I was generously donated three Nakamichi BlackBox series units, by the same person who donated the “System One” rack from a previous post.
The units were in great condition, and more than likely could have been used as-is. They also surprisingly came with their original owners manuals, which usually get misplaced or lost over the years.



The Nakamichi BlackBox series consisted of seven add-on modules for adding features to an audio system. A required module is the PS-100 power supply which powers the other modules. It supplies a regulated plus and minus 10 volts DC at 200 mA to the other modules using a 4 pin DIN cable. The PS-100 also supplies a mute signal for a short period of time after power up and power down thru the cable to modules that utilize the signal. The PS-100 only has one output connector, but the other modules have a IN and OUT connector allowing additional modules to be daisy-chain connected to each other.


The supply uses a 48 volt center tapped transformer with rectified and filtered plus & minus 24 volts supplied to the regulator section. Zener diode ZD401 is the voltage reference for the regulators with Q402 & Q406 pass transistors which are attached to a heatsink regulating the output voltages.
All the original electrolytic capacitors were replaced using Nichicon low ESR units. After cleaning and load testing the supply was reassembled and ready for service.

The BA-150 bridging adaptor module allows bridging a pair of stereo power amplifiers into a more powerful mono amplifier pair. When used with the Nakamichi 420 and 620 power amplifiers it allows an output of 120 or 350 watts per amplifier, respectively. This doubles the available voltage swing at the speaker load compared with the same amplifier used without bridging. The bridging doesn’t increase the current capability of the amplifier though, typically requiring a minimum speaker load of 8 Ohms.

The Nakamichi BA-150 is identical to the BA-100 with the exception of an internal power supply, and added DIN connectors. Once again all the power rail electrolytic capacitors were replaced with Nichicon capacitors. The audio signal circuit electrolytic capacitors were replaced with upgraded BP (Bi-Polar / non-polar) Nichicon capacitors of equivalent capacity.

The final module is the EC-100 electronic crossover BlackBox.
The EC-100 allows adjustable separation of low-frequency and high-frequency signals from an input which when routed to an amplifier allows driving a 2‑way (bi-amp) speaker system. The module includes a high-pass filter, a phase shifter, and an adder to accomplish the separation.

Just like the BA-150 all the electrolytic capacitors were replaced and I also used a pair of Polymer electrolytic capacitors to further reduce any noise and ripple at the power input section.

All of the modules were tested on the workbench using my newly refurbished HP 3314A function generator. I have ordered but am still waiting on the DIN interconnect cables to be able to put the BlackBox modules to use at the time of this post.