Check the listing for details. Advanced Technology Design Series Model 221 Power Amplifier. Color: Black, Condition: Used. Listed at 109.00 USD. The Advanced Technology Design Series Model 221 Power Amplifier by AB International is a high-quality 2-channel power amplifier designed for professional audio applications. With XLR audio inputs and outputs, this amplifier offers pristine sound quality and versatility. The sleek black color and rack-mountable feature make it easy to integrate into any audio setup. The AB amplifier class ensures efficient power delivery and excellent performance for a wide range of audio sources. Ideal for musicians, sound engineers, and audio professionals looking for a reliable and powerful amplifier solution. It has some scratches and the 2 knobs are missing, but they still work and turn. This was used in a large church and was working when it was replaced with updated equipment.