Circuit diagram of the 12AX7 tube microphone preamplifier

The following figure is the circuit diagram of the tube microphone preamplifier 12AX7. This circuit is difficult to build. You must have intermediate or advanced skills to build this circuit.

All capacitors with a value of 33uF are rated at 16V, while all others are rated at 50V unless otherwise noted. Resistors marked with a "#" are 1% metal film resistor types. The

"Drive" LED indicates how hard the tube is being driven. The "Mix" control allows for a mix of SS and tube coloration. Symmetry controls the relative amounts of even and odd harmonics, so a CCW tube mic preamp may sound punchier, while a CW may sound warmer. The 12VAC required for pin 5 of the 12AX7 can be obtained from point G, while pin 4 should be connected to point A.

There are two options for line level signals. First, if you know the TMP will always use line level signals, then you can change the two 33k 1% resistors to 1k 1% types and you're done. Or, if you want line level or low level signal options, you can sacrifice the polarity switch and rewire here as shown. Note that the 47k resistor is again of the 1% variety.