[2006-01-25]

I learned a lot from the first try at a microphone preamp, and this one turned out way better. I built it up in a 1U BUD chassis. Only 4 inches deep, so the layout was nice and compact. Totally point-to-point wired. Only the power supply is on an eyelet board. I used 18awg wire for a ground bus for the power supply and the circuit section. I used 1uF snubber resistors on the filament of the 6sl7.

Quick and dirty schematic. I screwed up the polarity of the bridge rectifier diodes. Ignore those please.

Probably the most unique part of the design is the use of the triode-mode 6v6 as a line driver. I wanted something with a nice low plate resistance, with very little gain, in an octal package. I tried to find a triode, but the 6v6 was convenient, and the "forward" (hi fi buzzword alert!) sound of a triode-mode 6v6 should sound nice for vocals, which is the primary purpose I have in mind for this recording setup.

The image above shows it with a 6sn7 and a 15095A input transformer. This is the minimum-gain version. The Altec 1566A that somewhat inspired this design uses a higher gain tube and an Altec/Peerless 4722 input transformer, giving it a great deal more gain than this configuration. I am currently using it into a low level unbalanced input on my laptop, which is so sensitive that it is easy to overload it. This low-gain setup, along with a voltage divider on the output lets me turn it to nearly 12 o'clock instead of a hair off of 0 with a 4722 and a 6sl7.

My favorite part of this design is the phantom power supply. There is a trend in hi fi design to use voltage regulator tubes fed with a current source as a heavily filtered, regulated power supply. The 48V supply in this amp uses a pair of 24V zener diodes and a large (20k) resistor to create a shunt-regulated supply. It is a very minimalist design compared to most phantom power supplies that use a voltage regulator or something.