Lol, Tom... and thanks everyone! RDavidson is a talented designer, it looks good without being too complicated to build. This was my first project using the CNC mill, what an amazing tool... CAD>Reality quickly and accurately. As an engineer I always handed off my work to mechanical designers who handed off their work to fabricators, doing it myself is a lot of work but it's fun and satisfying. At least I have Ryan doing Solidworks.
A few details:
- The felt padding is a gasket/vibration pad for the three power trafos mounted on the top sheet. The wood boxes are meant to resemble potted trafos, but they aren't potted because I wanted to use a passive convection cooling system for them since wood is an insulator. The trafo boxes are painted with a heavy coat of carbon conductive grouding paint and attached to ground via the wood screws holding them to the topsheet. There is a hole for wires and another for ventilation, then three holes on the top/back of the covers for an exit. The boxes stay cool to the touch after the preamp was on for 8 hours and the filament trafo (EI trafo between the 2 wood boxes) only gets mildly warm, it's hugely oversized which is mostly for looks. And if I want to add another 10 6SN7s I'm all set.
The felt on the interior is used to damp capacitors mostly. I just siliconed a little felt to them. Ceramic caps are microphonic so it''s a good idea to damp them imo... and while electrolytics are debatable it takes 5 min and $1 worth of materials to damp them.
- The interiors of the chassis and trafo boxes are painted with a couple heavy coats of carbon conductive grounding paint and a metal bottom plate will be used so it effectively has an enclosure. It doesn't make much of a difference in most homes but I figured I'd go the extra length for this one. I've watched distortion readings go down when a metal lid is placed on an open preamp.
- It can use 6SN7, 12SN7 or 12SX7 tubes, there is a heater voltage switch in the back panel. John Broski recommended 12SX7 so I have a pair to try out...
- The big blue caps are for the last stage of the power supply filter, which is split into separate RC filters for each channel. Each tube has it's own Clarity TC 175 uF ultra-low impedance film cap. The caps right after the rectifier are electrolytic but they have a film cap bypass and make only a very subtle, fairly insignificant difference in that position. The Clarity TCs make a massive difference where they are used in this preamp. The caps are hugely oversized but they aren't much more expensive and ESR goes down as size goes up.
- The big white rectangle near the IEC is a time delay relay for the B+ supply, it allows the heaters to warm up for 30 sec before high voltage is turned on.
- Now that all the CAD/CAM work is done it's possible I could make chassis for other folks using the same design without it costing a small fortune. It's not really my preamp as it's a collection of 3 kits, but it's a combination I think is as close to ideal as possible while staying in a reasonable budget and other folks could build their own if they wanted.
Oh yeah, Roy... this is the ebay seller I got the knobs from:
http://www.ebay.com/sch/m.html?_odkw=&_ssn=along1986090&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR12.TRC2.A0.H0.Xknob.TRS0&_nkw=knob&_sacat=0I had to specify whether I wanted indicators in the notes, so make sure you specify no indicators for encoder knobs. I'm happy with the fit and finish of the knobs, they are solid aluminum.
