Ampex 351 Mod

Ampex 351 Mod
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Ampex 351 Mic Pre Optimizaton Mod:


This mod turns a somewhat unusable Ampex 351 electronics channel into a truly impressive mic pre!  Capitalizing on a fantastic design by the brilliant Ampex engineers this mod eliminates drawbacks to practical use in the recording of music while preserving and even teasing out a bit more of the inherent impressive sound.  The result is totally usable totally pro sound that is unmatched by most mic pres.  This modified mic pre has all-around utility:  it is  punchy, has a warm, even hot midrange, and the highs are silky and shimmery.  This is not at all a “straight-wire-with-gain”.  It does improve the sound it amplifies. It calls attention to the instrument or vocal being recorded. In some ways it is brash, but in no way harsh.  It just plain impresses.



This Mod came about at the request of some customers who noted the fantastic sonic characteristics of the Ampex 351 tape recorder tubed channel electronic’s mic pre, but found some significant drawbacks that prevented them from practical use of the unit in music production as a mic pre.


The Drawbacks  sad:

  1. Too much gain and noise.  
  2. The mic pre of stock units has about 90 db of gain from input to output, which is about 30db more than is needed.
  3. The excessive gain results in having to ride the input level control near the bottom of it’s travel where resolution is poor.  
  4. The excessive gain also results in excessive noise, both hiss and hum. 
  5. No input pad 
  6. No phase reverse
  7. No phantom power
  8. A lack of low end if not terminated by 600 to 1200 ohms.


The Solution laugh:

  1. Some circuit components are changed to lower the amplification factor of both the input and output stages, greatly lowering hiss.  This has been carefully worked out so as to preserve the hot sound of the original circuits.
  2. The input transfer switch is modified from line – mic – unbal  to mic – mic w/ pad – hi-impedence input. The pad is tailored to the input impedance of the mic transformer so as not to sound dull.
  3. The EQ/speed switch is modified to be a phase reverse switch.  The phase reverse is done at the output connector and can be switched without glitch
  4. Phantom power is added via an added phantom power supply. The record switch is replaced with a lighted push-button switch for engaging phantom power.
  5. The playback level pot is replaced with a rotary switch for engaging/disengaging an internal 1200 ohm load as needed.  (Engaged when the output of the Ampex 350 is not connected to equipment with a 600 ohm input impedance.
  6. Along with all the modification work the Ampex 351 channel electronics are recapped.  Intensive cleaning is done to switches, pots, tube pins and sockets and connectors.



(Price is for modification labor and parts.  Does not include unit to be modified.)

(Repairs not included)

  • Brand: WPA
  • Product Code: WPA-Mod-Ampex-351
  • Availability: In Stock
  • $1,500.00

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Tags: Mod, Ampex, vacuum tube