Signal Preference: The Role Of The Mic Pre

In Uncategorizedby tfwm

In the world of pro audio, mic preamps are one of the few areas engineers can go beyond industry norms and experiment freely. While the rig may be covered in Shure SM58 mics, QSC PowerLight amps, Yamaha M7CL-48 consoles and JBLAE speakers, the mic preamp can be as esoteric as a $2500 DW Fearn VT-1 tube model or as accessible as a $199 ART Pro Channel.

The role of a mic pre is to take the lowLevel microphone signal and increase it to the point a mixing console, recorder or other line level device can process it. Due to the need for massive boost (up to 60dB) from mic to line, a preamp can have a dramatic effect on the overall quality of the signal. Mic preamps can also include additional controls for extended equalization and dynamics filtering beyond what is available on the console’s channel strip.

It is arguable the mic predetermines more of the sound quality in the signal chain than any device other than the microphone and speaker. Every mixing console contains a number of mic preampsequal to the number of primary inputs on the desk, apart from lineonly media inputs. Thus, a $2000 mixer with twenty-four mic preamps can allocate little more than $25 per mic preamp and still meet the overall construction budget. While a $25 preamp can sound good, there is simply not enough money involved to use the best components in each channel. Therefore, a separate microphone preamplifier is appropriate on important channels such as the pastor’s wireless and the worship leader’s vocal.

While the need for a mic preamp is objective, the choice among them is not. Unfortunately, mic preamps are the center of the subjectivity aspect of pro audio. Since the preamp tends to “color” or alter the incoming tone, what sounds clear to one person can be brittle to another. One listener’s “warmth” is another’s “mud”.

The best approach is to select a few contenders via price and front-panel control criteria and then audition three candidates in the intended facility with the existing equipment.

In order to arrive at some appropriate contenders, the myriad of options can be funneled through a questioning Process. First, what is lacking in the console’s preamp, and can it be addressed with an outboard device?

Next, what is the thrust of the need: tone or control? Finally, what preamps fit the available budget?

In order for an outboard mic preamp to alleviate an issue, the problem must be electronic in nature. In other words, if the acoustic properties of the room prevent clear reception of amplified speech, a mic pre is not the answer. Acoustic problems require acoustic solutions.

However, if the concern is a lack of definition in the pastor’s voice or a tendency for the worship leader to vary intensity beyond the norm, a mic pre may resolve the problem.

When compared to a mid-grade mixing console’s circuitry, a quality outboard preamp offers the distinct advantages of quieter operation, smoother transitional functions and extended control.

For instance, on older four-buss mixers, the channel preamps are useful when the incoming signal is high, thus not requiring extreme boost. But, when the signal is weak or the mic is distant relative to the source, the board’s high noise floor becomes audible. Thus, a mic pre can be inserted and used to quietly gain the signal to line and then hand it back to the console for routing.

This scenario plays out well with choir mics where the mic to source distance is an issue. With a preamp, the choir mics can be brought up without distracting noise, and the linearity of a quality mic pre’s response will create a satisfactory gainbeforefeedback ratio since there are no adverse peaks or dips to send the system into oscillation.

To answer the second question of tone or control, the need must be analyzed. If the thrust is to add “character” then a tube-based preamp is in order. Just as with guitar amps, mic pres can benefit from the “warmth” tubes add to the equation.

In reality, tubes simply distort in a pleasing manner to the human ear with even-order harmonics while solid state units distort in a “harsh” way with odd-order harmonics. The distortion is present in both versions when the preamp is overdriven; it just sounds better to us on a tubebased product. Tubes inherently require upkeep and replacement and should be acquired only when the commitment is made to service them long-term. For most applications, a solid state unit will suffice, especially in non-instrument situations.

The better question to ask is whether the features on a given preamp are appropriate for the intended job. On some older mixing consoles, the build quality and routing are fine but the channel strips lack a flexible EQ section. A good mic pre can add fully parametric equalization to the wireless lapel channel for elimination of ringing and plosives at far less cost than replacing the entire console.

Often termed “channel strips”, these boxes include line-level inputs and are typically derived from the layout of famous consoles. For presenters with poor level control from soft to loud, the preamp’s dynamic section can be used to compress the peaks while leaving the rest of the sound system in full-range mode.

The last question of budget is best answered through intelligent compromise. While some preamps are able to deliver vast sums of gain with little noise through an extensive EQ and dynamics section, the reality is most cost-effective mic pres must compromise one or more aspects of the design to fit a price point.

Therefore, the best approach is to determine the true essentials of The need and weigh those against potential models within the price range. For example, a church with a Steinway Model-D grand piano helmed by a concert pianist may want to look at a preamp capable of handling the breadth and depth of the tonal output from such a complex instrument and will then be able to ignore any preamp whose design focus is character or one with an elaborate EQ section.

The essential element is noisefree linear response. On the other hand, a church needing to bring presence to the sermon delivered through a Countryman E6 earset mic can eschew perfectly smooth response for a preamp with several bands of parametric EQ. A worship leader seeking to create a thicker quality to his voice when routed through the church’s Yamaha LS9/32 digital console may want to consider a tube mic pre for its intentional “rounding” of the voice.

There are a number of inexpensive yet excellent mic preamps on the market with a bent toward one or more design goals. Some low-cost pres to consider include the ART Pro MPAII, the PreSonus Blue Tube Dual Path, the Studio Projects VTB1, the Grace Design m101, Focusrite’s Trak- Master PRO, BLUE’s ovoid-shaped Robbie and M Audio’s DMP-3. On the higher end, there are some wonderful choices available, such as the API A2D, the Vintech 273, Great River’s MP-2NV, the Chandler Limited TG2, Avalon’s AD2022 and Neve’s Portico 5012 plus any offering from Universal Audio.

Mic preamps take the complex task of increasing gain and turn it into an art form. When used in the appropriate role and in a proper manner, they can eliminate poor sound quality and deliver stellar, clear audio to the congregation.