Custom Microphone Preamplifier "BATTHYB"

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

With an input transformer, two transistors, one tube, and twenty 9-volt batteries, BATTHYB is probably my most esoteric custom microphone preamp. Like VIHYB, it has an input transformer-coupled solid state gain stage followed by a vacuum triode stage and cathode follower output driver. However, BATTHYB uses Rod Elliott's 2-transistor feedback gain block (see descriptions of DTS and DMB channels) as the solid-state stage instead of VIHYB's INA103-based design, and its tube stages use a 12AY7 instead of a 5687. Each of these differences result in less current drawn and a more battery-friendly preamp channel.

The gain of BATTHYB's solid-state stage is either 12.5 or 30 dB, and is set by a toggle switch controlling resistance in the negative feedback path. Gain at the triode stage is up to 24 dB depending on the setting of a potentiometer feeding the grid. As with HYB and VIHYB, the cathode follower stage is directly coupled to the preceding triode stage, and a large non-polar film/foil capacitor provides the output coupling. BATTHYB's output impedance is about 900 ohms, which is significantly higher than that of HYB and VIHYB (owing to the differing tube characteristics), but still OK for today's typical "line" input impedance of 5 to 10 k-ohms.

BATTHYB's twenty 9-volt Duracell "Coppertop" batteries are hard-wired in series to provide a nominal +180 VDC for the 12AY7's plate circuit. The left image (above) shows the battery array with its safety-enhancing tape layer and plexiglas cover removed. The right image shows the unit with these insulating layers in place. The first battery's negative terminal (O VDC) is tied to the chassis/audio ground via a 315 mA fuse. I tapped the battery array at its nominal +36 and +54 VDC nodes to supply the solid-state and phantom power circuits, respectively; these potentials are dropped through series of 6 and 8 silicon diodes to approximately +32 and +48 VDC. Half or all of the diodes in each series may be shorted by closing four miniature (DIP) SPST switches to compensate as the batteries are depleted over time.

The BATTHYB's tube heater circuit does not use the internal batteries. Instead, it uses a jack for an external +12 VDC power supply. A second power jack is for an external +48 VDC supply; when BATTHYB's phantom power switch is in its "off/external" position, such an external supply can be used for phantom power-hungry mics. I included this feature to suit my Audio-Technica AT-4033, which draws too much current to be considered battery-friendly.

I tracked BATTHYB's "on" time and battery voltages during a 3-month period when it was used a total of 40 hours for monaural live recordings at a nightclub. You can click here to see a graph of my results (please return using your browser's "back" button). I extrapolated the data to conclude that the batteries should reach 90% of their rated voltage (or 162 V total) after 175 service hours. If the batteries are retired at that point, it will have cost about 23 cents per hour to operate this preamp, since the twenty batteries cost about $40. The "Energizer Bunny" would hop for joy if everybody had a BATTHYB!

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