PT2399 Mic Preamplifier and Echo Board

The PT2399 is a widely used CMOS digital echo processor IC, prized in the DIY electronics community for its ability to mimic analog “bucket brigade” delays with modern digital sampling. It converts incoming audio into a digital bitstream, stores it in 44 kb of internal RAM, and then reconverts it to analog after a specified delay to create echo or reverb effects.

Key Specifications & Features of Echo PT2399 IC

• Operating Voltage: Typically 5V DC (though many boards include a 7805 regulator for 9-12V input).
• Delay Time: Adjustable from 30ms to 340ms via an external resistor or potentiometer on Pin 6.
• Audio Quality: Low Total Harmonic Distortion (THD < 0.5%) and high signal-to-noise ratio (-90dBV typical).
• Applications: Ideal for adding “presence” to microphone signals and building guitar delay pedals.

The one I bought from a local electronics shop is a straightforward, single-channel board that operates on 12V DC. It comes pre-assembled with a microphone and potentiometer ports, making it easy to wire. I have wired input and output audio jacks as well as a couple of potentiometers, one for mic gain control and another for echo or reverberation.

There are three controls on the standard interface for most PT2399 echo and reverb modules.

1. MIC VOL (Microphone Volume)

This is the Gain or Input Level control.

• Function: It adjusts the loudness of the microphone signal before it enters the echo circuit.
• Purpose: If your voice sounds distorted or “clipped,” you need to turn this down. If the output is too quiet compared to your background music, you turn this up.
• Technical Note: In most DIY boards, this affects the “Dry” signal (your original voice) as well as the signal being sent into the digital processor.

2. REVERB (Echo Delay Time)

On PT2399 boards, this label is a bit of a misnomer; it actually controls the Delay Time.

• Function: It changes the “speed” of the echo.
• Purpose:
  • Low Setting: Creates a very fast “slapback” echo, which sounds more like a natural room reverb or “presence.”
  • High Setting: Creates a distinct, long-separated echo (e.g., “Hello… Hello… Hello”).
• Technical Note: This pot is connected to Pin 6 of the PT2399 IC. It varies the internal clock frequency. If you turn it too high (long delay), you may notice a slight digital hiss or “sampling noise,” which is a known characteristic of the PT2399 at its limits.

3. REP (Repeat / Feedback)

This stands for Repeat or Intensity.

• Function: It controls how much of the “Echo” signal is fed back into the input of the processor.
• Purpose:
  • Low Setting: You get a single echo that disappears immediately.
  • High Setting: The echo repeats multiple times, fading out slowly.
• Technical Note: Be careful with this setting. If you turn REP too high, the circuit enters “self-oscillation.” This creates a runaway loop where the echoes get louder and louder until they become a wall of distorted noise.

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Summary Table for Quick Tuning

Port	Controls…	Result of Turning Clockwise (Max)
MIC VOL	Input Strength	Louder voice, potential for distortion.
REVERB	Delay Gap	Slower, longer intervals between echoes.
REP	Echo Count	More repeats; eventually leads to infinite feedback.