Difference between “hi” and “low” on amps?

Question

What is the difference between "hi" and "low" input jacks on guitar / bass amps? Usually bigger amps have both of them (didn't see them on smaller ones really)..

Answer 1

The "Hi" input attenuates the input signal, usually by between 10-15dB (about half to a third the original volume). The "Low" input will not do this.

Different guitars and basses produce widely varying signal levels. This isn't just a passive vs active thing; plug a vintage-voiced Strat into the amp, then a PAF-voiced Les Paul; you'll have to turn the gain down. By contrast, the preamp of your amplifier works best with a signal in a particular range of strength; too hot, and you'll clip the preamp (which is generally not the best sound especially if you want a clean tone). Too low, and subtleties of tone become lost in the amp circuitry's "noise floor".

The different inputs, or the attenutation switch on a single input, allows you to "turn down" a hot guitar with a simple switch instead of involving other elements of the preamp. This has two benefits, both tied to being able to turn the other "downstream" controls to higher levels. First, taming a hot signal without an attenuator usually requires turning down the "Pre Gain" level. This knob, especially on a tube amp, will become very sensitive; small moves will produce huge volume changes. By attenuating the signal before this point in the circuit, the knob becomes less sensitive to small moves. Second, by allowing controls like the pre-gain to be turned higher, more of the original "native tones" of the instrument get into the preamp stage, and the tone and equalization controls have more to work with. Overall, this results in a "less muddy" sound, as subtle harmonics and overtones aren't lost in the noise floor of the amp.

Answer 2

Contrary to what the previous answers stated, the different sensitivity is not the primary difference between the "hi" and "lo" inputs. It is indeed different, usually by 6 dB, but you could achieve this with the gain knob alone.
More important is the different impedance of both inputs: a "hi" input has a very high inner electric resistance (in the range of 1 MΩ), that is, it accepts the voltage the guitar outputs while there needs to be hardly any current at all. The guitar pickups can then do their job (which is translating the string vibrations to electrical voltages) completely freely without having to also supply a lot of current; so they can do a lot of resonance, which gives a bright and open, but sometimes also annoying and "un-compact" sound.
A "lo" input, on the other hand, typically has an impedance of about 150 kΩ, roughly 1/7th of the hi input's. That means, when the pickups try to send their voltage to the amp there will always be a significant current the pickups also have to supply. Actually, it's still a very small current – something like fractions of microamperes – but guitar pickups are very weak sources of current. Voltage and current together means they need to push energy to the amp: this energy will be taken from their own high-frequency resonances, therefore a normal electric guitar will sound more mellow when plugged into the "lo" input than when it's plugged into the "hi" input. It will also be simply quiter because of the attenuation, but this, as I said, is less important.

Every pickup responds differently to the amp impedance: some will sound muffled and dull when plugged into a "lo" input, others shrill and disturbing with a "hi" input. So when you're using two different guitars live, it can sometimes be useful to use each input for one of them (you should still always unplug the one you're not playing from the amp).

Answer 3

On my Ampeg amp, the instructions said that if you are playing a bass with passive pickup(s), to plug into the 0db jack; and if your bass has active pickups, to use the -15db jack.

Answer 4

They assume different signal levels so have different levels of gain in the pre-amp. In fact on some amps the low input has a pre-amp but the high just goes directly into the main amp.

This is to cope with the fact that instruments can have very different output signal levels, but it can also be used to change the tone of the sound produced.