Well, not
exactly a Series II. I had my Spoiler routed out plus a bit of an undercut to add Series II style electronics. The routing was done by Glaser instruments here in Nashville, so it's as good a job that could be done short of the factory.
It's actually the EQ and mic preamp from a Harrison MR-4 console. I reverse-engineered it and then fit them both onto two separate PCBs. The big 4 knobs are master vol, treble frequency, bass freq, and mid freq. The 3 small knobs are treble, mid, bass gain with 15db boost/cut with center detents. The knob with the gray top is a 4 pos shelving/bell curve toggle which activates for the treble and bass controls (treble shelf, bass shelf, both, none). The triangle knob is of course, the pickup select, and the toggle switch next to the master volume is a midrange Q width switch.
The input section is completely different than any bass electronics I know of. Both ends of the pickups are sent to the hot and cold inputs of the mic pre (essentially a differential amp). There is no ground, it is completely floating. As a result, there is no loading effects or capacitances to ground at the pickup side. The pickup selector switch has two 2k resistors that add in series to each leg of the selector switch output in order to impedance balance the load in the middle position (DC resistance in ALL positions is ~8k).
The signal goes through the EQ and then outputs to a SSM2142 differential line driver which is then fed to pins 4 and 5 of the 5 pin XLR jack. This terminates to a 500 ohm trimmer pot at the power supply which doubles as a load matching resistor and common mode rejection , and then finally out to a 3 pin XLR jack which I feed directly in to a console channel strip. As a result the bass is completely silent at idle no matter how much gain you add. Last, the 1/4" jack is NOT for plugging into an amplifier. It actually is connected directly to the output of the pickup selector. This allows the pickups to be plugged directly into a console to get its own unique mic pre/eq combination.
This project was never intended to be practical. I am currently in school for an EE degree (I have done electronic repair for 10 years) and this was just an engineering project for me. It is very usage-case-specific and not meant for stage use. As of now, it only has a 3 pin XLR output from the power supply and can only be plugged into microphone inputs. I will probably add in an unbalanced 1/4" jack to plug into an amplifier. Though the signal is so unbelievably loud the bass would have to be plugged into the effects loop and directly drive the power amp. The bass is capable of outputting 10 volts RMS at clipping.
I am fully aware that the bass could be de-valued from these modifications, but I love this bass and I will never get rid of it. For me it is the ultimate studio tool. The bass sounds very unique, and while it has the typical Alembic character to its sound, it seems to have more harmonic richness and extended range of the high and low end. I think this is due to the floating (differential) input and output stages.
I had to be a little creative with the layout of the controls because I already had the previously existing holes from the stock Spoiler controls. As a result, three of the pots sit underneath the circuit board. It may appear crowded, but due to the relative heights and diameters of the knobs it is functionally not crowded; every control can be reached with ease.
