Type 2699 Microphone preamplifier

Microphone preamplifiers

Microphone Preamplifiers by Brüel & Kjær are robust and acoustically optimized for a wide range of environmental operating conditions. The high‐output current capability of our preamplifiers is able to drive extremely long cables without signal degradation - even when measuring high sound pressure levels present at high frequencies. The Brüel & Kjær CCLD and classical preamplifier designs fit our range of 1/2" and 1/4" microphone cartridges.


A condenser microphone must be combined with a preamplifier to provide impedance conversion, some filtering, and the capability to drive relatively long cables without significant signal degradation.

Preamplifiers are designed in accordance with two principles. Each has its own special features.

  • Classical preamplifier design
  • CCLD preamplifier design

Classical Preamplifiers

  • The classical preamplifier has an easy to understand concept. It is basically a unity gain amplifier with extremely high input impedance and very low input capacitance. 
  • The supply voltage can be either ±15 V DC or a single 80 V DC.
  • The output signal has its own separate wire, as do the polarization and CIC voltage.
  • Pin 5 is often used for transmission of TEDS data (so called Class II TEDS).
  • CIC (Charge Injection Calibration) is possible by injecting a signal (on pin 1 of the LEMO connector).

CCLD Preamplifiers

Despite its origin in the vibration transducer world, the Constant Current Line Drive (CCLD) principle is increasing in popularity in the area of sound and measurement applications. Different manufacturers market transducers using the CCLD principle under different names.

The benefit of CCLD is that the same wire is used for both the signal and the supply current. Using TEDS Class I even the TEDS data can be transmitted over that same wire (using a level controlled electronic switch as shown in the figure below).

Selecting a Microphone Preamplifier

Brüel & Kjær offer a large selection of robust and acoustically optimised preamplifiers that allow operation in a wide range of environmental conditions. The high output current capability of Brüel & Kjær preamplifiers allows the use of extremely long cables, even with high sound pressure levels present at high frequencies.

  • Preamplifiers are available in both 1/2 and 1/4 inch dimensions for direct fit with the most used microphones cartridge sizes.
  • Adaptors are available for 1 inch and 1/8 inch microphone cartridges
  • The most popular classical 1/2 inch preamplifier is Type 2669 which is available in several different versions.
  • Type 2669 combines a CCLD preamplifier and an A-weighting filter in one unit. This type can be easily distinguished from other preamplifiers due to the two engraved rings
  • When insert voltage calibration is required, Type 2673 is the obvious choice.
  • Type 2695, maybe due to its small size, (half the length of the extremely popular CCLD preamplifier Type 2671), is an often overlooked unit

This opens for the use of cost-effective coaxial cables and BNC connectors as known from general T&M applications.

A CCLD input can be connected to both microphones and vibration sensors (and many other sensors with CCLD output). Due to the working principle the signal is superimposed on a DC voltage. This DC bias voltage is typically around 12 V. Bias drift (over temperature or time) will reduce the dynamic range.

Due to the lower DC supply voltage (typically 20 – 28 V DC compliance voltage out of the front-end), there is some limitation in the upper limit for a CCLD solution. Another limitation of CCLD solution is that only prepolarized microphones can be used and that CIC is not available.

However, in many practical applications this is happily accepted in order to get the benefits of CCLD solutions, that is, ease of use and cheap cables!

The size of the DC bias voltage is often used by the front-end to provide some simple means of cable monitoring. A bias voltage below a certain value is interpreted as short circuit while a DC value above a certain value as open circuit.