- Solid State Mic Preamp - ART TPS2 Mod -


T have seen several implementation of mic preamp using integrated preamp IC, M-Audio DMP3 is one of them. I am also wondering how would different OPAMP impact the sound of a mic preamp, as I have been rolling OPAMP in my DACs. So when I saw a ART TPS2 mic preamp on EBay, I bought it with an intention to mod it to experiment different circuit layouts.

The Circuit

The link to the original TPS2 schematic is here.

ART TPS2 is a starved tube mic preamp. One 12AX7 tube operating at about 50v plate voltage to add color to the sound. As I intended to build a all solid state preamp, the 12AX7 and its peripheral component were removed.

For experiment, the left and right channel circuit designs are quite different. See the partial schematic as shown.

#Right Channel

The right channel input stage is mostly the original TPS2 capacitor input discrete differential stage. The major change is that the input stage gain is now fixed.

The second stage is conventional OPAMP based amplification stage. The gain of this stage is controlled by a precision potentiometer, and followed by a attenuator. Coupling capacitors are used because of the high gain.

The output stage is a THAT 1646 balance line driver. An OPAMP follower is used to make sure the low input impedance.

#Left Channel

The core of left channel is a THAT 1512 integrated preamp chip. I have experimented different input and output configuration with T1512.

  • Capacitor input and THAT 1646 output
  • Transformer input and THAT 1646 output
  • Transformer input and transformer output

In the end I settle on the transformer input + transformer output design. The output transformer is wired a separate 8 pin IC socket that plucks into the IC socked on the the PCB, so I can easily switch it to THAT 1646 output.

Construction

The TPS2 power supply are reused as it is, although I prefer separate regulators for left and right channels.

One multi-turn precise potentiometer is used at the second amplification stage for each channel.

The two input electrolytic capacitors are replace with high quality Nichicon UPW capacitors, which sound much more smooth.

Result

Both channels offer quiet amplification with gain more than enough for any microphones I have at hand.

The right channel with discrete input stage has a very open sound. I use OPA2606 OPAMP to add some warmth to the sound.

Capacitor input and THAT 1646 output on the left channel sounds thin and very compressed. Transformer input makes the THAT 11512/1646 combination a bit smoother, but still not my cup of tea. An output transformer really make it sound right - there is substance in the sound without too much of color.

- 2009 -