Measurements for the Proceed BPA 3

We inadvertently left out the sidebar of the measurements TJN did of the Proceed BP3 amplifier, which was intended to accompany FM's review of the BP3 and BP2 amps in our May issue.

Proceed BPA 3 Measurements
All of these measurements were made on the Proceed BPA 3. (The BPA 2 uses the same circuitry, but in a 2-channel configuration.) Unless noted otherwise, the measurements are for single-ended (unbalanced) operation.

The BPA 3's frequency response was flat from 20Hz to 20kHz, and nearly as good from 10Hz to 50kHz (–0.07dB at 10Hz, –0.23dB at 50kHz at 2W into 4 ohms). The frequency response in the balanced mode was slightly less flat: down by 0.41dB at 50kHz! The separation for the physically adjacent center and left channels was 93dB at 20Hz, increasing to 115dB at 1kHz. Signal-to-noise was 99dB (A-weighted, ref. 1W into 8 ohms, left channel measured).

The total harmonic distortion plus noise at low power measured under 0.01% from 10Hz to 50kHz. The squarewave response could hardly be improved on in the real world, and the IM distortion (measured with an input of 19+20kHz at an output approaching two-thirds rated power) was very low. The BPA 3's gain was 29dB unbalanced, 23.1dB balanced. (The balanced and unbalanced gain results in our October 1998 review of Proceed's AMP 5 were inadvertently reversed; that amp has essentially the same gain as the BPA 3.) The input impedance measured 80.6k ohms (23.6k ohms balanced); the output impedance was 0.07 ohm at 1kHz, increasing to 0.18 ohm at 20kHz.

The BPA 3's power output at clipping (1% THD+noise) with one channel driven measured 147W into 8 ohms, 233W into 4 ohms, and 200W into 2 ohms (left channel measured). All but the last measurement were discrete readings. When I attempted a discrete measurement at 2 ohms, the BPA 3's protection fuse for the left channel blew. The 2 ohm result was derived from a swept power-output curve.

With three channels driven and only the left channel measured, the BPA 3 reached clipping at 135W into 8 ohms, 209W into 4 ohms—good results. However, the amplifier did not quite double its power as the load decreased from 8 to 4 ohms, and did not increase its power output at all into 2 ohms relative to its 4 ohm output. Line voltages for these measurements ranged from 116V to 119V.