B1 Buffer Preamp

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#228
I like re-purposing things for audio projects too.

A couple years ago when my neighbor was moving out, she was going to toss out a wood box for 5 x 8 index cards. Even though it was pretty cheaply made, I hated to see it get tossed into the landfill so I grabbed it figuring I'd do something with it eventually.

The eventually came when a friend of mine wanted me to build him a simple buffer, rather like the B1.

Here it is just after I'd started to line the inside of the lid with 1/8" cork.

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Here it is while gluing in some 3/4" maple side panels that will serve as a ledge to mount the top panel inside the box.

The RCAs are mounted to a piece of tempered hardboard taken from an old clipboard. I had it laser cut so the holes for the RCAs would accommodate the "D" flat on them to keep them from turning in the holes. The assembly is then attached to the rear panel of the box with five brass wood screws.

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And here it is flipped around to show the business end of the RCAs.

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Here's the inside of the top panel.

The transformers are 1:1 bifilar input transformers. The pot's a Penny & Giles RF15.

The buffer's pretty much the same as the B1, except it's using DN2540 depletion mode MOSFETs and using source resistors. It's powered by a split supply and adjustable for offset so there are no coupling caps.

The transformers and perf boards are mounted to 6-32 T-nuts installed on the other side. The top plate is 3/8" Baltic birch.

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Here's the top panel set inside the box, which has been trimmed out with the 1/8" cork. The knob is a large (2" at the skirt), art deco-styled bakelite number from the 1940s that I'd found at Leeds Radio.

The counterbores for the T-nuts have since been filled in with drywall compound and sanded flush. The three holes on each side are for the mounting screws that will screw into the maple pieces.

The top will be covered with a nice "cognac" distressed leather and the screws will be brass oval head types with brass cup washers.

Still thinking of a way to dress up all the real estate between the knob and the back of the top plate. Maybe a nice vintage meter or something.

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#237
Naah, 2SK246 is a large pinchoff-voltage part with low transconductance, I wouldn't use that here (though possible). Just send me the 2SK147 and I'll send you some 2SK170 in exchange 😀

Go for the real part, it's still rather easy to get!

Have fun, Hannes
 
#240
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