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Created by
 Western Graphics
Last modified
 08 February, 2006

Analog Metropolis Hardware
Blue Displays, Control Pots, Front Panels, Jacks, Knobs, LED's, LED Clips, 

Overview This page provides details of the hardware design of the Analog Metropolis Modular system, and there is lots of useful infomation for other Synth DIY'ers.

Control Pots There are 3 types of control potentiometers used in the AM Modular. Some of the modules are Oakley designs, which have to use Omeg E16 ECO pots. These are conductive plastic with a quite tight feel, and are relatively cheap. The disadvantage is that they use a wide fixing nut which is not covered by the Modular knobs, so the knobs don't sit close to the panel as they should. They also have a shaft diameter of 6.0mm, which means the AM knobs aren't perfectly central on the shaft.... and they leak gunge..

I started out using Bourns 91A conductive plastic controls, which E-mu Systems used in the early 1980's. They are very expensive and have a very free "feel" when turned.  My Oakley VCO's use these knobs. The blue plastic casing looks fabulous, they have 6.35mm shafts, and the fixing nut does fit under the control knobs. However they don't seem accurate in terms of resistance value, and they have some lateral "play". So not my favourite.

I now use Spectrol 248J conductive plastic controls. These are now considerably cheaper than the Bourns version, and I think have a better feel. They are smooth and tactile, but come in a boring brown casing. They have 6.35mm shafts, and the fixing nut fits neatly under the control knobs. The connections are short and need extending onto the PCB when a pot bracket is used. Spectrol also do a 1/8" shaft version, which suits my ARP2500 knobs.


Knobs  The AM Modular uses replica E-mu Systems Modular control knobs, manufactured in Taiwan. The original knobs are still manufactured by Alco, and they are part number KN701BA ¼. The Tyco Electronics P/N is 6-1437622-4. These are rather expensive at $4 - $7 each, even when bought in quantities of 500. The picture shows some original E-mu Modular knobs on an Oakley MIDI to CV interface built by Riley Smith in California.

These are too expensive for me, so I searched the Internet and found a Taiwanese company that manufacturers high quality knobs. The Taiwanese knobs are near perfect replicas. They are the same size and design, the only differences are the base of the knob is aluminium (not black) and there are 2 grub screws not one (for fitting). You can see them in use on the Analog Metropolis Oakley MIDI to CV interface modules.

They are manufactured by the Taiwanese Knobs Enterprise company as part number 42005-3BA 1/4. They are solid aluminum knobs, straight knurled, with a black anodized body, and a natural top anodized with a black arrow head indicator. There are 2 set screws, and the knob OD is 19mm, height is 16mm, with a shaft hole of 1/4 inch (6.35mm).

Analog Metropolis Standard LED Colours Colour Purpose Example Frosted White White Noise Pink Pink Noise Frosted Blue LFO rate Ocean Green LFO rate Green Amplitude Red Gate Overload Yellow Gate Orange Gate Blue Filter Type Sequence UV Filter Type

LED's It is nice to have some colourful lights on a Modular, so you can see what's going on, and to make the synthesizer look cool. Originally I was going to use just blue LED's, but in the end I decided to use a range of colours, as LED design has improved so much recently.

LED's have come along way over the last 10 years. They are far brighter and more efficient, and a wider range of colours are available. It is well worthwhile tracking down the modern LED's which have clear lens and high luminosities (2000mcd +). The traditional (but cheap) Red and Green LED's are not as bright, and the colours are dull and mushy.

High luminosity LED's require a much smaller current to make them work, and in a synthesizer (typically a +15V supply) lighting levels need to be calm rather than torch light! Remember these LED's are designed to be used as brake lights on cars....You'll therefore need to increase the series resistor to at least 2k2, and often 10-15k, and even 100k for the most efficient LED's. If you use 5mm LED's in clips, the clip will absorb and diffuse the light energy, so the resistor value does not have to be so high. If you use naked LED's (e.g. 3mm types) then you'll need to calm them down with large resistor values.

LED Clips The Oakley modules use a LED panel clip that is very useful and cheap, so I have borrowed the same idea! The clips are plastic cylinders that pop into a panel hole and allow the LED to pushed in from behind. The manufacturers in the USA make quite a wide range of clips for both 3mm and 5mm LED's, different colours and flush or proud covers. You can buy them from Maplin, but Farnell have a wider range.

The clear clip works best as it will show multi-colour LED's and a wider variety of colours (e,g pink, UV). I haven't tracked down a blue clip in the UK, but they do make one. I use the proud "clear" clips, but the flush ones work equally well.

Blue Displays Many synthesizers and sequencers from the early 1980's used 7 segment LED displays to communicate Patch numbers or other vital information. These are easily replaced with cool blue 7 segment displays, such as the LiteOn LTD-5523AB, with 4300ucd luminosity and 3.8V forward voltage. This model has a 0.56" display height, but other sizes are available. You also need to change the series LED resistors from a typical value of 150 ohms to 22 ohms, to ensure the correct display intensity. You'll probably need to experiment to get the right resistor value, as it depends on power rail voltages . Don't forget to replace the red arcylic filter with a neutral one.

Jacks There has been endless debate about the merits of 1/4" versus 3.5mm jacks, and the balance of opinion tends towards 1/4" being the professionals choice. Well not in the AM Modular! 1/4" jacks are just plain ugly and large, they may work well but its like patching a switchboard not a synthesizer. I use high quality Schurter 3.5mm jack sockets and plugs. The socket just about fits a 3mm thick panel, and the plugs are solid metal construction rather than partially plastic.

The patch leads are made from RG58 cable.

Front Panels AM front panels are manufactured by Schaeffer in 3mm aluminium. They are 4U high and have 2 standard widths (90 and 135mm) so that they fit in a 19" rack. The total width of a set of 4U panels is therefore 450mm. Either four (2 of each size) or five (90mm) panels can be fitted in one 4U height of 19" racking. The panel height is 177.75mm, which is slighly smaller than actual 4U (177.8mm).

Take great care when designing panel sizes, to ensure they will fit your racking/cabinet. The first set of panels I made were 180mm and designed to fit seamlessly into Studiospares 4U steel rack cabinets. The extra 2mm+ means there are no unsightly gaps, but of course they don't fit into my 12U cabinets...(dooh!).

Take care with the Schaeffer panels, they can be easily scratched as they are soft aluminium. I use plain aluminium to match the Eu Modular but also because the coloured aluminium shows scratches.

The panels are mounted to 12mm x 12mm L shaped aluminium angle with 4mm machine screws and locking nuts. The angle is fitted to the 19" racking strips using "rack" standard machine screws and nuts, I like the plastic screws as they are lower profile. A piece of teak wood is cut to size to act as an end cap. Aluminium ones would be rather expensive!