It is fairly obvious that the circuit shown above adapts the TMP’s XLR input to the increasingly popular 1/4″ balanced lines, but there is more here than immediately meets the eye. Notice that the sleeve of a MONO phone plug grounds the ring of the STEREO jack and consequently the inverting input of the differential amplifier. This has the effect of single-ending the balanced input so the standard phone plug of dynamic microphones can be plugged in directly. And, the polarity switch still works, even for unbalanced inputs. The 1/4″ stereo jack could be mounted in a hole added to the TMP case and permanently wired to the panel mount XLR Mic Input J3, but use twin-ax for the connections if the new jack is going on the back panel. No damage will result if you forget and leave phantom power on while using a single ended input, but performance will not be up to par.
There have been questions about using the TMP with line level signals. This can be accomplished in any of several ways, including simply padding the line level down by about 20 dB. and making up the difference with increased OUTPUT LEVEL. Here are two other ways:
1) If you will always be using a channel for line level signals the gain of the balanced input circuitry can be decreased by substituting 1000 ohm resistors for the 33k value presently at R20 and R39. Note that these resistors should be 1%, metal film resistors like the ones supplied with the kit.
2) The Polarity switch can be sacrificed and reincarnated as a mic/line level switch as shown in the schematic above. Flip the switch down for Mic level and up for Line level signals. Since there are typically many places in the signal path where polarity can be reversed, the loss of this switch will probably never be noticed. The added 47k ohm resistors should be 1%, metal film types.
DC Filament Power
Heating the tube filaments with rectified and filtered DC power squeezes the last couple of dB out of the noise floor. This is most easily and economically done as shown in the before and after schematics:
The 1000 uF / 25V Electrolytic capacitor (Cnew) can mount directly on solder lugs 4 and 5 of the tube socket. The added 15 Ohm 1W resistor (Rnew) can fly from the junction of C3, D2 and R2 to the tube socket. It is very important that the ground side of the filament return to the “G” point on the circuit board and not some other ground point.
Note: The DC-Filament mod is only effective with tubes marked Made in China.