July 28, 2016
The standard three band equalization system has long been a mainstay of mixers and guitar and bass amplifiers. Even minor adjustments to an amp’s bass, midrange, and treble controls go a long way in shaping your tone. These EQ controls, sometimes called Low, MID, and HI on mixers, are very common, intuitive and effective. However, in a dense band mix, some extra flexibility may be needed to dial in your amps tone or fit each player in their spot for a cleaner mix. Many guitarists can quickly address these issues by engaging an equalizer pedal or toggling to a different pickup. However, using and understanding an amp or mixer with a mid sweep can open new doors of tone shaping without having to add any additional components to your setup.
A single mid sweep control actually contains two controls: one that chooses the frequency, and another that sets the level of how much that frequency is boosted or cut. This allows for a much wider range of tonal adjustment and offers a lot of versatility in shaping the characteristics of your midrange frequencies. Compared to a standard three-band EQ, which has a fixed frequency for the midrange control, an amplifier with a mid sweep control like the Carvin Audio BX700 or C1648 console lets you choose exactly what part of the midrange you want to control. An overly boomy or zingy midrange can be instantly adjusted without any changes to the bass or treble controls. And considering that with an instrument like guitar or bass, where the midrange character and content will dictate both how that instrument sits in the mix and how the audience hears it, it is very beneficial to have extra control of this range.
Parametric EQ: What?
The sweep mid control is a simplified version of a parametric EQ control called a semi- parametric or quasi-parametric EQ. A parametric EQ control has three controls - frequency, gain (or level were you boost and cut), and the band width (also called “Q”). The new control here is bandwidth. This is how wide of a range the control affects around the selected frequency on the frequency control. This could get very detailed, but this is why some mid sweeps sound different than others. In a mid sweep the bandwidth is fixed internally to one setting. So, depending on how narrow or wide it is set changes how it sounds and how it is used. This simplifies and targets the control to the intention of the product and lets you dial in a quick frequency to boost or cut. An example would be the mid sweep on an acoustic amp, which might have its bandwidth set more narrow affecting less of a range. This lets you cut a frequency that is feeding back without cutting a lot of tone. On a bass amp the mid sweep bandwidth might be wider to sound more musical when boosted. A boosted narrow mid sweep will sound peaked like a wah-wah pedal held in one position. A wah-wah pedal is basically a parametric EQ with fixed level (usually set to unity) and bandwidth (usually set narrow) and you are turning the frequency up and down with the pedal.
Dialing it in: Finding the Sweet Spot
While using a mid sweep control may seem complicated at first, it’s really quite simple. Let’s use Carvin Audio’s AG300 acoustic guitar amplifier as an example. This is very similar to a mid sweep on a mixer channel.
Outlined in red on this panel image are the two mid sweeps. The mid frequency, labeled “FREQ” is selectable from 100Hz to 2kHz, with 1kHz being the position at noon. 100Hz controls a lot of the girth of the sound; 1Khz controls much of your midrange attack, and 2kHz adds upper harmonics (for more info about the characteristics of different frequencies, check out our previous article here). In addition, all frequency points in between can be boost and cut, so your midrange content can really be custom tailored to achieve the sound that you want. The AG300’s mid sweep is still musical so it can be boosted or cut and to cut feedback you usually need only a little turn of the mid knob to lower the troubling frequency.
The quick way to knock out a troubling feedback frequency is to turn the mid sweep level control to max cut and then turn the frequency knob until the feedback stops. Then go back to the level knob and bring it up out of cutting to just before the feedback starts again. This will cut the feedback and not as much of your tone. The next time you can cut it to what worked before and just select the frequency.
Adjusting a mid sweep may take some getting used to and will take some time of experimentation to get a feel for the knobs. The benefit of the additional midrange control is worth it!
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