Gain adjustment of a microphone power amplifier across multiple frequency bands requires comprehensive consideration of signal characteristics, equipment design, and application scenarios. The core objective is to achieve clear signal amplification across different frequency bands while avoiding distortion, noise, and phase issues. The adjustment process must focus on five key dimensions: gain structure, frequency response, phase consistency, dynamic range, and equipment compatibility.
Gain structure adjustment needs to be layered according to frequency band characteristics. Low-frequency bands (e.g., 20Hz-200Hz) are susceptible to environmental noise interference and require a high-pass filter to set the cutoff frequency, reducing the effects of rumble and proximity effects. For example, when recording vocals, if the microphone is too close to the sound source, low frequencies will be excessively amplified due to proximity effects. In this case, a high-pass filter (e.g., 80Hz/12dB per octave) should be used in the preamplifier to roll off the low frequencies, while adjusting the low-frequency gain to avoid a thin sound. The mid-frequency band (200Hz-6kHz), the core frequency band for vocals and instruments, needs to maintain a flat gain to avoid excessive boosting that could result in a sharp or muddy sound. In the high-frequency range (above 6kHz), a balance must be struck between clarity and harshness. This can be achieved by attenuating high-frequency gain or using a low-pass filter to reduce hissing and cymbal noise.
Frequency response adjustment must match the characteristics of the microphone and mic power amplifier. Different microphones (e.g., dynamic, condenser) have significantly different output levels. Dynamic microphones typically require higher gain, while condenser microphones may require reduced gain or the use of a pad (attenuation switch) due to their high sensitivity. For example, when recording drum kits, the kick drum microphone (dynamic) needs high gain to capture the low-frequency impact, while the top-mounted microphone (condenser) requires low gain to avoid overload. The input impedance of the mic power amplifier must also match the microphone output impedance, typically requiring the load impedance to be at least 10 times the microphone output impedance to ensure efficient signal transmission and avoid low-end clarity degradation or high-end response impairment.
Phase consistency adjustment is crucial in multi-microphone applications. When multiple microphones are used to pick up the same sound source (e.g., multiple microphones for a piano), phase differences between frequency bands can cause sound cancellation or hollowness. At this point, signal polarity needs to be adjusted using a phase inversion switch, or phase calibration tools (such as an audio analyzer) should be used to measure the phase difference across frequency bands, and phase alignment should be achieved through delay compensation or gain fine-tuning. For example, when recording symphonies, the microphones for the violin and cello sections must ensure that the mid-high and low frequencies are in phase to maintain sound cohesion.
Dynamic range adjustment needs to balance signal strength and noise floor. With low-level signals (such as soft singing), gain needs to be increased to improve the signal-to-noise ratio, but the amplification of background noise must be avoided; with high-level signals (such as shouting), gain needs to be reduced to prevent clipping distortion. Automatic gain control (AGC) circuits can dynamically adjust gain, but may introduce a breathing effect (volume fluctuations caused by sudden gain changes), so manual or automatic adjustment should be selected depending on the scenario. For example, in conference presentations, AGC can ensure consistent volume among different speakers, but in music recording, AGC should be turned off to preserve dynamic performance.
Equipment compatibility adjustments need to consider the interface matching between the mic power amplifier and subsequent equipment (such as mixing consoles and recording equipment). If the microphone power amplifier's output level is too high, it may overload subsequent devices; if the level is too low, it will need to be further amplified in subsequent devices, potentially introducing noise. Therefore, a reasonable output level needs to be set according to the device specifications, for example, controlling the peak level between -10dBFS and -6dBFS to leave a safety margin for digital systems. In addition, the combination of wireless microphone and microphone power amplifier must ensure that the frequency band is free from interference. For example, choose the UHF band (400-830MHz) to avoid VHF band congestion, and use the frequency scanning function to avoid interference from mobile phone or broadcast signals.
