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.
Back to Custom Mic Preamp Main Page
Back to Clark Huckaby Main Page
Copyright 2005 Clark Huckaby. All rights reserved. Commercial use of this content without first obtaining the expressed permission of Clark Huckaby is prohibited by law. Distribution of copies beyond personal use and/or re-posting of this page is prohibited by law.