BlueBerry Bass Overdrive kit
This is the official BJF DIY Blueberry Bass overdrive kit. This circuit was deviced in the year 2000 with the aid of two bassists and this version is true to the two first Blueberry bass overdrives ever made and traced from the first unit ever made. The background story of this is that two bassists tagged along with their guitarists to my workshop and asked if I wouldn’t make them bass pedals seeing I made pedals for their guitarists. At the time this circuit was intended to be made only in two pieces and a rare tint labeled Mahogany but that in reality gave a rose/purple colour most similar to mashed blueberries was used.
The voltage amplifier
The heart of the circuit is a CA3130 CMOS OP amp, that is particular in that it internally is almost entirely made with CMOS transistors (except for the VAS Voltage Amplifier Stage that is a bipolar transistor). Due to the CMOS technology max power supply voltage seen at pin 6 is 15V’s. However this OP amp has an output swing close to supply rails which at 9V gives near 8V’s pk-pk output as compared to standard OP amps 5,5V pk-pk. Noise performance is medium but the VAS has been overcompensated to reduce wideband noise and R7 ensures a minimum resistance at input in case input is shorted to ground and its value roughly corresponds to source resistance of minimum noise. FET/CMOS input OP amps has very low input currents and can handle large impedances and high signal levels and under such conditions they show better noise performance than bipolar OP amps that in turn can have lower noise but only at lower impedances and lower levels and further FET/CMOS Op amps handle overload slightly better than bipolar OP amps.
THE TONE RESPONSE
The sound is sum of dynamic distortion created with multiple clipping points and filters to restrict overtones. Feedback network around the OP amp is made in three parts. Initially two clamping diodes, two low current LED’s with a forward voltage of 1,7V limit output swing and they will clip gradually and also limit current through clamping diodes D5, D6 making a gradual onset of distortion rather than a rapid. The point of clamping diodes D5, D6 is stale and low impedance and serves as feedback entry point via a bass boosting filter made with C8 and R11. At the other end R8 and C6 for a treble boosting filter that can be negated with C5. After clamper D5, D6 follows a lowpass filter and from there again a bass boosting feedback making a step slope. The net effect is bandpass filter where the bass slope can be moved and at max bass circuit becomes a lowpass filter. The highpass section is set above any fundamental note on a bass and the relative level of fundamental notes can be set with the Tone control. Initially all notes are amplified and distorted with the clampers but nearly all overtones are fed back and nulled via the summing of the differential inputs of the OP. This hinders the output slewrate and so at low input levels sound is close to bypass but as input level increases the circuit distorts but most of the overtones are filtered out and result is distortion with mostly the lowest order overtones and higher order overtones are filtered out.
Now output of the last filter is fairly high impedance and it is isolated with an amplifier made with a J Fet. So the purpose of T1 is to isolate the filters from the outside world.
THE CONTROL KNOBS
Now the very first Blåbär took output from the source at N1 via a 2,2uF capacitor to a 100K Linear volume pot and this was sufficient for most passive basses however output can also be taken from Drain at a higher level.
The circuit can be wired for Treble and Bass control where N1 and N2 each get a 50KC connected as reostat to ground and C14 made 22nF and R18 2K0 but originally there was only a 50K log wired backwards because a 50KC could not be had as tone control setting relative level of bass.
To get more gain R8/C6 can be adjusted
In mid 2010 the original tint was gone and this model discontinued to lack of blueberries. When model was resurrected sometime later the original schematic had gone missing and schematic got contaminated with another model.
Level of difficulty: 5 (of 5)
https://moodysounds.com/wp-content/uploads/2020/12/blueberry-bass-od-kit-eng-1c.pdfThe manual for Blueberry OD kit (English)
The manual for Blueberry OD kit (Swedish)