The microphone preamplifiers in most low-cost audio mixers and recorders are now optimized for high-output condenser microphones. However, dynamic and ribbon microphones produce output levels that are too low for normal use by these devices. There is now a simple solution to this problem.
A relatively new category of products is on the market known as mic activators. These in-line devices use 48 volts of phantom power to inject about 25 dB of additional gain to boost the output of low level microphones. The added gain is transparent and noise free.
Whether it be popular dynamic broadcast mics like the Shure SM7B, the Electro-Voice RE20 or the Sennheiser MD 421-II, or classic ribbon microphones with low-output signals, these activators can save the high-cost of using specialized high-gain preamps with these microphones. The activators can also be used to boost signals in extremely long mic cable runs — whether in studios or on location.
Perhaps the best known of these activators is the Cloudlifter from Cloud Microphones, based in Tucson, Arizona. From single to multi-channel models, the Cloudlifter adds 25dB of gain to mics, boosting the tonal character of the microphone without adding color or noise to the signal. It is ideal for low-gain or noisy microphone preamps.
A basic single-channel Cloudlifter (CL-1, $149), comes in a small blue all-metal box with XLR connectors on both ends. On one side — the signal to the mixer with 48 phantom power activated — is connected. The other connector goes to the microphone itself. It’s as simple as that.
Other Cloudlifters offer extras, like the CL-Z model ($299), with a variable impedance control from 150 Ohms to 15 kOhms with a choice of 12 or 25dB gain. A two-channel model, the CL-2, is priced at $249 and a four-channel model is priced at $499. All the units use discrete, ultra-transparent JFET circuitry in their design.
A competing product is Radial Engineering’s McBoost ($199), a similar device that adds a three-position load switch and high-pass filter to the 25dB signal booster. Like the Cloudlifter, McBoost is designed to elevate the output of dynamic and ribbon mics to bring them up to a useable level for mixing consoles that may not provide sufficient gain.
McBoost uses a discrete class-A circuit design with a dual-transistor JFET circuit. Each transistor is measured and batch-sorted to comply with the design specification to certify performance.
The circuit board is then enclosed in a 14-gauge, I-beam, reinforced steel enclosure to prevent outside stress from torquing, which could lead to premature part failure. A book-end cover creates protective zones at each end to safeguard the switches, potentiometer and connectors from the rigors of heavy use.
Like the Cloudlifter, the McBoost is used in-line with standard XLR connectors and 48 volt phantom power. Users can select between three impedance settings to optimize the response and then reduce low-frequency clutter with the three-position high-pass filter. It can also provide extra drive for long mic cables.
Finally, the FetHead from TritonAudio is the brand name for a series of low-noise, in-line microphone preamplifiers that boost mic signals. The electronics are housed in a metal in-line barrel with a balanced female XLR input to male XLR output. Different models of the FetHead boost signals by 18dB, 22dB or 27dB. The prices range from $90 to $100, depending on the amplification level.
Mic activators have proven popular not only to boost the level of mics. Some broadcasters use them with cheaper dynamic microphones to enhance the mic’s sound and bring out frequencies that were once too low to notice. They also work well with broadcast OB trucks, enhancing audio in extremely long mic cable runs of hundreds of feet from the mobile unit.
With the overall cost of audio gear dropping, some manufacturers cut costs by optimizing the performance of microphone preamps for popular condenser mics. The gain boost from an activator allows these preamps to work more efficiently and sound better, without adding noise.
Mic activators are now a popular “fix it” tool in sound operators’ toolkits. In the right situation, they can make a major difference in sound quality.
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Microphone preamps are very simple. The basic function is to boost the low level signal from a microphone to a more robust line level. The signal can then be used for broadcasting, editing, recording or sound reinforcement.