Tech Bench - Mod Squad: Going Active

October 28, 2013
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Clockwise from top left: Sadowsky Onboard Preamp, Nordstrand 3B Preamp, Michael Pope Design Flex Core, Aguilar OBP-35K, EMG JX Preamp
ONE OF THE MOST COMMON MODS PERFORMED
on electric bass is the transformation from passive to active. It’s a relatively basic operation: Take one passive electric bass, add a preamp, shake and bake, and you’ve got an active bass. But there are potential pitfalls that could derail this simple plan, including some that might require an unexpected permanent change to your instrument. Taking into account that every preamp has its own character, it helps to have a sense of the variables before you mod. Obviously more gain and some form of active tone control are what you’ll get, but there are several types of active EQ circuit, and understanding how they work will help you choose one best-suited to your needs.

An important first step is to examine the instrument in question. For our discussion, let’s assume it’s currently a passive bass. It’s quite possible that a preamp install will require some routing, either for the battery compartment, to make room for the preamp itself, or both. Many passive instruments already have roomy control cavities, but with classic P- and J-type instruments, installing most preamps means surgery. If it’s not vintage, or has already been modified to the point of no return, cutting space for the preamp is not a big deal, but I highly recommend using a professional instead of DIY. For J-type basses, the Audere JZ models and John East’s J-Retro are two preamps designed to fit in the existing (and very small) control cavity, including the battery. The EMG JV and JVX are active pickups with electronics that will fit in a standard Jazz Bass control cavity as well, though adding one of their active EQ systems would require routing. John East also makes the P-Retro for surgery-free install on P-style instruments.

Beyond making it fit in your bass, you’ll have to decide what type of active EQ circuit to use. One popular breed is the bass/treble “boost only” preamp, like the Bartolini NTBT, the Aguilar OBP-1, and the Sadowsky Onboard Preamp. Another type of 2-band setup is boost/cut, with each tone control having a center detent that is considered flat—clockwise will boost, counter-clockwise cuts. A boost only design like the Aguilar OBP-1 gives you the entire pot rotation to spread out the frequency gain, in this case +18dB. But a boost/cut EQ like the Aguilar OBP-2 gives you half the rotation for the same 18dB boost, making it feel more sensitive. An added benefit is the 18dB of cut—a feature that can help you adjust to boomy environments, or roll off highs for a darker tone.

A common EQ configuration is the 3-band bass/ mid/treble setup, but there are several variations on this theme as well. Typically, 3-band EQs are boost/ cut, but the midrange control is where we see some differences. A basic version gives you three separate controls: bass, mid, and treble (all boost/cut), volume, and if there are two pickups, blend. It is common to see the bass and treble controls share space using a concentric pot with a knob for each frequency band. Many preamps offer several midrange choices; for example, the Hanson LH-3 has four user-selectable mid frequencies controlled by internal DIP switches. Another selectable midrange scheme uses a mini-toggle switch on the control panel to select two or three different frequencies, but it requires drilling a small hole in the bass, or control plate. Using a push/pull pot for the midrange switch is a drill-free solution. Another midrange option gives you a concentric pot with knobs for boost/cut and frequency sweep, as found on the EMG BQC or J-Retro preamps. With this semi-parametric setup, you can dial in a specific frequency to work with instead of presets. However, due to the nature of the control, it may be hard to find the same exact spot on the rotation every time.

As you can see, there are many ways to set up your EQ, and several manufacturers offer the same preamp in a number of different control layouts. A Michael Pope Design Flex Core preamp can be configured with an online tool that helps determine your preferences from a large number of possibilities—a useful and educational approach. Another thing to consider when installing a preamp is having the option to play in passive mode or not. Most preamps have passive bypass mode, sometimes activated by a mini-toggle switch, or more commonly with a push/pull pot— usually the volume control. It will prevent you frwom getting stranded when your battery dies during a set (although depending on the amount of gain you push in active mode, you may suffer a severe drop in volume). If you happen to like your instrument’s passive tone, rest easy—you’ll still have it. One drawback of some preamps is the absence of any tone control in passive mode, but many companies offer this feature as an add-on, like the Sadowsky VTC (vintage tone control), the J-Retro Deluxe, and Audere.

Beyond all the configurations and features available, you’ll want to look at the manufacturer’s frequency specs. Frequencies are expressed in Hertz (Hz) and kilohertz (kHz)—the break point between them is 1,000Hz, which can also be expressed as 1kHz. The fundamental frequencies of open strings on a 4-string bass are: E = 41.20Hz, A = 55Hz, D = 73.42Hz, and G = 98Hz. Of course, you detect much more than the fundamental of each note; many overtones combine to produce the tone you hear as your bass. By examining the specs, you can get a basic sense of how the preamp will affect your sound. For example, Nordstrand lists its preamp specs this way: The bass control is centered at 50Hz, and gives you 12dB of boost/cut; the mids are listed as ±12dB @ 1kHz, and the treble is set for ±12dB @ 4kHz. We can see that the lows are tuned fairly close to the fundamental of the E string, while 1kHz is a relatively high frequency for the mids that brings out note articulation, or scoops some clank for slap. The highs set at 4kHz will add a subtle sheen. These are fairly conservative and well-placed specs, with the intention of leaving the instrument’s character intact. The overall gain is adjustable for ±10dB via an internal trim pot, which helps you balance out the flat level with passive mode. But be careful—the more gain you add with the trim, the less headroom you’ll have at the amp input if you run a lot of EQ.

Now let’s look at Glockenklang’s onboard 3-band preamp: Lows are ±14dB @ 40Hz, mids are ±18dB @ 550Hz, and the highs give you ±18dB @ 18kHz. The overall gain is 0, making it the same level as passive when the EQ is flat. Examining the differences between the specs, we can surmise that the Glockenklang is going to provide more of a slightly deeper low-frequency boost, the mids will be thicker and chunkier, and the highs are tuned super bright. Ultimately, it’s up to you to determine what your tone goal is with a preamp; do you want your bass to sound the same, just louder? Perhaps the Nordy would be a good fit. Do you want more bottom and treble from your bass? The Glock has rump and high end for days. Of course, it’s not quite as simple as that; your tone is a combination of all the elements in the signal chain, but at least you can get a general idea of what you’re buying armed with this information.

One last thing to consider is whether you want 9 or 18 volts powering your preamp. You get more input headroom from 18-volt systems, which decreases distortion if you dig in; it also requires space for two 9-volt batteries. Many preamps can be wired for either, so you can experiment if you have the space available.

Active electronics can give your axe more versatility and volume, and can help you manage your sound in different acoustic environments. And, if you can install without routing, it’s a totally reversible operation. So, go get active!

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