Bass Guitar Amplifier Circuit Diagram

Preamps are one of the main factors that distinguish amplifiers from one another. By altering and strengthening guitar’s input signal before sending it to power amp, preamps add depth and texture to sound of amplifiers.

Some people like to include a valve (‘tube’) in the preamp stage for aesthetic reasons, yet this adds nothing that cannot be accomplished using modern transistors, JFETs and MOSFETs with appropriate opamps. While valves might add extra noise and unreliability they also contribute extra reliability.


A preamp is the starting point in any bass guitar amplifier circuit diagram, transforming mechanical energy from string vibration into electrical signals which can then be fed down long cables to other electronics like power amplifiers. Furthermore, it ensures that these signals don’t get too hot, which would cause clipping and result in losing any subtleties which make your sound unique.

Preamp input signals come from a 1/4″ audio jack which provides mono audio signals without distinguishing between left and right channels. It connects directly to a resistor at its tip while its sleeve connects directly to an inductor for amplitude-control of signal; additionally a simple variable resistor known as the volume knob allows you to regulate strength of signal sent from preamp to power amplifier while another pot changes frequency by raising or lowering tone control pot accordingly; higher settings will reduce treble frequencies further and vice versa!

Bass guitars produce low frequencies, so it is crucial that their tone controls provide sufficient control of these low frequencies to prevent their treble from being overshadowed. You can customize these controls to achieve optimal balance for your playing style by using tone controls to adjust them as necessary.

Preamps typically feature gain controls to boost or attenuate input signals, and series or parallel speaker output jacks enabling you to choose how your preamp output reaches speakers. Parallel wiring connects each positive terminal of each speaker in parallel wiring mode – as does series wiring which links positive terminals together in series wiring mode.

Preamps are at the core of guitar amplifiers and represent an essential step in music creation. A well-made preamp will help you get the most out of your amplifier while offering you an expansive variety of tones suitable for every musical genre.

Power Amp

The power amp takes the relatively weak guitar signal from the preamp and transforms it into high enough current to drive a speaker motor and create loud noise for bass players. Care must be taken when designing this component to prevent distortion; maximum output depends on what the speaker can safely handle before melting down.

As is true for preamps, an amplifier requires a ground connection in order to electrically “ground” its circuit – this connection should connect directly with its chassis; on your bass amplifier this same line carries Overload LED that detects any instantaneous peak above 8V and lights up accordingly.

A typical amplifier connects the ground line to both a bypass capacitor (often labeled “CF”) and cathode resistor of an EL84 tube, creating a small DC voltage on its cathode that inhibits electron flow until a guitar signal hits the grid and swings it positive, such as what occurs with V1A Grids.

Signal from the grid passes through a 470 ohm cathode load resistor that is bypassed with another.02uF coupling capacitor rated for 600V+ before entering a 1 Megaohm Volume Pot that acts like a variable voltage divider (potentiometer), with its wiper marked by an arrow. Its top center pin connects to R13 Negative Feedback Resistor which sends part of Output Transformer secondary signal back through V1B cathode resistor R13 negative feedback resistor which sends part of Output Transformer secondary signal back through to lower overall gain and eliminate buzzing.

Some people still appreciate the sound of amps using valves, but most will agree that modern transistor or opamp preamps provide just as much or more power and tone than their cheap valve predecessors in equal-sized enclosures. Valve power amps require large output transformers with at least four output valves per amp and their reliability is notoriously poor; for these reasons most bass amplifiers utilize solid-state power amps combined with speakers in one enclosure; this type of arrangement is known as a combo amplifier.


Bass guitar amplifiers must provide speakers with enough current so as to overcome their high impedance. Most bass amps incorporate a power and excursion limiter circuit (usually R2/R3) which limits maximum output to prevent damage to speakers or the amplifier itself; it should always be used as it serves an essential safety measure regardless of type or brand of amplifier used.

A speaker is essentially an electromagnet with a cone that moves in and out as its voice coil polarity changes, producing sound waves heard as bass, midrange, and treble frequencies. Some models utilize ferrite magnets while others may opt for lighter alternatives like neodymium which still produce sound waves with equal fidelity.

Circuit diagrams of bass amplifiers often display a bypass capacitor on their preamp tube cathodes to allow DC current through while still permitting AC current to flow freely, increasing gain by allowing more electrons from plate to cathode; an essential feature for bass guitar amps due to high input signal peaks.

In the Vox AC4 schematic, a 270k (2.7 million ohms) cathode resistor and 25mF bypass cap are connected in parallel between preamp and power tube cathodes for cathode resistance and electron flow through their respective cathodes, providing DC voltage. Because it also acts as a capacitor, bypass caps allow AC electrons to flow more easily than resistors would, giving rise to another form of DC blocking capacitor labeled as “DC blocking capacitor”.

The guitar signal passes through a phase inverter and two power tubes, each equipped with its own grid control system; output transformer primary windings are connected to both. A guitar signal on one grid affects current flow in another grid which then impacts speaker field coil magnetization as voice coil polarity shifts back and forth.


There are various methods of adding extra effects to a bass guitar amplifier. Adding a tweeter, for instance, can increase the range of frequencies that can be projected; variable EQ may also be included to allow fine-tuning of bridge pickup tone; output from preamp may even be split off and sent directly into stereo amp, giving two outputs.

Most bass amplifiers start off with a straightforward preamp circuit, as illustrated here on the right. Here, the guitar input signal is amplified through V1A triode tube acting like a current amplifier to increase its size for easier processing by subsequent stages of an amp.

Phase Inverters produce two mirror-image signals 180 degrees out of phase, which are fed to Power Tube grids for push-pull amplification. A guitar signal passing through tube plate induces low impedance signal in Output Transformer primary winding thereby driving speaker’s voice coil and cone.

Zener diodes installed at the preamp output protect the subsequent circuitry from transient voltage spikes, while also helping to stabilize output voltage when speakers are switched on or off.

People sometimes mistakenly believe that valve preamps add something special to a bass amp’s tone not possible with transistors or op-amps; however, this is often not true and many amps using valve preamps only use it as an embellishment or marketing gimmick.

A typical bass preamp typically incorporates a high-pass filter to filter out low frequencies that cannot be amplified and only consume power, typically having a 24dB/octave rolloff in its first stage of filtering; if this proves too much for the user, an 18dB/octave cap may be employed instead, with ultimate rolloff reduced by 18dB/octave rolloff. Finally, most preamps contain bass/treble controls with variable turnover frequencies so users can tune the output sound to suit themselves for personal taste or studio work.