an on/off switch to turn on the effect (as with lower- to mid-priced amps), or they may have one or more knobs to control how much compression is applied to the bass tone (typically a ratio and threshold knob or just a single knob). Some 2000s-era amps may have an electronic tuner and a mute button, to mute the sound of tuning during a break between songs without having to change the volume settings. On some amps, vertical sliders may be provided to control a graphic equalizer, which gives the bassist control over a number of frequency bands. Higher-cost amps for professionals with an XLR DI out jack may also have a "ground lift" switch (to be used in case of a humming ground loop), a DI out level control knob, and a switch which determines whether the DI out signal to the PA or recording mixing board is pre- or post- the amp's internal preamplifier and equalization circuitry. The pre-/post- switch enables a bassist to decide whether to send the audio engineer just the signal from her bass, or to send the signal once it has been pre-amped and equalized by her amp settings. Some higher-cost amps may have a parametric equalizer (or a semi-parametric equalizer) for some frequency ranges (typically the middle frequency range), which can be used to modify the bass tone to suit different styles or performance venues. Some bass amps have a 15 or 20 dB pad which can be used to attenuate "hot" signals, such as basses with an internal preamplifier (depending on the model of amplifier, some brands may provide two inputs (high and low gain) instead of providing a "pad". This pad can be turned on using a button. Some bass amps have an even stronger pad, a 40 dB pad. Some bass amps may have additional controls for onboard effects such as bass chorus or a knob for controlling a multi-effects unit (which might include a suboctave generator, chorus, reverb, fuzz bass etc.). Some 2000s-era amps may have a knob to control digital amp or speaker emulation settings (e.g., emulating the tone of a huge 8x10" speaker stack or a vintage tube amp by famous makers, such as the Ampeg SVT). Input and output jacks Bass amps come with a range of different input and output jacks, depending on the cost of the amplifier and its intended purpose. The least expensive