In early April 2026 I put out a video with Neil from the Retro Collective about the HDD Synth MKII prototype which was a great chance to get feedback on and gauge the level of interest.

From this I finalised the work on the MKIII design, which was essentially a professionally assembled version of the MKII (using surface mounted components) with some additional bells and whistles.

On April 20th 2026 I submitted this design to PCBWay for manufacturing. This was my first time using their PCBA (PCB Assembly) service, which in itself was a learning experience I'll write about another time.

Unfortunately some of the lessons I've learnt from this is that the design is relatively expensive to manufacture. It's also taking a long time, due to the nature of the components used along with the low quantity of boards being assembled.

Whilst I've been waiting for the MKIII to be assembled and shipped to me I've revised my approach with the MKIV prototype, which has quite different design goals.

Those being:

• To be as physically small as possible
• To be as cost effective to manufacture as possible
• To also function as a CompactFlash adapter

These are all design goals I've been able to hit... so I'd like to introduce the MKIV prototype

Small

Physically it has the dimensions of a 2.5" drive, so it's quite compact.

This makes it both cheaper to manufacture, whilst also being more portable and easy to fit into a larger range of machines. Power is provided by an FDD (Berg) connector, which you can easily find adapters for so that you have drive it from other 5v sources in your computer.

Part of the size reduction comes from losing the speaker mount. This means you can either mount a speaker yourself wherever you'd like in your case or you can use an optional 3.5" drive bay mount that allows a speaker to be fitted.

Cost effective

For the MKIII I enjoyed putting in place lots of controls and indicators that are visible through the ISA bracket out the back of the PC. I love all this control and feedback, however it very much adds to the cost.

For the MKIV these same outputs can be optional added through the headers on the board. Meaning it operates by default in a headless mode, but if you want to install it in a 3.5" drive bay you can route these controls and an OLED screen to the front of your machine.

It also means that an ISA slot adapter can be created that houses the controls and provides power to the HDD Synth device, for those who want the original ISA experience.

CompactFlash adapter

Since publicly announcing the HDD Synth quite a few people have voiced the desire for a CompactFlash version that would act as a full drop in replacement for an existing HDD.

I really liked the idea of this, so began research and found that for a simple implementation it's fairly straightforward mapping the IDE bus to the CompactFlash adapter. 

However I've also learnt that the easiest implementation is the slowest. It uses PIO and runs at 8.33MHz versus the enhanced DMA/UDMA versions that run up to 33MHz using DDR. Getting the higher speed implementation requires much more advanced PCB design as trace length and impedance become very important. I had a few attempts at this, however I've reverted to the PIO implementation to start with - which is how most IDE to CompactFlash adapter appear to be designed.

It's also worth mentioning that this functionality is entirely optional, so it can still be driven by an HDD LED input and not use the CompactFlash as a HDD.

Working on the HDD Synth project has been a real voyage of discovery. 

I've learn so much more about the ISA bus, electronics and circuit design than I ever thought I'd know. Which isn't to say I know that much, but certainly much more than I started.

It's also been rewarding to see how my designs in KiCad have become neater as I've learn to use the features of the software to create cleaner and easier to understand designs.

I'm really looking forward to seeing what people do with their HDD Synths once it's released. All the hardware and software designs are Open Source, so it's very much there for the community to improve and extend.

So what does all this mean?

I plan to send the MKIV off to be manufactured in the near future which will give me a good comparison of the true costs of the ISA based MKIII versus the smaller and cheaper MKIV

My gut feeling is that the MKIV approach is the way to go. Getting something released that's as small, flexible and good value as possible.

It can still be used in a ISA configuration, either by creating an adapter board or by manufacturing the chips directly onto an ISA PCB. 

My email address is listed at the bottom of this website, so please do get in touch with your thoughts.