The Peasant decided to see if he could play a bass drum beat along with singing and playing other instruments, so he took the bass drum pedal from the Solid State Percussion Synthesizer and tried it out while playing the banjo. This seemed to be working pretty well, so it was decided to build a small battery-powered module to electronically produce the bass drum sound.
The circuit required for this needed to be fairly simple, with low current draw and portability being two of the main design goals. Thomas Henry's excellent Bass++ design easily meets those requirements and so was chosen as an initial starting point. Since this new project was based on the Bass++ design, the name "BassBox++ was chosen for it.
The circuit was first bread-boarded and modified to better suit the application. Firstly, The pitch CV input was removed, as it wasn't going to be used in this project. So J2, R28, and R21 were simply left out.
Next, the supply voltages were changed to +/-9 volts, so that the whole thing could be run off of two 9 volt batteries. To compensate for the lower voltage R5 was changed from 1.5K to 1K to keep the LED brightness approximately the same. To reduce power consumption, the TL072 was replaced with a low current TL062 chip as well as raising the values of R1, R13, and R17 to 220R, 50K, and 100K respectively.
As this module will be used almost exclusively for bass drum sounds, the value of C2 was doubled to 0.01uF to lower the oscillator frequency range.
Separate envelope generators were desired for the pitch and volume functions, so Q1 and associated circuitry was duplicated. (Includes D1, C11, R4, 10, and 33.) The decay potentiometer values were also increased to the more standard 1 Megohm, allowing C11 to decrease to 1uF. This was done on both envelope generator circuits.
Triggering this circuit with a piezo disk caused some annoying popping sounds to be generated from the "impact" circuit, so a 2.2K resistor and a 0.1uF capacitor were added between D3 and R11/C9 to filter out some of the transients and lower the overall impact tone. Similarly, the "shell" VCA output also had some bad popping sounds, especially during short decays, so a RC filter was added to the current source. This entailed changing the value of R18 to 18K, adding a 2K resistor between it and the 2N3904, and adding a 0.47uF capacitor from where the two resistors connect together to ground.
Finally, some type of waveform control was desired, such as what the older Thomas Henry ADV Bass and UD Bass circuits had. So a diode-based variable clipping circuit was inserted between the oscillator and the VCA sections. To do this R24 was changed to 10K, followed by a 100K potentiomenter, which then connected to two opposite-polarity diodes connected to ground, and finally a 5.6K resistor connecting to IC1 pin 14.
Here is the modified schematic for the project.
Below is a picture of the parts chosen for building the BaseCase++. The case is a small plastic hinged box, saved from the landfill, that was just the right size.
For the first construction step the circuit was built up on a small piece of perf-board. Everything fit on to the board with just enough space!
Then the pc board was mounted into the case, along with the input and output jacks, and a plastic divider was installed to create a separate section for holding the two 9 volt batteries.
Next, all of the front panel parts were mounted and everything was wired together. For convenience, a few components were wired between front panel parts instead of being placed on the actual pcb.
Here is a view of the front panel with the control labels applied.
And here is a couple of pictures of the finished project. The transparent case gives this project a very unique and intriguing look!
This project was one of the lucky ones that actually worked perfectly from the first power-up! The modifications were a significant improvement to an already excellent design, particularly the new waveform control. Now it's time to go make some noise!