VIHYB Mic Preamp
My prototype VIHYB mic preamp is shown
above. VIHYB stands for
Hybrid Microphone Preamplifier.
I will soon be building and selling enhanced versions of this unit. Please
stay tuned to this page for more details. For now, here are some of
Selectable input coupling and impedance: Each channel has
a 6-position rotary switch for your choice of either capacitor coupling (6600
ohms) or five transformer-coupled settings (nominally 250, 625, 1000, 1700,
or 2500 ohms).
Mono Bridged Mode: With the flip of a switch, the normally
2-channel VIHYB becomes a mono mic preamp with a balanced signal path from
input to output.
Short, high-quality signal path: Sealed relays with gold
contacts handle all signal switching functions (including polarity, mode,
and impedance). Each channel's relays share a compact ground-plane PCB with
the solid-state gain stage and part of the tube circuitry.
Hybrid Gain Structure: The first stage is solid-state,
using a Burr-Brown INA103 instrumentation amplifier IC. A three-position
toggle switch sets its gain at 0, 20 or 40 dB. The second stage uses one
triode of a selected USA-made N.O.S. 5687 tube and has continuously variable
gain up to 22 dB. Thus, overall gain is 0-62 dB in three ranges. The other
triode of each channel's 5687 is used in cathode follower mode as a low-impedance
output line driver.
External power supply: VIHYB uses an external power supply
so that the preamp itself may have an uncrowded layout in a rack-efficient
one-rack-unit enclosure. This also eliminates magnetic coupling between the
power and audio transformers as a source of noise. Each external power supply
can serve up to two VIHYB rack modules (four channels).
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