, a gain control element, and associated timing circuits. When the input signal exceeds the threshold, the gain is reduced according to the compression ratio. A 4:1 ratio, for example, means that a 4 dB increase above threshold results in only a 1 dB increase at the output.

Attack and release times control how quickly the compressor responds to level changes. Fast attack times catch transients but may cause pumping artifacts; slow attacks allow transients through but provide smoother operation. Release times must be carefully matched to program material to avoid audible gain modulation on sustained notes.

The gain control element may be a VCA (voltage-controlled amplifier), an optical attenuator, a FET operating in its resistive region, or a variable-mu tube circuit. Each type imparts different sonic characteristics, from the transparent precision of VCAs to the smooth, program-dependent response of optical designs.

Sidechain Processing

The sidechain is the signal path that controls the gain reduction. External sidechain inputs allow one signal to control the compression of another, enabling ducking effects where music automatically reduces in level when a voice is present. Sidechain equalization can make the compressor more or less sensitive to specific frequency ranges, useful for preventing bass frequencies from triggering excessive compression.

Gate Operation

A noise gate monitors signal level and attenuates the output when the input falls below the threshold. This effectively silences background noise during pauses in the program material. The depth control determines the amount of attenuation applied, ranging from a few decibels to complete silence.

Attack and release times are critical for natural-sounding operation. Fast attack times ensure that the gate opens quickly when signal arrives, preserving transient attacks. The hold time keeps the gate open for a specified period after the signal drops below threshold, preventing the gate from chattering on signals with rapid level variations.

Expander Characteristics

Expanders provide gentler control than gates, applying gain reduction proportional to how far the signal falls below threshold. An expansion ratio of 2:1 means that a signal 2 dB below threshold appears 4 dB below at the output. This gradual action often sounds more natural than the abrupt switching of a gate.

Downward expanders, the most common type, reduce the level of quiet signals. Upward expanders increase the level of signals above threshold, effectively boosting dynamics. Both types find applications in noise reduction, broadcast processing, and special effects.

Passive Crossovers

Passive crossovers use inductors, capacitors, and resistors to divide frequencies after the power amplifier. The simplest first-order crossovers use single components per driver but provide only 6 dB per octave slopes, allowing significant overlap between driver outputs. Higher-order designs with steeper slopes (12, 18, or 24 dB per octave) provide sharper separation but introduce more phase shift and component complexity.

Component quality matters significantly in passive crossovers. Air-core inductors avoid the saturation and hysteresis distortion of iron-core types, while film capacitors outperform electrolytics in linearity and long-term stability. Power handling requirements demand appropriately rated components, particularly for the woofer section.

Active Crossovers

Active crossovers divide the signal at line level, before power amplification. Each frequency band then drives a dedicated power amplifier and driver. This approach offers several advantages: drivers are directly connected to amplifiers without passive components in the signal path, amplifier power is used more efficiently since each handles only its assigned band, and filter characteristics can be adjusted without changing heavy, expensive crossover components.

Active crossover designs typically use Linkwitz-Riley, Butterworth, or Bessel filter alignments. Linkwitz-Riley crossovers, with their fourth-order (24 dB/octave) slopes and in-phase outputs, have become the standard for high-quality systems due to their flat amplitude response through the crossover region. Time alignment controls can compensate for driver mounting offset, ensuring coherent wavefront arrival at the listening position.