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Electret Mic Booster Schematic Circuit Diagram

Exploring TL431-Based Audio Amplifiers Online

Those who have delved into online searches for intriguing circuits have likely come across a TL431-based audio amplifier. This circuit operates on the premise that any comparator can function in a linear mode if it’s appropriately attenuated with substantial negative feedback. While the TL431 is commonly labeled as a programmable or adjustable zener, it is fundamentally a comparator equipped with its own 2.5 V reference, neatly contained within a TO92 package.

Electret Mic Booster Schematic Circuit Diagram

Enhancing TL431-Based Audio Amplifiers

Many TL431 amplifiers available online suffer from low gain due to excessive negative feedback (nfb) without proper compensation. Additionally, some circuits mishandle biasing the control input, further diminishing performance. The presented circuit tackles the low gain issue by incorporating an AC shunt in the feedback path and utilizing an electret microphone for the input. The stable 2.5 V set on the control input is well-suited for an electret mic. Initial prototypes used a 35-ohm loudspeaker as a load (RL) and a Vcc of 12 V. However, using an old 130-ohm telephone earpiece as a load can reduce stress on the TL431. The AC shunt C2 (100 µF) must be a high-quality component with specified equivalent series resistance (ESR) to prevent RF sensitivity. Optionally, components C1 and R1 can be added for unshunted feedback to reduce noise; 1.5 kΩ and 5.6 nF serve as suitable starting values.

Flexible Set-Up and Voltage Control

The initial configuration depends on the current drawn by the electret mic and the RL value, ranging from 200 to 2,000 ohms. R2 allows the TL431 cathode to swing despite the AC shunt, with a 1.2 kΩ resistor found satisfactory. P1, a 47 kΩ trimpot, sets the voltage drop on RL. When working with moving-coil speakers, a balance between voltage swing and pre-biasing should be achieved. Once the operating point is determined, P1 can be replaced by an equivalent fixed resistor.

Versatile Applications and Unique Capacitor Testing

This circuit offers notable advantages. It functions well with a twisted-pair setup, and the output can be accessed at the wiper if RL is a pot at the power supply end. Salvaging a JFET from an old electret mic allows various piezoelectric elements to be used as transducers. Brass disc sounders, quartz discs from clock crystals, phono crystal cartridges, and even piezoceramic pellets from flightless cigarette lighters provide different outputs, significantly expanding the range of potential applications.

An intriguing application lies in testing the microphonic sensitivity of regular capacitors. Disc ceramic types generate output with minimal tapping. While rolled metalized foil types also produce some output. This unique capability enhances the circuit’s utility, making it a valuable tool for diverse electronic applications.


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