Audio Circuit Diagrams

Noise Injector Schematic Circuit Diagram

Exploring Audio Experimentation

The circuit noise injector is designed primarily for individuals interested in experimenting with audio. It allows users to explore various aspects, such as identifying differences in their audible noise threshold with and without music or assessing the impact of a slight noise presence on a specific CD’s quality. Additionally, given its ability to generate white noise, the circuit finds utility in test measurements. These applications include comparing the sounds of different loudspeakers, evaluating filter characteristics, and conducting similar audio-related assessments.

Noise Injector Schematic Circuit Diagram

Stable Noise Generation and Amplification

The measured characteristics, illustrated in Figure 2, demonstrate an almost flat amplitude distribution averaged over 64 measurements. The effective noise signal output reaches a maximum of around 100 mV (with both potentiometers set to maximum) across the frequency range of 22 Hz to 22 kHz.

Noise Generation and Amplification Circuitry

The noise is generated by reverse-biasing the base-emitter junction of a PNP transistor (BC557B) to create zener noise. In the prototype, T1 exhibits a voltage of approximately 10 V. P1 is utilized to set the noise level audibly, while the output level can be fine-tuned using the logarithmic potentiometer P2. For measurement purposes, P1 can be set to its maximum position. The noise undergoes amplification through two opamp stages.

The level of generated noise can vary significantly based on the transistor manufacturer or type. Utilizing two amplification stages in series provides flexibility and increased bandwidth. Custom filter characteristics can be implemented around IC1a and IC1b. The gain of both stages is maintained equal to maximize bandwidth. The amplified signal is then directed to a straightforward summing amplifier (IC2). Employing a stereo arrangement ensures both channels receive the same noise signal. If expansion is desired, each channel can have its own noise generator, necessitating a dual potentiometer for P2.

Stabilized Supply and Characteristic Table

The well-known NE5532 opamp is chosen for amplification, although any other reliable dual opamp would suffice. These opamps operate on a standard symmetrical ±15-V supply. To prevent potential positive feedback via the power supply and minimize power supply noise effects (given the opamps are non-inverting), the supply for the noise diode circuit (R1 and T1) is separately stabilized by IC3 (7812). Additional filtering for the ±15-V supply is implemented through C8 and C9. It’s imperative to place IC3 as close as possible to R1, T1, and IC1. Coupling capacitors C1 and C2 are essential to prevent the DC component of the noise signal from appearing at the outputs. The provided table details specific circuit characteristics, considering a bandwidth (B) of 22 Hz to 22 kHz and a reference level of 2 Veff.

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