Speaker Project CS-8

This is a 1.5-way all-passive 8-litre closed-box speaker project for current-drive (Current Speaker 8). Besides the baffle step, the bass resonance has also been compensated with passive circuits, so the feeding transconductance amplifier can have flat frequency response. Detailed design principles and process with the optimized driver models and simulation results are disclosed in section 11.5 of the book.

The driver used is a 4½-inch, 4-ohm device Vifa PL11WH09-04, whose operation range extends on current-drive up to 10 kHz, making it thus possible to omit the tweeter. The lower −6 dB cut-off frequency obtained is 56 Hz, which is in this size class a fairly competitive result. 

The driver's rated resonant frequency is 67 Hz and the mechanical Q value 1.8, which numbers also hold true surprisingly well. Two drives accrue effective area 116 cm², which is yet fairly adequate for civilized listening.

The schematic is shown below. Driver A gets current mostly only at low frequencies, while driver B operates in the whole range. To adjust the frequency balance, A receives, however, a little current also at high frequencies via the 33 Ω resistor.

The resonance compensation has been realized with the series RCL network connected across driver B. In this case, it is sufficient to apply the compensation only for driver B because the resonance prominence of A remains quite minor even as such.

To be continued