BiAmp CPA 650

The BiAmp CPA is what is termed a commercial power amplifier. She is mainly used in permanent installations for background music, paging and announcements. She is not meant to compete in the PA market neither can she be used in concerts.

In the commercial amplifier market, she has carved herself a reputation for being solidly build, reliable and coupled with good performance.

BiAmp CPA 650 Power AmplifierFig 1 – BiAmp CPA 650 Amplifier Schematic

Fig 1 is the schematic of the CPA 650. She uses a mirror image topology, that is the top and bottom halves are similar. The input signal drives the NPN and PNP differentials simultaneously (Red line). The collector then drives the VAS which the drivers are attached to. The drivers drives the bases of the output power transistors. So far, it’s pretty straight-forward.

Summary

Judging from the transistors used, I do not recommend this CPA 650 for HiFi. Her treble will sound “harsh”. Probably fine for subwoofers but keep her away from the mids and the highs in particular.

As for the power outputs, two pairs of 2SC3856 and 2SA1492 is cutting it really thin. This is an amplifier with a rated power of 325W into 4Ω, running off rails of +-75V. I feel safer with 4 pairs. Moreover, the filter capacitors are only two pieces of 10,000uF / 75V serving two channels. For a commercial amp, I suppose it’s fine but certainly not for continuous high power applications like in sound reinforcement or even HiFi. One redeeming feature of this CPA 650 is she’s fanless. She is convection cooled by two large heatsinks on either side. Great to mod for domestic use.

Fig 2 – BiAmp CPA 650 PSU