This project is about a single-channel microphone echo generator circuit. This circuit builds around the Holtek HT8970 echo effect generator IC. It offers complete control of the echo effect by providing delay, mix, and output level adjustments.

HT8970 is an echo/surround sound effect generator IC with 20Kb SRAM. It also contains the built-in VCO stage, AD, and DA converters. The chip costs less than 0.5 US$ and is commonly available in the market.

The given schematic offers a maximum of 320ms time delay and works with a single-rail 9V DC power supply.

In addition to the HT8970 integrated circuit, this circuit uses an NJM4558 op-amp to amplify the input and output audio signals. HT7150 voltage regulator uses in this design to power up the HT8970 IC. The HT7150 is a low-power 5V regulator IC from Holtek, the same maker of the HT8970 IC.

The circuit designs to work with 500Ω dynamic microphones. For microphone connectivity, this circuit provides a standard XLR connector. The output audio signal of this echo generator can directly couple to an AF power amplifier or a PC sound card.

We successfully tested this circuit with the EC042 dynamic microphone at the testing stages. To power up this circuit, we use a 9V DC power supply. As we observed, the maximum current drawn by this circuit was around 50mA during our test phase.

The PCB design of this circuit uses only through-hole-type components. The dimensions of the PCB are 84.5mm × 71.0mm.

All the schematic, KiCAD PCB design files, and BoM of this circuit is available to download at this google drive location.

Overall this echo effect generator provides a natural echo sound effect. The only drawback we noticed with this IC is the significant distortion in the sound at the peak delay point.