I am looking for a comprehensive explanation of the different audio signal levels, including their typical values in dBu and Volts RMS, their typical applications, what standards exist (if any), and what input and output impedances are expected in different applications.
Here is the start of an answer. I will update to make it more comprehensive when I get spare time. Please edit with any suggestions / corrections! It would be good to get more accurate ballpark ranges for each level and conversions into the appropriate dBu / dBv number.
In order of voltage from lowest to highest -
The level you get from most microphones. Usually connected with a balanced XLR cable to a desk with a preamp in it, which amplifies it to line level for mixing. Approx 1-50mV, depending on the sensitivity of the mic.
From the magnetic pickups in electric guitars and basses, as well as the output of some electric keyboards. Usually connected via a 6.35mm mono TS "jack" aka "guitar lead" to a guitar amplifier or DI. A DI converts the signal to balanced XLR at about the same level. The voltage is somewhere between mic level and line level. That is to say, there is no standard, and the outputs from different devices can vary significantly, and there is some overlap with mic level. A ball-park estimate is 70-150mV.
Instrument level is also characterised by a very high impedance, and therefore cannot be used to drive anything that requires current.
Some pickups (e.g piezo pickups) operate at such a low level and at such high impedance that they must be preamplified by a dedicated preamp before they can even be plugged into a DI or guitar amp!
Used to route audio signals between mixing devices, tape recorders, home cinema equipment, your Nintendo 64 etc. There are two loosely defined standards for line level:
- "Consumer" line level, around 750mV. Commonly associated with the red and white stereo RCA connectors, but also used with 3.5mm stereo jacks. Because of the use of jacks, line level is often confused with headphone level, but a line-out cannot be used to drive headphones as it cannot provide enough current.
- "Professional" line level, around 1-1.5V. Connected by a variety of connectors including 6.35mm mono TS Jack, balanced XLR, stereo or mono RCA. Commonly used in studio recording setups to interconnect mixing desks with effects, compressors, tape recorders, etc.
Usually in the same voltage range as line level, but able to drive much lower load impedances i.e it can provide much more current. While it is not possible to drive headphones with a line-level output, it is possible to safely drive a line-level device with a headphone output (your iPhone does it regularly!)
This is the output of a power amplifier, and there is no limit to how high the voltages can get. The output should also be able to provide a large amount of current (and therefore drive low impedance speakers). Usually presented as bare positive and negative speaker terminals which are screwed or clamped onto bare speaker wires.
Just wanted to add a little more about typical values for signal level and impedance. Take all of this with a grain of salt because these things vary widely between different pieces of equipment. These are all just ballpark ranges.
These standards have changed over time too. For example, it used to be that most professional line level gear had an input and output impedance of 600Ω. Modern line level gear usually has a very low output impedance and a high input impedance for bridging instead of impedance matching. This makes it easier to drive line level inputs since you don't need as much current.
Typical signal flow in a studio goes from a microphone (mic level) to a mic preamp (up to pro line level) to a mixer (also pro line level) and then to a power amp (speaker level). I'm skipping a lot in between but that's the basic idea of how this stuff flows. Everything in between happens at line level and that's where they do all their processing and whatnot. I'm neglecting anything digital right now since we're talking about analog signals.
Instrument level signals usually go through a hi-Z amplifier like a guitar or bass amp which consists of a preamp (instrument level > line level) and power amp (line level > speaker level) driving a speaker. Sometimes there are effects pedals in between that bring it up to roughly the range of consumer line level to intentionally clip the input of the amplifier (distortion/fuzz effects etc). In recording studios, the speakers of the guitar/bass/other amps are often mic'ed and then that mic level signal goes through everything I mentioned above.
Alternatively, a hi-Z instrument level signal can be run through a DI (aka direct box) which uses a 12:1 step down transformer to bring the signal down to roughly mic level in terms of signal level and impedance. Then it can be treated like a mic level signal and run through a preamp and so on.
In a consumer setup, your sources are usually consumer line level (game console, TV output, smartphone headphone jack, etc) which you bring up to speaker level through your receiver or power amp etc.
You have to be careful about mixing consumer and pro line level since they are not directly compatible. You might end up clipping a consumer device's input if you give it pro line level, or you'll have a weak signal if you do the reverse. That's why people buy things like this. It's also not difficult to build an interface like that. To go from consumer to pro line level, all you need is an amp with about 12dB of gain (high input impedance and low output impedance of course). To go from pro to consumer, you need about -12dB of gain which you can easily get from a 10k:600Ω transformer (4.1:1 turns ratio). [Don't try this backwards to go from consumer to pro, you will get tons of noise and loss due to the step up in impedance]. Impedance isn't usually a huge concern here since most line level gear has very low output impedance and high input impedance.
A lot of companies make stuff that's considered "prosumer" now which has a nominal level of roughly 0dBu (.775V RMS) or so, in between pro and consumer line level. That stuff varies a lot as well and it's designed for people who want to make decent recordings at home or on the go. Those people might have a mix of pro and consumer line level devices which is why prosumer offers them some flexibility.
Some people get really hung up on the connector and think that if they see an XLR, that automatically means the signal is mic level. That is not the case. Your connector is just a couple pieces of metal in a convenient shape. It tells you nothing about the signal level or impedance really. That being said, it would be very unusual to see an ELCO connector hanging off a microphone for example. Even though signal would pass, not many mic preamps have those connectors so it's usually standard to see an XLR. But an XLR could also carry a line level signal or anything else you solder to it within certain limits. XLR is just an example, the point is don't make any assumptions based on a connector alone.
Mic, pro line, and sometimes prosumer line level devices are often impedance balanced and differential which is why their cables (XLR/TRS/DB25/ELCO) usually have 2 conductors and a shield for each channel.
Instrument, consumer line, speaker, and sometimes prosumer line level are usually unbalanced and single-ended.
But balancing is a whole different discussion.
There are plenty of exceptions to all this. Just to name a few:
- Hi-Z microphones- usually similar to instrument level and often use a TS plug. Plug straight into guitar amps. Common for blues harmonica and stuff.
- There are balanced electric guitars out there but they're uncommon
- Even a pro line level device is often single-ended internally
- Transformer coupled inputs/outputs can be operated in a balanced or unbalanced configuration. It's not uncommon to see a 1/4" jack that accepts either TS or TRS for example (other circuits can do that too).
- Some guitars/basses have active pickups giving them an output roughly closer to 1V or so with much lower impedance. They don't necessarily play nice with some effects that expect a higher source impedance (e.g. Fuzz Face)
- Even some pro line level devices put out a little less than +4dBu and clip before +24dBu
- Some (cheaper) prosumer devices use a resistor from the ring of the TRS (pin 3 of the XLR) to ground to "balance" the output. That's impedance balanced (so you still get the noise rejection) but single-ended. Don't assume anything that says "balanced" is also differential, especially in the prosumer world.
- Speaker level varies very widely. A concert venue might be pushing 100s of watts. Nominal level is really just for test signals.
Hope this helps!