01
Abstract & System Introduction
This documentation details the technical design, architectural framework, and operational analysis of a low-voltage audio processing circuit. The system integrates an acoustic transducer input stage, an active preamplification module, a digital delay line processing unit, and a low-power audio power amplifier stage. Designed to operate under a single low-voltage direct current (5V) rail, the circuit provides real-time analog signal conditioning paired with digital time-domain manipulation — echo and delay effects. The primary application of this topology is in portable public address (PA) systems, karaoke mixers, dynamic microphone conditioning units, and embedded voice processing systems.
02
System Architecture & Signal Path
The complete signal chain follows a sequential architectural flow — each stage hands a progressively cleaner, larger, or more processed version of the original signal to the next.
STAGE 1Input
Captures acoustic waves via the microphone and filters out the DC offset.
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STAGE 2Preamplifier
A low-noise BC547 stage boosts the millivolt-level mic signal to line level.
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STAGE 3Echo / Delay
The PT2399 digitally samples, buffers, and re-blends adjustable time-shifted repeats.
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STAGE 4Power Amp
The TDA2822 amplifies current and power to drive a low-impedance load.
2.1 — Input & Transducer Biasing Stage
The input stage interfaces directly with an acoustic transducer — typically an electret or dynamic microphone. For electret configurations, the circuit provides the necessary DC bias voltage via a current-limiting resistor network. This stage isolates the raw acoustic AC signal from any superimposed DC component before routing it to the active amplification section.
2.2 — Pre-Amplifier Stage (Low-Noise Voltage Amplifier)
Because raw microphone outputs sit in the microvolt-to-millivolt range (roughly 1mV–50mV), an active pre-amplifier stage is mandatory. This subsystem uses a BC547 NPN bipolar junction transistor configured as a common-emitter voltage amplifier, scaling the weak input signal up to a line-level amplitude sufficient to drive the analog-to-digital conversion stage of the processor that follows.
2.3 — Echo & Delay Core (Digital Spatial Processing)
The core signal processing is executed by the PT2399, a dedicated CMOS digital echo/delay processor. It samples the analog pre-amplified signal, converts it via a high-resolution internal ADC, stores the digitized audio frames into an integrated 44Kb SRAM delay line, and blends the time-shifted output back into the analog domain using internal operational amplifiers. Dual-potentiometer control loops allow dynamic adjustment of both the mix ratio (echo feedback volume) and the clock frequency governing the sample retrieval rate (delay time).
2.4 — Output Power Amplification Stage
The post-processed audio signal passes through a final conditioning stage driven by a TDA2822 dual/bridge power amplifier IC, configured here to drive a single mono output channel. This stage provides the current and power gain needed to drive low-impedance external loads — headphones or small speakers — without loading or distorting the output of the processing core.
03
Subsystem & Component-Level Description
Three functional blocks make up the board. Each is broken down below at the component level.
3.1
Input & Pre-Amplifier
- Microphone interfaceBias current is fed from the 5V rail through a 1kΩ resistor. A 100nF capacitor blocks DC, passing only AC audio onward.
- BC547 preamplifierA 100kΩ resistor sets stable base biasing; the 47kΩ collector resistor sets voltage gain — enough to drive the digital stage.
- Volume controlThe first 50kΩ potentiometer acts as a variable divider, manually setting input volume into the processor.
3.2
Digital Echo & Delay (PT2399 core)
- Sampling & storageThe internal ADC samples audio from Pin 16, stores it in 44Kb of SRAM, and an internal DAC outputs the delayed signal.
- Delay pot10kΩ on Pin 6 adjusts the internal VCO frequency, changing how fast memory is read — and the delay length.
- Echo pot50kΩ feeds a portion of the delayed output back to the input, building repeating echoes.
- Filter networksThe 104 and 471 capacitor networks form multi-stage low-pass filters, clearing quantization noise from the digital sampling.
3.3
Power Amplifier (TDA2822)
- Power amplificationThe mixed dry/wet signal passes through a 10µF coupling capacitor into the TDA2822, which amplifies current capability.
- Output networkA 100nF capacitor and 100Ω resistor stabilize the amplifier, protecting the output line from high-frequency self-oscillation when driving a speaker or headphones.
04
Circuit Wiring & Grounding Guidelines
To keep digital switching noise and hum out of the audio path:
01Decoupling capacitorsPlace the 100µF electrolytic capacitors as close as physically possible to the power pins of the PT2399 and TDA2822.
02Star groundingRoute every ground connection — the high-gain analog preamp ground and the digital ground pins alike — to a single central power-ground node to eliminate ground loops.
03Shielded cablingUse braided audio cable for the external potentiometers and microphone lines to keep electromagnetic interference out.
05
Estimated Electrical Specifications
| Operating supply voltage (VCC) | 5V DC (nominal) |
| Quiescent supply current | ≈30mA – 55mA |
| Input impedance | ≈1.5kΩ – 2.5kΩ |
| Output impedance | <32Ω |
| Maximum output signal swing | ≈3.2Vpp before clipping |
