inside2 Published December 16, 2012 - size: 700 × 732 in How to build the Apheared 47 Headphone Amplifier for Grado Headphones ← Previous A closeup of the guts. Notice the OPA2134 opamp chip. I also tried the OPA2132, OPA2227, and OPA2228. Any dual opamp with the same pin assignments will fit. Read More...