The Fishman Platinum PRO EQ integrates a sophisticated analog preamp with a fully featured DI box. Its pre/post EQ switch lets you choose whether you send a straight, uncolored low-Z signal to the board or one that you’ve tweaked using the unit’s extensive onboard EQ controls. Photo by Mark Marshall
The Great Equalizer(s)
Because the guitar is a midrange-rich instrument, it makes sense that we want to adjust the frequencies in this region. But that’s not all: An active DI may have a high-pass filter (sometimes referred to as a low-cut filter) that only allows frequencies above a specified point and eliminates those below it. For example, if you set the low-cut to 120 Hz, it will remove frequencies below this threshold. How aggressive the filter is at removing those frequencies is typically preset on DI preamps.
There are a couple of good reasons to use a low-cut filter. For one, it can reduce feedback. Even though the acoustic guitar has a prominent midrange, it doesn’t mean there aren’t sub-frequencies happening. There could be a party going on down there that you’re unaware of until you plug into a PA. Rowdy low-end frequencies can cause feedback or muddy your sound. Depending on the guitar and the pickup you’re using, bloated low end can mask the rest of your tone. That’s where a high-pass filter comes in handy. Used properly, it won’t thin out your instrument’s voice, but simply unmask it.
You’ll find a variety of EQ-shaping tools on active DI devices. The flexible 5-band EQ system on the Fishman Platinum PRO EQ ($299 street) consists of six knobs: low cut, bass, a sweepable midrange with boost or cut, treble, and brilliance. Another popular active DI, the L.R. Baggs Venue DI ($299 street), has a 5-band EQ that includes bass, presence, and treble knobs, as well as tunable low-mid (100 Hz–500 Hz) and high-mid (500 Hz–2.5 kHz) controls that give you the ability to boost or cut frequencies in these regions.
For many players, a 5-band EQ with one or two sweepable midrange bands provides ample tone-sculpting power. But if you need more midrange control, some devices go further. For example, Grace Design’s ALiX ($625 street), which has its roots in high-end studio equipment, boasts fully parametric midrange controls. A parametric tone control lets you adjust the frequency, gain, and Q (the amount of surrounding frequencies that also get cut or boosted). It also has a low-pass filter (aka high-cut filter). This comes in handy if you need to put a ceiling on the upper frequency content coming from your instrument. I find this to be the case with resonator guitars, which can be piercing when plugged in, depending on your pickup system. A low-pass filter helps to control that harshness, and it’s the first adjustment I make when plugging in my National resonator.
As anyone who has plugged in onstage can attest, feedback is the enemy of an acoustic-electric guitarist performing at a live venue. Active DI boxes offer several nifty tools for dealing with feedback.
One is a phase inversion switch. This reverses the polarity—the direction of the waveform—of your guitar signal. Flipping this switch can help reduce booming feedback by “changing its relationship to the sound coming from the amplifier,” as described in Fishman’s Platinum PRO EQ manual. The manual goes on to say, “One phase setting usually provides better resistance to feedback than the other and will vary depending on the instrument and playing environment.” So the trick is to experiment by flipping back and forth to see what position delivers better feedback reduction. A phase switch may sometimes—although not always—drastically affect your tone. So before I reach for EQ knobs, I’ll start with the phase switch. Once I find the right tonal character, I’ll move on to adjusting EQ.
Among its many features, the L.R. Baggs Venue DI offers a flexible EQ with two bands of sweepable midrange boost or cut, an adjustable gain knob with a 4-segment clipping meter, and an FX loop. Photo by Mark Marshall
Another feedback buster is the notch filter, which provides a very deep, narrow cut at a specified frequency that’s prone to feedback on most acoustic-electric guitars. Here’s how L.R. Baggs describes their Garrett Null anti-feedback notch filter, which provides a -21 dB cut at 1/8 octave, in the Venue DI manual: “Sonically it is virtually invisible and it can be used to sweep the primary feedback range of an acoustic guitar (60 Hz–320 Hz) to cut out a frequency that is causing feedback or ringing.” The trick to using a notch filter is to get your guitar to start feeding back at soundcheck. Then, as it’s howling, turn the notch filter knob until the feedback disappears.