Custom Microphone Preamplifier "HYB"

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

I named these two mic preamp channels HYB for "hybrid." They combine an IC front-end gain stage with vacuum triode gain and output driver stages. A capacitor-coupled Analog Devices SSM2017 chip-based stage (similar to that of the SS preamp) comprises the front end. In the HYB preamp, a three-position toggle switch sets the gain of the solid-state section to 0, 20 or 40 dB. The next stage is directly coupled and uses one triode of a 5687 vacuum tube for 0-22 dB additional gain (as set by a potentiometer at its grid). Thus, overall gain is continuously variable within three overlapping ranges: 0-22, 20-42, and 40-62 dB. The remaining triode of the 5687 tube serves as a direct-coupled cathode follower, and drives a capacitor-coupled line output with authority. The VIHYB preamp design is basically a HYB preamp with an option for variable-impedance transformer input coupling.

I married a new aluminum 19-inch rack panel to a recycled enclosure from a gutted BBE "Sonic Maximizer" to house the two HYB channels. The solid-state (SSM2017) portions of both channels share a single ground-plane perf board. The vacuum tube stages and associated point-to-point wiring are grouped together for the two channels. This schizophrenic layout led to the color-coded front panel design, with input jacks/controls for a "yellow" and a "pink" channel separated as you can see in the picture. Adjacent to each channel's front panel-mounted XLR input jack are toggle switches for +48V phantom power (top), polarity (bottom), and "stage 1 gain" (to the right). Knobs for "stage 2 gain" are on a separate color-coded patch of the panel. I used a standard (more intuitive) grouping of controls for this preamp's descendent, the VIHYB prototype.

The HYB channels require an external power supply, the connections for which are on a barrier strip at the rear of the enclosure. Several of my preamp modules shared the same power supply. The HYB channels require +180 VDC for the plate circuits, +12 VDC for the heaters, +/-15 VDC for the chips, and +48 VDC for phantom power. HYB drops the heater supply voltage through a series of three silicon diodes so that the 5687 heaters run at about 10 VDC. This vastly increases the lifespan of these great old-stock tubes without compromising their performance very much. I optimized their operating points based on their characteristics at reduced heater voltage. Cathode bias for the HYB design uses Ni-Cd batteries. In the VIHYB prototype, I set up one channel so that battery bias could be readily compared to the more conventional cathode resistor method.

The HYB channels were a great partner to my studio condenser mics. I used.them frequently with AKG C-1000S mics on drum overheads and acoustic guitars. This preamp has a warm tone with a great high-end and low noise.

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