I used DRA Labs’ MLSSA system and a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the Monitor Audio Platinum 300 3G’s farfield frequency behavior and dispersion. For the nearfield responses, I used an Earthworks QTC-40 mike, which has a 1/4" capsule and thus doesn’t present a significant obstacle to the sound. I also used this microphone for the in-room behavior. The impedance magnitude and electrical phase angle were measured with Dayton Audio’s DATS V2 system.
Monitor Audio specifies the Platinum 300 3G’s free-field sensitivity as 88dB/2.83V/m, which was confirmed by my measurement. The Platinum 300 3G’s impedance is specified as 4 ohms, with a minimum magnitude of 4.0 ohms the impedance remains between 4 ohms and 7 ohms for most of the audioband (fig.1, solid trace), with minimum values of 4.2 ohms at 124Hz and 3.75kHz. The electrical phase angle (dashed trace) is occasionally high; the effective resistance, or EPDR, therefore drops below 4 ohms between 27Hz and 330Hz and below 3 ohms between 66Hz and 128Hz and between 2.4kHz and 5.9kHz. The minimum EPDR values are 2 ohms at 85Hz and 1.85 ohms at 4.2kHz. The Platinum 300 3G is a relatively demanding load for amplifiers; tubed designs will best be used from their 4 ohm output taps.
