Music - studio and stage notes - technical side

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✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Analog audio stuff

Voltages and impedances

Many analog audio levels are not so well standardized, some have changed over time, some quietly. So assume each can be a factor of two off and can require a little knob twiddling at least.

phono

phono input

many vinyl players would expose the not-yet-amplified output of phono cartridges

typically on RCA plugs

unamplified output

order of a milliVolt -- There are two common types (and historically we have tried many more things)

Moving Magnet (MM) pickups give ~2.5mV,

Moving Coil (MC) give ~0.2mV

MC versus MM is one of those debates. Higher-end is usually MC, but quality also significantly depends on other factors.

cartridge output impedance: varies, order of 500 Ohm or lower (verify)

separate phono pre-amps will amplify this to (typically) consumer line level (and impedance)

phono amp input impedance is order of 47k Ohm (verify)

inputs marked 'phono' mean an integrated pre-amp

plugging vinyl players into a line-level (or mic level) input will be very quiet

...or, if you manage to amplify it enough, very noisy

plugging line outputs into a phono input is likely to be distorted

and you may manage to blow the pre-amp

consumer microphone level

consumer microphone level (mono, often on 3.5mm TS plugs)

on the order of ~10mV signal output (verify)

...because these are typically electret mics, and that's the order of what a 1cm electret (with FET) will output

devices with 3.5mm TS mic input will

very usually provide bias voltage for such electrets to function

apply a small-to-moderate amount of amplification to get the still-weaker level to the same level as line

impedance

mic output - high-impedance microphones are typically cheaper, e.g. the common electret mic is often 1-2kOhm but some 10kOhm+ (verify)

PC mic in impedance: 1..10kohm (varied over time and with cards)

technically you could do both line-level input and mic inpu ton one socket with some extra switches, but separate plugs is easier to explain to people

consumer line level

consumer line level (often stereo, often on RCA or 3.5mm TRS plugs)

on the order of 0.3V (~310mV RMS, ~440mV peak, 0.9V peak-to-peak)

also known as -10dBV (mostly in situations that also mention +4dBu pro levels)

but has varied somewhat over time.

I've seen amplifiers with a sensitivity of 250mV, older ones with 150mV

Some recent devices are moving to higher voltages - amps may choose to deal with up to 1V, or 2V in the case of DVD, Bluray(verify) (perhaps in imitation of pro line level?)

when the the other end is not aware, you may need to attenuate the output, and/or keep the amplification low, to avoid distortion.

impedance(verify)

line in impedance is often ~100 Ohm(verify). Possibly higher, up to 1 kOhm(verify)

line out impedance is often ~10 kOhm(verify). Possibly higher, up to 1 MOhm(verify)

Keep in mind this is not the same as headphone out.

Professional microphone level

professional microphone level

...that is, the mono balanced-audio thing often connected on XLR3 plugs (there are others choices)

The standard known as +4dBu means ~1.2V (RMS) sets the maximum you can expect to not distort when put on that wire

but the voltages that most mics output are typically considerably lower

a bunch are on the order of 10mV

some as little as ~1mV (not very typical)

some as much as ~200mV (not very typical; even 100mV is a bunch)

impedance:

mic output: most are in the 50..200 Ohm range (and usually higher in that range), with some deviations (see more notes around here)

mic preamp/mixer input: order of 1..2kOhm (impedance bridging setup to them)

Professional line level

professional line level (mono, often balanced and on 6.35mm TRS)

is +4dBu, so up to ~1.2V

...and in practice may bet something between ~200mV and 1.2V; this can be more flexible

outputs of interfaces, mixers, modern synths, and

various input stage are designed with headroom, meaning they can deal with a bunch more

Instrument level

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

instrument level

instruments will typically be unbalanced, often mono, often on 6.35mm TS

has no standard, though is generally in a predictable range, often being somewhere between pro mic and pro line level

...in part because it includes things like guitars (not amplified) and e.g. synths (amplified)

instrument output impedance: up to dozens of kiloOhms (if e.g. a passive guitar pickup), down to maybe half a kOhm (if active)

On paper this is also +4dBu, so up to ~1.2V, but this only sets a maximum.

And yes, if it comes from a powered instrument (e.g. a modern synth) it may be up that high, and need no gain. Enough hardware does this that you need to accept 1.2V

...but you can argue that the entire concept was for unpowered instruments (electric guitars, basses, and some vintage synths)

in which case assume this may be as low as 50mV, and needs ~20dB of gain

In fact, guitars can be considered a their own variant of instrument level, because of the impedance part.

guitar pickup impedance is often quite high (see notes below on pickup impedance), and their levels may be fairly low

so a direct box (a.k.a. DI) to make it XLR is a better and easy way to put it on a longer cable to a mixer

guitar amps expect high impedance from a directly connected pickup (typically closeby)

DI boxes tend to have a thru on the input side (which are two plugs wired together and typically entirely equivalent) so that you can both

the DI (towards the mixer) accept that high-impedance guitar

a stage-side amp (used as a monitor) accept that same high-impedance guitar

...and don't have to choose between the two

a few guitar amps may have a built-in DI

in general there can be good reasons to mic the cab instead of using an output (primarily the speaker's sound)

Other levels

Less standard / more varied:

• headphone level (on 3.5mm TRS)

roughly commercial line level, but less of a standard, and can easily be a little higher.

headphone amps tend to aim to drive at least a few milliAmps into a ~30-60Ohm headphone (verify)

there are ~4Ohm headphones, but you really woudn't plug those into everything (likely to distort, and also damage more standard amps)

there are 250Ohm-600Ohm headphones, but these need their own preamp (the idea is that you can design a specific amp with slightly better THD because you put less load on that amp)

Car audio tends to be on the order of 2V, sometimes 4V (verify)

(a headphone amp is sometimes a good cheat to connect consumer-level things to this)

consumer speaker wires

not really about voltage - they will be proportional to the amplifier's/speaker/s ability and related to the imedance, but for a rough idea, for ~100W speakers you'll see up to a few dozen volts

impedance

speaker load is often around 8 or 4 Ohm (sometimes 2, sometimes 16)

amplifier output impedance is typically very low, say 0.1 Ohm (this is also why the whole 'match your speaker impedance exactly to your amp impedance' thing is nonsense in a literal sense -- but with lower-impedance speakers you should limit how much you turn up the volume, because the maximum sensible power output happens earlier - and above that you get both distortion (THD increases with load) and risk of damage)

pro speaker wires

not really a thing. Most speakers are connected by one of:

XLR: carrying typical XLR line signals to active speakers

Speakon-connected: already-amplified signal to a passive speaker

6.35mm TS: already-amplified signal to a passive speaker (Sometimes avoided, to avoid smoky mistakes on mixers)

Note these cables are different from TS instrument cables, basically in that instrument cables use a thinner core-and-shield, while TS for speakers should be two beefier cores (and shielding is pretty irrelevant)

Speakon carrying already-amplified signal to a passive speaker - somewhat preffered to the previous because it's a one-purpose plug

See also:

• https://en.wikipedia.org/wiki/Line_level#Nominal_levels

Theory you may not care about

Semi-sorted impedance stuff

Other notes

On microphone impedance

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.