After a lot of testing, I am pleased to release V2 of the card!
The focus for this release was on reducing the output noise:
- New component values for the step-up voltage regulator: The values of some components were fine-tuned (again) in order to place the switching frequency of the step-up voltage converter way out of the audio band. These new values can also be retrofitted on a V1 board and will improve things dramatically (See V1.1).
- Alternative voltage step-up module: Cheap step-up voltage boost modules can now be used instead of the TL497-based circuit. Such modules, which can be easily sourced from Chinese portals, are much more modern and use a higher switching frequency, resulting in (near-)zero switching-induced noise. Using them also speeds up the assembly process, as many components can be skipped.
- Linear Regulator: If you are really paranoid about switching-induced noise, you can use the above alternatives as a first-stage voltage regulator and then make a second stage through a linear regulator, which should "eat" any remaining noise and provide the SID with very stable power.
- Separate grounds: A separate audio ground plane was introduced and connected to the signal ground plane through a single ferrite bead, preventing noise from spreading from the latter to the former. A second ferrite bead was inserted on the incoming +5V power rail in order to make it cleaner.
- Alternative BJT footprint: An alternative footprint for the output amplifier transistor was added, allowing for the mounting of the 2SC1815 BJT used in the original C64 audio circuit.
- SID clock delay circuit: This allows use of CD74HCT4520B chips in place of the CD4520B.
- Clock divider reset: Makes sure the SID clock always has the correct phase upon startup/reset.
- Short-circuit protection: In the original Solder design, a short-circuit can happen on the joystick port if it is written to. Series resistors were added in order to avoid this situation.
Please note that this release comes with update GAL equations, so please make sure to update your GAL!
As usual, be sure to read the assembly notes carefully and enjoy!
Released files
Note that the released gerber files are password-protected. The original reason behind my projects was to allow anyone to make their own retrocomputing accessories cheaply. Instead of this, what I achieved was actually fuelling greedy people grabbing my work, making boards for a couple of bucks and the selling them at outrageous prices ($15 for a badly-soldered OpenC64Cart that costs $2 to make is just ridiculous).
I still believe in free software and open hardware, and that is why my projects will remain as such. I am no longer providing ready-to-use gerber files, but you can still generate them from the KiCad project or ask me for the password, if you think you don't fit the above statement.
If all you want is to get boards made, I would really appreciate if you did it as explained in the Support section in order to get them cheaply while supporting the project.
Note that you are NOT allowed to use these boards commercially, but I won't care if you sell them AT COST. Let me repeat that: AT COST.