- 12AX7 12AU7 Mic Preamp - Nady TMP1 Mod -


The initial motivation for an all-tube microphone preamp was to tame the harshness of Heil PR40 microphone. PR40 is excellent mic, but it does not work with solid-state M-Audio DMP3 mic preamp very well. I wanted a mellow tube preamp to round out the edge of PR40.

However, I did not want to build the phantom power, hi/lo volume control and phase switch from scratch. Especially I need to find a suitable enclosure for the preamp. After some research, I decided to mod a cheap Nadi TMP1 mic preamp. I found a brand new TMP1 in EBay about $80 shipped.

TMP1 is a starved tube mic preamp. It has one 12AX7 tube operating at about 50v plate voltage. So the main TMP1 circuit is of no use to me at all.

The Circuit

The design is a typical three stage mic preamp. The input transformer which converts balance input to single end, is followed by common cathode amp stage. A second common cathode stage provides enough gain, even for dynamic microphones. The output stage is a SRPP driving output transformer.

To eliminate any possible negative impact on preamp's sound, the TMP1 VU meter is not connected.

I selected 12AX7 and 12AU7 tubes because of their low heater current. The TMP1's 12v power outputs have very limited capacity.

Power Supply

To build the tube preamp, I removed all TMP1 components except the phantom power and dual 12v power supply. The dual 12v power output are used for tube heaters.

I used a 220v/9v toroidal transformer to step up the 13v output from TMP1's tranny. I decided to utilize a voltage regulator reduce possibility of hum. A LR8N3 IC based high voltage linear regulator produce a very clean 300v DC power without a bulky choke. The schematic is here.

The TMP1's main transformer appears to be over rated. It becomes really hot after extended usage. So I have drilled some holes on case for ventilation

Result

The result, is really a surprise, not at all as I'd expected. I'm immensely impressed by the air this preamp produces. The high is splendid, but not harsh. When paired with SM7 mic, it sounds fantastic for female vocal.

This preamp is totally free of hum. When compared with solid state preamp, it has higher level of white noise, but by no means it is interfering with the recording.

But I don't like the sound of PR40 through this preamp, it is totally unnatural. It will be another project to find a perfect preamp for PR40.

- 2009 -