Project 904A
Here it is! And it works! |
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Input output stage board The extra board is the regulator board giving +12V and -6V |
Filter Board |
91-018 Board Nice big fat orange capacitors! |
Detail of front panel this is a really nice quality laminate |
91-069B Board showing regulator board |
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Fri Jun 14 2002 At long last I have finished my 904A filter project - (I hear a huge collective sigh). This has been a bit of an obsession with me over the last few months. I acquired the front panel, graphics and bracket for the moog 904A lowpass filter some time ago and just put dummy pots and sockets on the front and stuck it in a small cabinet for display only. This bugged me and I needed to make the thing functional. I was toying with the idea of putting in a modern filter board in there just for functionality but was talked out of it by many list members, including the maker of the ladder filter kit. Ok so lets build the original! I had a copy of the 91-069 board, which I did some time ago when I had one in my possession but couldn't get hold of the 91-018 filter board. A few requests on the mailing lists and I got a result. Someone had indeed scanned them and I could etch a copy. Components were difficult to source to; I wanted an authentic sound so I had to use the original 2N4058 and 2N3392 matched transistors. I had no overlay for this filter so I had to go off the service manual schematics to identify the components - this has inevitably driven me mad but I could confirm the identity of the parts with some VERY VERY kind and patient people on the mailing lists 'Analogue Heaven', 'Synth-DIY', 'moog' and 'Oldsynth'. They know who they are and their help is so greatly appreciated. Problems of capacitor value and that 2.5 ohm trimmer were eventually sorted out and although I am a barcket short (!) I worked out a mounting solution. (I think I now have a bracket on the way from a good friend) I built an add on board with 7812 and LM337T regulators to give me the +12V and -6V directly from my +-15V power supply just by plugging it in. So after probably over a year since first getting the front panel I had everything assembled and ready to test - I couldn't bear it. - Nothing worked! Non of the voltage readings made sense - agghhh! I checked a scan of a real 904A - it turned out I had missed a tiny section of track out linking +12V with most of the circuit - doh! Anyway - it works and sounds fantastic! There is something special about this circuit - I have a few ladder filters, which sound very much the same, but this does have that certain extra! Audio DemoA quick sequence to show off the filter doing bassy things.Just a sequencer, saw, ADSR and twiddleing. Had to be WAV, mp3 sounded awful! Its 4.28MB but worth it! SOme manufacturers of ladder filters are listed below: Oakley Sound SystemsThe MultiLadder VCF - The original multitapped Moog ladder filter.The SuperLadder VCF - issue 3 out soon I have built both of these and they are superb and have some unique features. Synthesis TechnologyMOTM 490A faithful copy of the 904A to capture the irregularities of the old transistors in the ladder. Sounds great and available now. Synthesizers.comQ150Another tranny ladder ready built this time - sounds great Moog Custom Engineering904AA Faithful reproduction Analogue SystemsRS 100I have one of these too and it sounds great EFMvcf2fnot built this one but I have built the diode ladder version. Thank You!Again - infinite thanks to all the people who have helped me with thisproject with their infinite patience - it is all very much appreciated. You know who you all are. Kevin Lightner for scans, knobs, trimmers and transistors which had their "vbe matched to within 0.001 volt and the temp changes during matching were nulled via retesting a day later when it was cooler. They're all spot on to each other" (I had his last set, he has no more!) 
Tony Allgood for lots of encouragement and help with my limited electronics understanding. Thanks to Juergen Haible for encouragement to go as close to the original as possible |