What is a Bass Preamp?
A preamp is just what it sounds like it’s the part of the bass amp that comes before the amplification section.
A bass preamp, or bass pre, only slightly amplifies your bass signal. The signal from your bass is pretty weak. Try plugging in a pair of headphones directly into your bass’ output jack and you will barely hear it. The preamp will raise the input signal to line level. Line level is an optimal audio signal level used in electronics. It basically makes the signal easier to work with in the electronic realm.
After boosting your bass signal to line level, the next thing the preamp does is shape the tonal characteristics of your bass signal with signal processors. There are numerous ways this is done depending on the bass preamp and its features.
Your bass signal travels through various stages as it passes through all of the electronic circuitry of the bass preamp. Next we’ll discuss the various bass preamp stages and features you may come across and what they do.
Bass Preamp Inputs
The first thing you will do is plug your bass’ output to the input of the preamp with an instrument cable. You may see a single input, a single input with a pad switch, or two different inputs.
If your bass is active (see passive and active basses), it’s output is louder, or hotter, than a passive bass. In that case, the bass signal might be too loud and often needs to be attenuated (lessened). If you have an active bass and a single input with a pad switch, set the switch to attenuate the signal (especially if it is distorting). It may say "-10dB" pad or something.
If you have two inputs marked passive and active, plug your active bass into the active input, not the passive input. Otherwise your signal may be too loud and could clip, or distort. It’s not the good kind of distortion, either.
If you have only one input with no switch, you have no decisions to make!
The next knob of your bass preamp will usually be a gain knob. Gain is another way of saying level. Think of the gain knob like an internal volume knob. The following stages might respond differently to different amounts of gain. For instance, turning the gain way up might produce a pleasant type of distortion like you hear in rock. The following preamp stages would then tweak and alter that distorted signal.
The next stage of the bass preamp is commonly the equalizer stage, or EQ. EQ allows you to emphasize or de-emphasize particular frequencies. First, let’s make sure you have a basic understanding of frequencies.
Frequency is how often (frequently) a sound wave vibrates. Frequency is measured in Hertz (Hz) which means cycles per second. The low E of a 4-string bass rings at 41Hz. That means 41 vibration cycles each second. The slower the vibration, the lower the pitch. Human hearing ranges from 20Hz to 20,000Hz. You may see kHz meaning kilohertz, or 1000Hz.
At first you might think when you play a note on your bass it is only one specific frequency ringing out, but there are other frequencies, too. The main frequency (most of what you hear) is called the fundamental. For instance, that low E fundamental rings at 41 Hz. But, other frequencies ring out at the same time as well. These other, subtler frequencies are called overtones. Overtones are what give each instrument it own unique tone. You could play the same note on bass as you could a pipe organ, but they’d sound different. This is due to the unique set of overtones each instrument produces on top of the fundamental tone.
EQ allows you to emphasize or de-emphasize certain frequencies and alter the tonal characteristics of your bass tone. You can increase, or boost, a frequency. Or, you can decrease, or cut, a frequency.
Many times the frequencies of your bass preamp’s EQ section are predetermined. You might just have a bass, mid, and treble knob each set at a certain frequency. Other times you may have an adjustable, or sweepable, frequency range. This allows you to select from a range of frequencies to cut or boost. You might then have a bass, sweepable mid, and treble on your preamp. The more EQ knobs you have, the greater control you have. Most preamps have 3, some have 5, and others even more.
Parametric EQ means each knob (usually bass, mid, treble) has another knob or switch for selecting the specific frequency.
A graphic EQ is a little more visual having up and down sliders for each frequency. Graphic EQs take up more space on the faceplate and are less common.
You will need to experiment with your EQ settings to find what you like and what sounds good with your instrument and the mix of other instruments on stage. You may discover what sounds good by itself, doesn’t sound good when combined with your guitar player’s tone or the room acoustics.
Bass Boost and Brightness
You may come across a few other knobs and switches which tweak the EQ in various frequency ranges. These might be bass boost or brightness. The former boosts a low frequency, and the latter boosts a higher frequency.
continues on bass preamps part 2...