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The self-field ac loss of a monofilamentary Bi-2223/Ag tape has been measured using an improved lock-in amplifier technique. To investigate the dependence of the ac power loss on the phase-angle error of the lock-in amplifier, theoretical calculations of the ac power loss of a flat superconducting strip in a perpendicular ac magnetic field were performed. The expressions derived also predict the dependence of the apparent loss on the size of the lead extension of the voltage contacts. For zero phase-angle error and an infinite lead extension the Norris formula is obtained. The voltage wave-form for ac current amplitudes greater than the tape's critical current has been measured and calculated. Theoretical predictions agree well with the experimental data.