switchboard and mainboard joined

TB303 clone_RE303

Been busy working on this synth lately, making the RE303. The good chaps at din sync have reproduced the legendary Roland TB303 circuit board, and I just had to get one. In my build I managed to get very lucky with parts finding equivalents, lots of “new old stock” and some rare components that are out of production. Japanese transistors, sumida coils and Hitachi ICs were among the gifts and finds.

It was not too difficult to make, and I only had a few issues during the build. In the power supply I put a 15K resistor instead of 1.5K causing a low voltage on the power rail. After some serious head scratching, I eventually got to the bottom of it. All the time spent staring at the components and I couldn’t see the wood for the trees!

The hardest part was fitting the board inside the case as there was no margin for error. On aligning the potentiometer controls you could not afford to be out at all, there was no 1mm tolerance, it was very unforgiving. However, after some persuasion it fit correctly and then it was testing time. Wow! This thing sounds absolutely incredible, so much so I actually prefer it to my original TB303. My TB had appeared on a few records in the past but building this has really given me the bug for making music again!

Power supply

RE303 power supply

This section caused me the biggest bother as my mistake from the resistor had me testing transistors and diodes to make sure they weren’t leaky!

Voltage Controlled Oscillator

This section produces the waveform that is synthesized. The sawtooth wave is produced by the circuit and the square wave is actually sampled from this! Well, I say sampled – it is not held in digital memory as such it is reconstructed from the sawtooth wave. This is why it has a very distinct and unmistakable sound.

Oscillator build

I had to jump start the oscillator to test it was working, I was relieved to see the scope jump to life. In fact, that was one of the most special moments during the build – apart from finishing it and jamming on it.

Testing VCO

Voltage Controlled Filter

The VCF on the TB303 is one of the most incredible sounding analog filters. It is an unusual 18db design and is based around transistors configured as a diode ladder. In the VCF there are rare parts 2SC2291 and 2SC1583- which are dual package transistors. My 2291 measured hFE 450 on one side and around 490 on the other. This is a real mismatch but the end result sounds absolutely incredible.

VCF built

Paul Barker, who’s brainchild the RE303 project is, suggested in the build that we could change R97 to a 10K in parralell with a 220K to drive the resonance and accent a bit harder. To Paul’s ears and his extensive experience working on TBs, this is what can make the difference between a good machine and an incredible machine. I think with the imbalance in the 2291, I have somehow ended up with an incredible machine. I was going to try this dice roll hack but as I am so impressed with my machine, I don’t want to alter it in anyway!

VCF Waveform

Voltage Controlled Amplifier

The only part that was unobtainium during the build was then BA662 transconductance amp which is long out of production. Luckily a talented engineer from the USA managed to remake a clone with SMD components. This part is crucial for triggering the volume envelope but not the VCO or VCF. In this way, not having the original VCA was not so critical to the overall sound, and to my ears it actually sounds a tiny bit more tight and punchy, and actually exceeds the specs of the original part.

I’m sure there may be a few purists out there who might take issue with this, but having tried virtually every 303 replica on the market  and having owned a TB303 for over ten years, I can say that it sounds close enough and I am more than happy with how it sounds.

VCA build

Digital section and switchboard

These were quite straightforward, however, I used thick jumper cables and this meant it was a nightmare to fit in to the case. Add to that that I soldered in the 40 pin IC socket before reading the case instructions meant I actually had to trim down the cpu legs to fit it in the case!

Assembled switchboarddigital build

When I finished I tested it before it went in the case…

switchboard and mainboard joined

Had a nightmare aligning the pots and buttons with the holes…

placing circuit in caseRE303 finishedRe303 and TB303

And voila! One assembled small sleek silver box next to its older sibling – the grandaddy! The controls are different as my TB has had the “Borg modification” done to it which gives loads of output options and full midi control. The RE has less of those options but to be honest I always thought the borg mod was a waste of time. It was already modded when I bought it, if I start removing stuff off it, I will end up with a swiss cheese effect on my case 🙁 As the filter is cross modded with the oscillator it is quite noisy even when switched off. With the amound of background noise it produces my RE really sounds fresh and punchy in comparison.

Sound comparisons

First thing I did after a jam was to make some objective A-B sound comparisons between the two units. The main differences are the punchiness and also the squeaks with the envelope mod fully clockwise and decay on minimum.

Punchiness – As the VCA and lack of modification are different on the RE303, I noticed that it sounds much cleaner than the TB in a good way. It actually sits much better in a mix and it actually sounds more full. It sweeps through the filter range beautifully and I actually prefer it to my TB303.

Squelching – When cranking the env up full and decay down to minimum, it produces that classic squelchy noise we have learnt to love over the years. There was a slight difference between the two units. My TB sounded slightly more rounded and the RE was slightly more edgy here. The RE seemed to lack a tiny miniscule bit of resonance here, this might be the upshot of the imbalance in the 2291. However, This difference was very minimal and I would challenge anyone to guess which was the RE and which was the TB on a blind test.

The original TB303 was not built with audio fidelity in mind and there is a lot of variation between old units anyway. I would say mine is well spec’d and I feel chuffed that my build turned out so well. Whilst I had to listen hard to find that nit picking difference, I am pretty sure I could put it down to the age of the thing. The TB is 36 years old so the di-lectric in the caps and PN junctions in the transistors have settled in nicely. The TB is smooth and refined. This RE in comparison sounds suave and aggressive.


Having owned most 303 remakes such as MB33, FR777, bass station, revolution etc. I was always left slightly disappointed when comparing it to a real 303. However, for the first time of not being a real TB 303, this clone of the original circuit sounds just perfect to me and I am absolutely over the moon with how great it sounds. I hoped it would sound nearly as good and I have exceeded my expectations on this project by making something that sounds remarkable to my ears. Well, I guess you should hear it for yourself and not take my word for it! I added some distortion and reverb but I’m sure it will remind you of being at a party somewhere during a mispent youth!


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