Music - studio and stage notes

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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

older 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)

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

inputs marked 'phono' mean that pre-amp is integrated

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

the unamplified output is confusable with consumer line level on the same RCA plugs

avoid connecting non-phono and in particular line-level outputs to phono inputs, it'd be distorted at best and you may manage to blow the preamp

plugging phono directly into line-level (or mic level) input will be very quiet (or if you manage to amplify it enough, very noisy)

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 pics put on that are 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)

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 ~1.2V typical. See notes anove.

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

Instrument level

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)

guitars could be considered a their own, more varied variant of instrument level

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)

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

Practicalities on stage / in studios

Plugs and practicality, on mixers and instruments

XLR3 is pro mic level, which is always balanced/differential, always mono.

mono, because one signal requires a differential pair (and the third pin is shield, not ground)

if you want to carry stereo over XLR, use two cables (depending on the devices you're interconnecting, this may be easier with two balanced 6.35mm TRS instead).

6.35 mm is two different things:

6.35mm TS is unbalanced, mono, often instruments and often called instrument level. Guitar plugs are the same thing, though there is a difference between levels put out by guitars and instruments)

Effect pedals are typically unbalanced, instrument/guitar level

Tip is signal, Sleeve is ground/shield

unpowered instruments may have rather lower voltage levels - but still close enough to gain without much trouble

powered instruments may be somewhere inbetween(verify)

6.35mm TRS is pro line level

(typically) balanced/differential ~1.2V mono, and mixes may mark this as "balanced".

Tip and Ring is the pair, Sleeve is shield (not shared ground)

this seems mostly used for interconnects

(rarely, and recognizably) an insert Y lead to two TS plugs, to put an effect on a mixer insert socket

(rarely) unbalanced stereo. To see this on a device is an exception, and will be noted

Mixers tend to accept both TRS balanced and TS unbalanced, because it's not very hard to design them that way - and avoids some weird cases.

If they do both on the same socket they usually mark that (e.g. "bal/unbal")

Note that unbalanced inputs are not always isolated, so connecting unbalanced things (other than floating instuments) could create common mode issues.

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.

DI

✎ 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.

DI box, DI, DI unit, Direct box. (some people argue over whether it stands for direct input, direct injection, direct induction, or direct interface. Most people don't care.).

Functionally

Takes a high-impedance, unbalanced signal (often pro line level 6.35mm TS).

Outputs a low-impedance pro-mic-level balanced signal (often pro mic level XLR).

Passive DIs do so with a single audio transformer, so need no power, which makes them a no-worry functional addition.

In the cases where passive DIs run into issues (undue sound coloring, noise), you want active DIs to fix that, but they do take power (battery, adapter, and/or phantom power) so take a little more thought.

They are necessary for passive, high-output-impedance pickups (which mostly describes electric guitar pickups (magnetic), and contact mics if you use them) to carry more than a few meters without picking up a lot of noise and/or coloring the sound.

When your instrument is a low-output-impedance, active instrument, this may not be necssary. It may still be practicall in that stage snakes tend to take XLR only.

it might carry far enough without, though you might still like a DI to have everything be XLR, and to avoid ground-related issues.