Dynamic microphones are the most popular for instrument amplifiers because they’re rugged, and many models are designed to handle high SPL. The venerable Shure SM57 is still a valid (and popular) choice for guitar amps but many modern models have gained ground, including the Audio-Technica Pro25ax, Shure BETA 57 and 56, Audix i5 and f5, Blue encore 100i, and AKG D40. Larger diaphragm models such as the Sennheiser MD421 also remain a favorite.
Side-address dynamics made their debut years ago and work great on amps. The mic design has the front of the diaphragm facing 90 degrees in relation to the body, with the Sennheiser e609 being a popular side-address model for amps. A variation of side-address models are units with a head that swivels so they can be used in front- or side-address configurations.
Another approach is using side-address studio-style large-diaphragm condenser designs on guitar amps, with the thought being that condensers respond a bit quicker while offering an expanded high-end frequency range. A while back I worked a show with an MXL MCA SP1 on a guitar amp and it sounded quite nice.
No matter the type of mic, placement and EQ are critical to getting the desired sound. On most gigs, the guitar amp is one of the loudest things on stage so stray sounds bleeding into its mic is not really an issue. That said, bleed into any stage mic should always be taken into consideration.
I normally mike guitar cabinets from the front, placing a dynamic about 2 inches away from the speaker cone and about halfway between the cone’s center and edge and center.
Amps may contain one, two or four speakers, and there may be more than one cabinet connected to the “head” where the electronics and amplification reside. On multiple speakers and cabinets, my approach is to mike one cone unless the amp is a stereo amp like a Roland JC120. In those cases, I place a mic on each speaker to pick up the stereo effect.
Usually, moving a mic closer to the center of the speaker results in a brighter sound. And on cabinets with a speaker port, I position the mic at the side of a cone and away from the port to avoid picking up any port sounds.
Moving It Around
Before addressing EQ, I normally roll off everything below 70 Hz and above 16 kHz. With multiple guitar players (and therefore amps), each should be EQ’d a bit differently so that each can stand out and also have a place to “sit” in the mix. With a player who uses heavy distortion, it can help to cut more of the low end in the 70 to 150 Hz region so there is room for the kick and bass guitar in the mix.
More than few times, I’ve placed a mic at the rear of an open back cabinet. Once it was because the stage was so small that a stand and mic in front of the cabinet was in the way. In a couple of other instances, show directors didn’t want to see “messy mics” on the stage and/or in their video shots. One time the guitar player brought out an amp that had a metal grill that buzzed, so placing a mic at the rear of the open back was a matter of survival. When miking an open back cabinet from the rear, be sure flip the polarity switch (called “phase” on some consoles).
It’s also a good idea to carry some “shorty” boom stands. The smaller size is perfect for amps that are placed on the stage deck, especially small combo amps. Another mounting option that works great is called an “amp clamp.”
Available in many types and models, they all do the same thing: grip a speaker cabinet or combo amp while positioning a mic in front of the amp. The Audix Cab Grabber is a great example of an amp clamp. Many manufactures also make inexpensive Z-shaped bars that can slip through a guitar amp’s handle or be placed under the speaker cabinet to position a mic without the need for a stand.
Bass amps present challenges because they go far deeper and get louder in the low frequencies than many “standard” instrument mics can handle. Guitar fundamental frequencies run from 82.4 Hz on the low E string to 320.6 on the high E string. Four-string bass guitars fundamental tunings are 41.2 Hz on the low E to 98 Hz on the high G, with five-string basses adding a low B at 30.8 Hz and six-string models adding a high C at 130.8 Hz.
Mics need to be able to handle high SPL at low frequencies, so kick drum mics are often employed to mike bass cabinets. Models such as the Shure BETA 52, Heil Sound PR40 and AKG D112 “football mic” (so named because of its distinct head shape) are often seen in front of bass cabinets.
Dual-element mics can also work well on bass amps. They combine dynamic and condenser elements in a single housing and allow the user to mix and blend to taste. By mounting the capsules right next to each other, they’re aligned to ensure phase coherence between the two individual outputs. The Audio-Technica AE2500 and Lewitt DTP 640 REX are examples.
“Combo” amps with a single 15-inch (or smaller) speaker or speaker are popular with bass players for practice, as well as wedding- and jazz-type gigs. Single cabinets with 15- or 18-inch speakers are also used by some bass players, but more common are ones containing four 10-inch speakers.
The theory is that the smaller speakers can reproduce the high end better than a single larger-diameter cone, and by placing four in a cabinet, they can combine and push out the low end. Many of these cabinets also include a high-frequency tweeter in the center to help reproduce harmonics.
Bass players may use more than one type of cabinet, such as a 4 x 10 atop a single 15- or 18-inch box. The two may be used with a crossover network, so both may need to be miked to pick up the entire spectrum. I place mics on bass cabinets the same as on any other cabinet, about halfway between the cone edge and center of the driver and about 2 inches from the cone. This usually results in the mic being only about 0.5 inches away from the grille. Some techs, however, prefer to move bass mics back a bit, away from the grille, to pick up more of the cabinet and porting.
The Direct Route
More common for bass is to feed it direct via a DI box. In its most basic form, a DI takes an unbalanced high-impedance signal (i.e., from an electric guitar or keyboard) and converts it to a balanced low-impedance signal that can run down the snake and into a mic preamp in a console channel.
If the high-impedance signal is not converted to a low-impedance balanced signal, it can pick up noise. Also, running a high-impedance signal at longer distances (like down a snake to front of house) can attenuate higher frequencies.
DI boxes are available in two styles: passive and active. Passive designs use a transformer that performs impedance matching and balancing, essentially creating a “magnetic bridge” for audio to pass. They don’t require a battery or phantom power. The quality of the sound depends mainly on the quality of the transformer with good ones sounding far better than cheap ones.
When given a hot input signal, passive styles saturate instead of clipping like their active counterparts, resulting in a “warmer” sound that some may actually strive for. Better passive designs use a shielded transformer to help reject any interference from entering the unit.
Note: Don’t place a passive DI atop an instrument amplifier because the amp transformer’s magnetic field can interfere with the magnetic field of the DI’s transformer, causing noise. Most DI models add a ground lift switch and an output or loop-thru that facilitates connection of the instrument to the stage amplifier.
Active models utilize electronic circuitry that requires power, provided via a battery or more commonly via phantom power from a mixing console. They’re much like preamplifiers and can also offer features like ground-lift switches, high-pass filters, mono summing, polarity switches, and equalization circuits. Solid-state electronic designs are common in live audio production, but there are some tube DI models that see some usage on stages with the thought being that the tube circuitry adds a bit of “warmth” to the signal.
A common rule of thumb when I started out many years ago was to use a passive unit for high-output instruments like electronic keyboards or guitars with powered pickups and use an active unit for low-output instruments. I still basically follow this but it’s not an absolute rule anymore because many active DIs now have input pads to reduce hot signals while many modern passive units work quite well with low-power signals.
There are several manufacturers that make both active and passive units, with Radial, Countryman, and Whirlwind popular choices (for good reason). My own preference for bass is an active DI that has a pad switch so I can reduce the input signal to the box if the performer has active pickups on the instrument.
A common and effective technique is to mike the bass cab in addition to getting a DI feed so that the two can be blended into a solid but natural tone. For players that use foot pedal effects in their signal chain, I’ve found that both a DI and a mic on the amp is a great way to go. With just the mic, it can tend to be effects heavy, and with just the DI, there’s none of the intended effects.
Around In Circles
With keyboards, I prefer passive DIs. If the player has multiple units, I may deploy a rack-mounted DI that places four devices in 1RU. Most manufacturers make multi-channel units, and they help cut down on stage clutter. The exception to my “DI on keys” approach is a Hammond B-3 with a Leslie speaker or other rotating speaker instrument.
While there are many different models of Leslie speakers, past and present, the most common one is the model 122. It’s in a wooden cabinet that has slots around the front and sides to let the sound out, but most players remove the backs of the upper and lower rotator sections to let even more out.
Usually I deploy three mics on a Leslie cabinet – two condensers for the top rotor and a larger diaphragm dynamic for the bass rotor. The dynamic resides about 12 inches away from the bass rotator, with the condensers for the top at opposite corners at the rear. Wide panning of the top mics can deliver a rich, full sound. However, if console channel space is at a premium, a single mic can be used on the top horns, placed about 12 inches away from the rotors.
There are some modern versions that consist of real rotating horns for the tops but use a stationary speaker(s) for the lows, with digital effects to recreate the bass chorale and tremolo sounds. In these situations, I deploy two mics for the real rotating horns as with a 122 Leslie, but then position a large-diaphragm dynamic as I would on any bass speaker.