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OmnidirectionalCardioidBidirectionalBock Audio 251

Multi-Pattern Tube Condenser Microphone

The Bock 251 is a high end, modern version of the vintage Telefunken Ela-M 251 or Elam 251 mic.

It uses a proprietary CK12-style capsule hand-made to David Bock’s specifications by MBHO. Bock describes the capsule as a “1'' diameter, dual asymmetrical backplate CK12 type.”

David Bock

It is based on the original [CK12], with some modern improvements like more accurate machining for better consistency.

The 251 has a proprietary hand-wound multi sectioned audio transformer, a NOS GE5670 “five-star” triode tube in the mic amplifier, a hand built outboard power supply, and a custom low capacitance audio cable.

It provides three distinct pickup patterns. In Cardioid mode, the rear diaphragm is disabled to ensure optimal tone and signal quality.

The Bock 251 is not a perfect cosmetic recreation of the original. “My job was to get the essence of the sound without all of the cost,” said David Bock at the Audio Underground Roadshow in September, 2008.

David Bock

The amount of disrepair and malrepair abounds in the world of the 251. Especially when we started making that mic eight years ago, you never knew what you were going to get if you rented a 251, at least in Los Angeles. I rented 10 from Audio Rents; every one was different.

Some had been modified for super-thin diaphragms, which was almost the equivalent of sticking a Dremel tool in your ear.

What do I model my mic after if they’re all different?

…We made some improvements with our 251 [as compared to the original]. The original 251 had a very tiny transformer that started rolling off [the low end] at 80Hz. That means when you put it in front of a drum set you don’t get quite the impact. My transformer is an inch and a half in either direction. It doesn’t lose low end. it doesn’t lose definition in the low end. It’s flat to 8 Hz.

Like all Bock Audio mics, the 251 is hand-built and tested in Southern California.

Permalink: Bock Audio 251

The mic was released in 2007.


Pickup Patterns Pads & Filters
Omnidirectional (19 mV/Pa; 10 - 18,000 Hz)
Cardioid (19 mV/Pa; 10 - 18,000 Hz)
Bidirectional (19 mV/Pa; 10 - 18,000 Hz)
Capsule Dimensions Impedance SPL/Noise
Diaphragm diameter: 25mm
200 Ohms (Low) Max SPL: 94 dB
Weight Length Max Diameter Interface(s)
716g (25.26oz) 216mm (8.50'') 52mm (2.05'')
  • 3-pin XLR male (1)
Power Specifications
  • Includes tube power supply

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