Custom Microphone Preamplifier "DTT"

Click here for schematic of the DTT mic preamp; please return using your browser's "back" button.

I found the new-in-box vintage Thordarson input transformer for this channel (along with those for DTS) in my dad's inventory (hence the "D" in this channel's name). Channel DTT couples a Thordarson T-5A2 "mic to push-pull grids" input transformer with an old-stock RCA 12AU7 vacuum tube for a combined 40 dB of gain. With "push-pull grids," each of the 12AU7's matched triodes amplifies an inverted version of the other's signal (i.e., a balanced signal is maintained through the tube stage). The balanced signal is next passed to a rotary 12-step, 5 dB/step dual attenuator before being amplified a further 20 dB in a OPA2134 op amp-based differential amplifier/output buffer.

A key feature of this design is that the differential amplifier ignores (cancels out) noise common to both balanced signals, such as power supply noise from the tube gain stage. However, harmonic distortion from triode amplification is also cancelled out, linearizing the amplifier's performance, and thus potentially negating one of the goals of tube amplification. I therefore incorporated a toggle switch before the differential amp to select between "linear" (listening to differential signal of both triodes) and "non-linear" (listening to just one triode) operation. Of course, the input transformer cancels out common mode noise in the mic and cable, so the DTT preamp remains immune to such noise in "non-linear" mode. Both modes sound excellent, especially on electric guitar amps miked with a SM-57. In fact the DTT channel turned out to be my favorite preamp for recording the electric guitar.

Channel DTT shares a rack tray with the TT and DTS channels, and requires external power supply connections of +180 VDC (plate circuits), +12 VDC (heater), +/-15 VDC (op amp), and +48 VDC (phantom power). Most of its components are on a single ground-plane perfboard, although the output coupling capacitor hitches a ride on the neighboring DTS board.

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