Custom Microphone Preamplifier "DMB"

I wanted a "minimalist" battery-powered stereo mic preamp for recording some live shows and DMB was the result. "DMB" stands for "dual-mono, battery-powered." This unit is (literally) built around two military surplus input transformers (of the type I use in VIHYB). Its ground-plane perfboard contains two 2-transistor preamps using the basic Rod Elliott design which I discuss more fully with my DTS channels.

An aluminum plate holds two XLR input jacks and is screwed to the threaded mounting holes on either end of the transformers; batteries are taped to an extension of this plate to the rear and are stabilized by elastic bands. The ground-plane perfboard is secured to the transformers via screws and spacers. Each channel has an on/off toggle switch recessed behind the XLR mounting plate in the space between the transformers. A trim pot in the feedback path of each channel's Rod Elliott circuit sets the gain.

Each channel originally used four 9-volt batteries soldered in series (36 VDC), supplying both the gain circuit and phantom power bias. This worked out quite well with for my AKG C-1000S mics. I made some great live recordings using the DMB direct to DAT with these mics held in a X-Y coincident-capsule configuration. I added the two extra batteries you can see on the photo when I wanted to supply phantom power to my Audio-Technica  AT-4033 (which needed something closer to 48 VDC) via one DMB channel. I found out that the AT-4033 draws a lot of current from its phantom power source and (unlike the C-1000S) dosn't lend itself well to battery power. I later made the monaural BATTHYB, which can use an external power supply to bias mics, and as such was a much better partner to the AT-4033.

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