There is a current thread on the SDIY list regarding who has the oldest DIY synth. You might remember Jim Patchell's 1978 DIY modular.
Geoff built the Sound Synthesizer designed by G.D. Shaw featured in Practical Electronics.
"I built the Practical Electronics Sound Synthesiser featured in the February 1973 issue of the magazine. I think it was the first DIY Synth to be published in the UK. For those who have never seen this, the designers page is here (Shaw's Corner), click on 'New Stuff v Old Hat' and scroll down to the front cover of the magazine"
From the site:
"An interest in the physics of sound meant that 1972 saw the design of a Sound Synthesiser. The prototype instrument is shown on the far left. Practical Electronics agreed to publish the design providing that it was made in modular form and equipped with a keyboard. The resulting instrument is shown on the front cover of PE for February 1973 and the series ran for a total of thirteen months."
Be sure to check out the Practical link below for more. Also check out this site.
Update via McKansas in the comments:
"Hi. I also built the 1973 Practical Electronics synth . Added the VCF that was published later. It had a sample and hold module that is still unrivalled. There were two simple but incredibly useful differential inverter/non-inverter/mixer amps. The ramp generators are very interesting eg: one ramp through the non-inverting and the other ramp output to the inverting input- this result to osc or filter. All my modules still work. This was a well thought out design. It was of course intended as a 'sound workshop' rather than a keyboard synth. The only glaring omission was the VCF. ... also had a great Ring Modulator..and voltage controlled stereo output amps."