Amplification signal path for bass

Most performance technology involves amplification. Here is an overview to signal path –how sound goes from your mind to someone else’s mind by way of air, electrons, and moving speaker parts — for electrifying your bass.

Definition of the major parts of a bass amplification rig:

Amp: typically this means “power amp” and it is the thing that is responsible for making your speakers loud. It will have several numbers associated with it. Wattage (500 watts, for example) and ohms (2ohm or 4ohm or 8ohm). The ohms won’t be as easy to find as the wattage number, ohms are usually listed near the speaker out jack. Lower ohm number gives you greater flexibility in choosing cabinets.

Pre-amp: This is an amplifier that brings your bass signal up high enough so that the power amp (see “Amp” above) can see it. This device often includes an EQ and sometimes other fun knobs and lights.

Cabinet/Cab/Cabs: A box that has speakers in it and nothing else. This box will contain some collection of speakers: 4 10″ speakers, 1 15″ speaker, 1 12″ speaker and so on. Often bassists will use a shorthand when describing a cab: 410 or 4×10 means 4 10″ speakers, 115 or 1×15 means 1 15″ speaker. Cabs will have maximum power ratings (750watts for example) and an ohm rating (4 ohms, for example).

The above three items make up the important parts of your amplification system. They can be combined like so:

Amp+Pre-amp+Cab all in one box is a combo (a combination of these things, or all of these things combined into one box). Nice things about a combo are that they are relatively easy to move around and all the ohm/watts/speakers electrical mumbo jumbo has been worked out for you. You will likely want a combo as your first system because it will be easy to deal with. The downside of a combo is that you are stuck with the amp/pre-amp/cab combination. But if you buy a combo you like then this isn’t an issue.

If the amp and pre-amp are in one box together this is called a head. These can be put in one box by the manufacture (like the old Bassman heads). In this situation you plug your bass in one end and then there’s a speaker out that gets plugged into the cab. Or you can select your own pre-amp and power amp and put them in a box yourself. The difference between these two approaches is that the one you do yourself gives you more flexibility while the one that the manufacturer does prevents you from making errors in electrical engineering. Also, remember that you need a cab if you have a head–the head doesn’t have any speakers.

A stack is when you have more than one cab and a head. For example, I have an Eden 410 and an Eden 118 and I drive them both with a Glockenklang head in which Uwe has put both the pre-amp and power amp in the same box. This is what we might call “a righteous stack.” It’s the sort of thing that is best suited for an arena stage and it’s absolutely ridiculous that I own and play through this.

You may also hear about an onboard preamp for electric bass. Unlike the pre-amp mentioned above, the onboard pre is inside the bass and requires a battery. Sometimes this is referred to as active electronics. Sometimes it can replace the pre-amp mentioned above but usually it can’t. It’s simply an additional tone-sculpting tool with knobs on your bass instead of back at the amp.

You may also hear about a DI or DI box or Direct-Injection. Many of the pre-amps mentioned at the beginning of this post have a “DI out” and there are also specialized DI boxes. You use these if you want to plug straight into a PA system. Your bass guitar on it’s own may not have enough oomph to plug right into a mixing board. DI takes care of that. Sometimes (but not very often, more common in acoustic focused settings) a venue has a PA and the band plugs directly into it and plays. In an instance like that you may just want a DI box and then you don’t need an amp or speakers or any of this. But I bet you’ll want a combo really.

DI boxes are fairly common for upright players and often have special circuitry to deal with piezo pickups (a whole different topic I’ll get to in another post).

Signal path for Bass

This is the complete signal path for playing bass. Every one of these steps is capable of changing the way your tone works, some more than others. The places where the sound is transformed will have tremendous impact on the sound–these are the borders where the sound will lose things or gain things.

  1. Your mind. The origination of the sound is in your mind and is your intent to make that sound. Here the sound is transformed from not existing to existing (albeit only in your mind).
  2. Your fingers. The intent of sound-making goes to your fingers which have to move and touch in special ways to generate the sound. The sound is transformed from your intent into a biological/mechanical action.
  3. The strings. The making-sound intent passes from your body and makes the strings vibrate. The sound is transformed from your biological/mechanical actions to a vibrating mechanical sound.
  4. The pickups transform the mechanical vibration of the string into an electrical signal using either magnets (as in an electric bass or Pierre Josefs pickup) or pressure (as in a piezo element). The signal is now in the electrical world.
  5. The electronics. Whatever is connected to the tone/volume knobs  associated with the pickup filters the signal.
  6. Instrument cable. The electrical signal leaves the bass via the instrument cable and heads toward the amplifier.
  7. Pedals (optional). The signal goes into pedals and is further filtered and adjusted. They will leave via another instrument cable.
  8. Pre-amp. The signal enters the pre-amp is raised to a level that the power amp can see it later. There may be additional tone filtering options here before the signal is sent along. The signal may also be split here with one going to the power amp and the other going to a DI. We’ll follow the power amp in this example.
  9. Pre-amp cable. This is just another instrument cable that connects the pre-amp “out” to the power amp “in.” If you have a combo or a manufactured head then this cable is likely inside the box and you don’t have to think about it.
  10. Power amp. The electrical signal arrives here and is made very very loud.
  11. Speaker cable. This looks like an instrument cable but it isn’t. If you look at it, it should say “speaker cable” on it somewhere. It needs to be a little beefier because the power amp made the signal beefier.* This connects the power amp “out” to the cab “in.” *Gross oversimplification.
  12. Cab. The electrical signal passes into the speaker (if there is no “in” jack don’t worry, they are all “in” just don’t plug more than one power amp into the cab). Here, the electrical signal is transformed into mechanical sound again–the speaker compressing and rarifying air in front of it.
  13. The air. The mechanical sound of the speaker travels through the room, bouncing off hard surfaces and being absorbed by soft surfaces.
  14. A listener’s ear. The mechanical sound that travelled through the air and arrived at a listener is transformed into a neural signal by the eardrum. The way a listener’s ears work in conjunction with their mind will determine how they perceive the sound you make. If grand-dad is hard of hearing for example. Or if little Joey hates all that noisy “new music,” for example. Both of these will determine how they experience your sound (the first is more about the function of ears, the second more about the function of mind–it is in our best interests to help audiences cultivate both).

The final step in the chain is the most important one and, strangely, the one that may be least susceptible to any technological or musical adjustment we might make. But there it is anyway.

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