Sunday, 7 February 2010

Aikido Line Stage Build Begins!

Finally got around the designing and building the chassis for the Aikido line stage. Have a look at some of the photos during the planning phase. The chassis will be the usual timber sides and metal plate on top with the tubes and regulator heat sinks sticking through the top plate. The knobs and connectors are all on the front face.

I have decided to use XLR connectors in lieu of the 'Audiophile' type uber-expensive RCA sockets. It was actually the Eichmann plugs that gave me the inspiration. They were talking about the eddy currents, etc that happens when there is a sleeve type connection. And well, that does kind of make sense. But why not go one step further and remove ANY sleeve or ring type connection in the signal chain? The XLR are pin connection only, and being a professional audio product, designed for high numbers of inserts and removals. It is also silver plated connectors, with cable locks. All this had to around $10 each. Further, with the 3 pin standard connection, I can run stereo RCA, with the grounds tied together (which is what happens anyway inside the amp), and thus half the number of cables in the system.

You can see the generic layout of the chassis, with plugs all front access. Transformers and power stage at the back. 3 inputs and 4 outputs. The outputs will be arranged so that 1 will have a High Pass filter (around 150 Hz) and the other 3 will be full range with the RC f-3db point set around 1 Hz.

Close up of the DACT Stepped Attenuator. There will be enough room around for a potential upgrade to the Acoustic Dimension 41 step ladder attenuator.

Close up of the DACT Selector. It will switch both channels and ground. Since it is a 5 position switch and you can see I only have three inputs, it will be arranged that the two unused positions will act as mute function between each active position.

Some additional parts ordered to replace the standard kit components. Tantalum resistors in the important areas and Takman resistors in others. Note also the large Elna Tonrex caps for the heaters. Moulded Teflon sockets for all tubes and Obbligato film capacitors. The choice of output stage capacitors will be Mundorf Tin foil, with a potential for one of the output to have some kind of oil cap (Teflon, Copper, Silver/oil or Beeswax).

The power transformers, one for heaters (Hammond 185D12 6.3/12,6V 43VA) and one for B+ (Hammond 185E230 115Vx2/115Vx2 80VA). The IEC inlet has both dual fuse and EMI line filter. The heater transformer is slightly over-rated. With 5687 and 12AU7 as the tubes, the current draw should be around (5687 @ 12.6V = 0.45A and 12AU7 @12.6V = 0.15A) 1.2A total draw. With an available 3.4A supply from the 185D12, there should be enough head room.

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