Custom Microphone Preamplifier "TT"

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These two channels use modern Jensen JT-115-KE input transformers coupled to two consecutive vacuum triode gain stages. My goal was to achieve gain with a warm, transformer-colored, vintage-like tone. A single potentiometer between each channel's triode stages varies overall gain from zero to maximum. Initially I had biased each channel to use 12AX7 tubes, but this provided a vast excess of unnecessary gain (to be perfectly non-quantitative!); for less gain, the more practical of the TT channels is presently modified to use a 12AU7.

TT channel input coupling follows Jensen's "Standard Microphone Input Circuitry" schematic, which provides two-position toggle switches for phantom power, impedance selection, -20dB pad, and signal polarity. These channels use TL072 op amps configured as unity gain buffers (with padded inputs) to drive the output jacks. The TT channels sound nice and tubular, but have more noise than necessary due to wasted gain (check out the 20 dB pads ahead of the output buffers!). This was essentially my first tube preamp project and I realize now that I was somewhat cavalier with the gain structure. The channels are slated for further modification or redesign. At Transient Sound I used the TT channels for non-critical tracks such as scratch vocals, or anytime I was short on input channels.

The two TT channels share a rack tray and external power supply hookup with the DTT and DTS channels. As a cover for this entire rack unit, I used part of an aluminum door kick-panel purchased from the local Home Depot. Liberal use of aluminum angle brackets was integral to the mechanical layout of this rack module.

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