ETI 4600 Owner Rick Jelliffe's Page

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Rick Jelliffe

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Rick Jelliffe
Present Owner

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  I was given the ETI 4600 in an 80% finished state in about 1982, and after years of work it is now in about an 80% finished state! I got it thanks to a friend, Peter Laughton, whose brother was moving on to better things: I am so grateful to them and never had a good chance to thank them).

Sometimes it seems that no matter how much I do, I never progress :-) Well, not quite. It actually makes noises and the modules are just being tested, and is about 95% finished now.

The synth was mugged by punks. The house I was staying in had previously been occupied by terrifying punks who held a legendary party and removed all interior walls, windows, toilets etc. The repairs, as it turned out, were dodgy: the roof of my bedroom collapsed (I was not there, fortunately) all over the synth, smashing some keys and covering it with dust. I thought it was dead, and I put it into storage at my parent's banana farm, as one does.

A couple of years ago, after gallivanting around the world, I started to make software synthesizers (using SynthEdit): one of VSTs I made, the Neumixturtrautonium got on a UK magazines best 10 VSTs for the year. Recently, another design idea of mine for FSK-based synthesis was realized as an 8-voice synthesizer using FPGA technology: cool. So software, firmware, and, with the 4600, hardware synths! I decided to see what the condition of the 4600 was, and to start skilling up to fix it. I was happy to find it was actually much better than I expected.

I decided I wanted a working synth so some out-of-date or missing things won't be original: I am replacing the keyboard mechanism with a MIDI keyboard (and a PAIA MIDI to CV converter: I'll build a little Herz converter), I don't have the inductors for the EQ so I am looking at using a dedicated EQ chip (bought but not installed), and I bought a PAIA reverb kit with Accutronics springs.

So far I have made only minimal customizations. The first issue was that there was a crazy amount of beehive noise, so I recently entirely rewired it using shielded cable, which has wonderful results. But I used a too-thick shielded cable, and that then needed extra strain relief and doesn't tuck in neatly the way other people's wiring manages to do. I actually rewired the matrix to be on headers, so that the shielded cables plug into a little PCB rather than directly. This allows me to piggyback other connectors, in particular because I plan to send all the outputs to another modular externally (I've made a big 25-jack shielded cable for this, with opto-isolators at the other end.)

The second fix was to upgrade key op-amps. The 301A is a little sad by modern standards, but the 4600 has a common design problem that the data sheets for it give a rule of thumb setting for the external compensation capacitors, but if you do the calculations, these are really only appropriate for op-amps used to amplify the signal, not attenuate or buffer. The result is that the top octave or so is dulled. By the time you go through the mixer, VCF, VCA and output, you could easily have 4 or more of this filtering. So I replaced all the input op-amps in the mixer and most op-amps in the output stage with a more modern op-amp (just TL071, nothing special) removing the compensation capacitors for them. In the VCOs I replaced IC8 TL71 and IC2 with TL81 and removed the appropriate caps. I didn't want to replace all 301s, just in case they colored the sound in some nice way (think, ARP 2600.)

One of the things I found the most fun is the most trainspotter-ish: finding good knobs. I got some great tear-drop Bulgin knobs from the web, I wish I could find more. And I tracked down a couple of sources of Cliff knobs (which are pretty much unavailable now), as used on the EMS VCS3: some of these I have hand-painted. These were cheap knobs at the time, but since the visual design of the original 4600 panel was obviously inspired by the vertical layouts of the EMS synths (which in turn owe their design to IBM 360 and other 60s computer consoles) they go well. I am adopting a yellow, black and silver theme, like the EMS Synthi E perhaps. The other cool thing about the Cliff knobs is that they look like little Daleks when viewed from above: clearly the synth is a grown-up version of the Tardis games of my boyhood...

Other fixes included breaking out the power supply to make it easier to add some extra modules, putting some extra .1uF decoupling on op-amps and putting diode protection on the matrix, so that future connections to outside modules might be safer. I have installed switches and made up the circuit to add 5600-style sync, and I have put in a couple of extra pots on the envelope generators to fiddle with a few extra stages. (The cunning plan at the moment is also to add LEDs for each of the three stages of the three envelope generators, for better blinklichten.) I haven't got any VCFs working yet, so I may put in some EMS diodes filters or CGS Steiners just to get the thing functional.

At one stage, I considered just ditching all the circuits and just connecting the panel knobs directly through ADCs to a PCs, and
simulating the 4600 in software. I made a Velleman kit that could drive a data capture board, but I decided against it: digital is very quick to get results but analog is fun!

The one remaining component I have not been able to source is the dreaded 4416 analog switch. There is a workaround using an 4066 in a kind of "dead bug" configuration with its legs splayed out. (Another owner showed me about this, and I hear this is what Ken Stone did with his 3600 oscillators.)

I don't know if people are aware, but the 4600 PCBs are still available on order! RCS Australia has carried them since the 70s and is supposed to be closing down this year (retiring.)


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