Drum Synth Development

I hope this personal account does not come across as arrogant, I felt physically sick after being told I’d copied someones design without due credit and have been pretty stressed since! This post is intended to show that I did undertake a large body of work, honestly If I had copied it my life would have been alot easier. It took some convincing (business wise) that all the time I was spending on this project would give me the experience I needed going forward and the case was a creative outlet for me. The content is hosted here because its a platform I am familiar with, I’m open to ideas.

Currently, I do give schematics to customers through the account dashboard and by direct request.

My Background

Whilst at university studying electronic engineering as a mature student I developed some kits instead of part time work. The plan(ha!), was to get a job after university and stop making kits. But it turned out I liked making them, even with a job and my wife was keen to work from home whilst looking after our son, she now runs the business day to day and maintains the website.

My boss and manager at work also happen to have vast electrical and analogue synth knowledge between them. This helped.  They told me not to respond, but to be fair they saw the prototypes, helped me analyse parts of it, and pointed me at similar interesting schematics and synths along the way.

My experience in circuit board tools is limited to KiCad(Rakits), Orcad & Allegro(Work), Proteus(University). I use Gerber Viewers to check the output of programs and sometimes delete a rogue bit of silkscreen.

I’ve started reaching out, making contact with other members of the kit making community which has led me to question whether my designs should be open source, partially or fully.

Project Development

End 2015 > Late 2016

Project Inception, I was looking for ideas, a simple analogue synth to cut my teeth on. I found the Amdek PCK-100 demoed alongside a hand clap unit (WIP) on YouTube and shortly after a whole swathe of information regarding original mods and a full original schematic, jackpot!

I moved house in the middle of this lot, had a new full time job and a 3 yo to contend with which is why it took me so bloody long!

I did some prototyping on perf board and breadboard, managed to get it mostly working with the original parts (lm2904, 4558 etc.) but then decided to get clever an optimise.

Stuff I found in my research directory, it lists all the extra mods and controls I added to the unit.

amdek_pck-100_assembly – Original PCK-100 Stuff

amdek_pck-100_cv-mod – I have literally no idea who drew this or where exactly it came from, but it was in my ‘research’ directory. More on this below. *CV Mods*

amdek_pck-100_mods – Original PCK-100 Stuff

amdek_pck-100_settings  – Original PCK-100 Stuff

amdek_pck-100_specs  – Original PCK-100 Stuff

BA6110-datasheet-with-SIP-9

*CV Mods* – The VCO used in the drum synth is a classic design based on application notes released by various semiconductor companies such as the application listed in the LM358 Datasheet page 18. The configuration to add more CV inputs to the VCO is a typical summing amplifier configuration, adding an AC coupled or FM CV;for example would just be a resistor and capacitor instead, this is commonplace in analogue synthesis.

Current mirror – I found the current mirror while browsing some forums for a ba662 replacement. It was lifted from xoxbox schematics. I added a trimmer because the matching was going to be crap at best. (Actually selecting the op amps gets rid of a lot of the unwanted offset and makes the mirror current much easier to tune, more below)

Pot Sweep – This easily took me a couple of weeks of toying with using analogue switch IC’s and switches. Until I spent a lunchtime working on it with my manager and we figured out you could do a complete polarity switch with a DPDT and an extra resistor. I also wanted to use a 1k pot, you cant seem to obtain 500 Ohm board mount potentiometer. Using a larger pot was easily solved by fudging some other resistor values (double the 4k7 feeding the pot).

Decay/Trigger –  I also couldn’t find a 1 Meg pot as used on the original schematic so I instead opted for a 500k.

Prototype schematic This didn’t work, not even close. It was my first attempt with KiCad too, I got it to a semi working state.. with a load of mod wires and time. I got the op amp power supply connections the wrong way around, but there was something else. Even with something to adjust the current mirror the sweep/envelope would be long on some units and short on others. I think the original MUST have used selected op amps for the trigger input conditioning, even the original circuit would not have worked properly without selecting an Op Amp (LM2904) for negative VIO (Voltage input offset), that’s why one of the op amps in my kit are marked with a white pen (poorly, after being tested by my wife [no seriously she cant draw a straight line]).

Early 2017

Prototype artwork – Junk! I still learned a lot from it. Also featuring ‘retro’ dots, disliked by the wife and strangely don’t feature on the production design.

After this I significantly changed the schematic to match much more closely the dual op amps used in the original so that I could select one of the LM2904’s. Turns out TL074/TL072 are pretty awful single supply op amps unless you babysit them, they were canned. The trimmer was changed to a 50k and the footprint changed for a right angle variant. I didn’t like the push button DPDT switches, I also didn’t think the square triangle mix was worth the extra complexity.

After this I got a board back that did work, sort of. There were a couple more resistor value changes and then a couple of months (at least) figuring out how to get nicely made PCB cases. I got the sweep switch around the wrong, sodding, way.. The first lot was a nightmare, I had to do a lot of filing to get them to fit right. It wasn’t until after the first 100 that I made some significant changes to the PCB panels to make life easier.

There was another production run after this where 100 Kit PCBs went to landfill and I cried, literally.

Finally I had some success, everything worked nicely, controls were easy to trim. It wasn’t until my fingers were bleeding (lots of trimming and filing) that I had the confidence (and experience) to do a revised case a few months ago.

RakitDrumSynth_REVB_Schematic Please, feel free to compare this with your schematic I have literally no idea how similar they are. The license and watermarking are a bit of a nonsense and is representative of my internal struggle. I should note that while I spend my time designing the kits, it is really my wife’s business. So all decisions are jointly made which complicate matters.

Bollocks

I discovered Jason’s kit on Tindie some way into the design of the case artwork for the ‘finished’ PCB revision and was annoyed with myself, frankly, for not being more thorough while I was assessing product viability. My target price had always been between £60-70 for an assembled unit and £45 for a kit throughout the project, I still feel that is what my kit is worth when compared to the price of my other kits. I’m able to charge relatively little for the  assembled drums because I get the bulk of the work done (surface mount & PCB fab) in China so it actually saves me time (=money), some people have commented on how cheap it is and some how expensive it seems.

Thoughts

TLDR: I had my head up my arse. I’m trying to be more open in general (I tend to work in isolation, but I’m working on it).

I should have done more research throughout the project, I would have been aware much earlier of Jason’s design and done something completely different. I thought I was reviving something unique for most of the project and whats more there was original scanned documentation… that really appeals to me. There was no way in hell I wasn’t going to release this thing after I spent over a year (albeit in fits and starts) fighting with it, it had very much become an obsession.

I genuinely believe I did not copy Jason’s work, but I can see how it looks and I didn’t do myself any favours regarding the website description. I really struggled with this and I did read your description a more than a few times, it definitely made its way onto the page and I apologise for that wholeheartedly, any feedback regarding this welcomed.

I purposely and consciously did not post my clone on Tindie as I feel it was my mistake for not carrying out due diligence in the latter stages of development.

I do not intend to have a fight about, well, anything. And I’m starting to wonder where I fit in in general (me personally), I’d be gutted if I was ostracised from a group that might attend the same events as me in the future.

Xoxbox current mirror – I believe I checked the license and it was very permissive, still I do hear what you are saying and would be keen to continue the discussion on open source.

I might use parts of this page as a road to product sort of post.

I hope we can meet at some event in the future and talk electronics, music, kits. It took me some time to compile all of this random information. I hope it comes across that I genuinely do give a shit and that I am open to suggestions or just straight up being told to bugger off. I also welcome discussion and am open to help troubleshoot in any capacity I am able.

Thanks

Darren Blaxcell