The 350 Modded Mic Preamp by The Ampex Corporation

The Ampex 350 Modded Mic Preamp is a vintage tube-based microphone preamp derived from the electronics of the Ampex 350 a classic 1950s professional tape recorder amplifier originally designed for the record/playback circuitry of reel-to-reel machines.
Modders take the original 350 tube circuitry and update/modify it for modern studio use as a dedicated mic preamp.
This includes recapping, rewiring, adding phantom power (48 V), pad switches, input/line options, and improved power supply so it can be used with contemporary microphones and recording gear.
These units were not originally designed as mic pres Mods are essential to integrate phantom power and practical modern connections.
Performance varies depending on quality of the mod and whether restoration/upgrades were done well.
Sound character tends toward vintage warmth and coloration rather than ultra-transparent.
Noise characteristics: properly serviced units can achieve a fairly low noise floor for vintage tube gear.
In stock form the 350 was a tape electronics module not a standalone studio preamp so modifications are necessary to make it practical for mic use.
The result is a tube mic preamp with a warm, vintage character, capable of high gain and low noise performance when properly serviced.

Specs: (Modded Version)

• Tube-based preamp: uses vintage tube amplifier circuits from the Ampex 350.
• Characteristic warm vintage sound with tube coloration and saturation when driven.
• Mic input with pads: often includes -20 dB and -40 dB input pads for handling hot mic sources or line signals.
• Input signal options: original mic input or line input selectable.
• Record level control: now serves as the main gain knob for adjusting preamp gain.
• 48 V phantom power added for condenser mic compatibility (a common mod).
• Input transformer loading mod: improves linearity and usability for both mic and line levels.
• Updated power supply: added or improved for reliability, possibly with external converter depending on region.
• Recapped & rewired: old capacitors replaced; modern parts used for longevity.
• Unnecessary tape machine stages removed: focus on a clean mic preamp signal path.
• High gain capability: enough to drive low-output mics.
• Balanced input and output: modified to reasonably modern standards, typically into 600 Ω terminated lines suitable for modern consoles and converters.

Controls After Mod
Phantom On / Off Enable/disable 48 V phantom power
Input Transfer / Pad Select -20 dB / -40 dB pads or input source
Gain (Record Level) Adjust preamp gain
VU Meter Visual output level monitoring
Power On/Off Master unit power switch