Click here for schematic of the DTS mic preamp; please return using your browser's "back" button.
I found the new-in-box vintage Thordarson input transformers for these two channels (along with the one for DTT) in my dad's inventory (hence the "D" in the name for these channels). The two DTS channels each use a Thordarson T-6A3S transformer which has a low impedance secondary appropriate for coupling to a solid-state gain stage.
For gain I used a very simple discrete two-transistor feedback design that I found on line by Rod Elliott. I substituted 2N3904 transistors for the BC549 units in Elliott's design, used (large!) non-polarized coupling capacitors, and changed the gain-determining negative feedback loop slightly. These preamps have low noise, sound good, and with transformer input matching have "hair." In my hands their usable gain range was limited to about 15-38 dB. I liked this basic gain stage so much that I also used it in DMB and in the solid-state front-end of BATTHYB. I recommmend everybody check out Rod's highly informative Elliott Sound Products web site. Look there for his article called "Low Noise Microphone Preamplifier" (Project 13).
I built the DTS preamps into a rack tray next to my TT and DTT channels. A large ground-plane perfboard hosts most of the DTS circuitry along with their own +30V regulated power supply. The power supply connects to an external (wall-wart style) transformer via a jack in the back of the rack module. A +48 VDC external power supply connection for phantom power is shared with the other preamps in the rack module.
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