Music - studio and stage notes - technical side

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Analog audio stuff

Voltages and impedances

Many analog audio levels follow fairly strong conventions, but few are well standardized, and some have changed over time, some quietly so.

So assume a lot of these figures can be reasonably a factor of two off, and this mostly requires some knob twiddling and is fine.

CONSUMER	voltage	output impedance (of sound source)	input impedance (e.g. mixer, amp)	notes
consumer line level unbalanced	up to ~0.3V ("-10dBV")	roughly 100 Ohm (old equipment sometimes ten times that)	1k to 10kOhm (verify) (some cases up to 20..50kOhm, old equipment even higher)	Impedances were never very standardized. Voltages seems to have increased a little over time, from maybe half that(verify). Some modern devices (DVD, Bluray) allow 1 or 2V but this is its own thing. Keep in mind that consumer out is not the same as headphone out.
consumer mic (3.5mm mono TS) unbalanced	~10mV for electrets after FET buffer (before is <1mV but you often can't even get at that)	1-2kOhm, some higher (10k+(verify))	1 .. 10kOhm	e.g. for cheap electrets and PC mic in. The variation leads to some suboptimal combinations. Sockets that are 3.5mm and intended to take microphones will typically provide plug-in power and many (but not all) mics want it. (technically you could do both line-level input and mic input on one socket with some extra switches, but separate plugs is easier to explain to people)
headphone unbalanced	varies, maybe up to 2V	varies, may be a few Ohms	varies, order of a few dozen ohms	usually aim to drive at least a few milliAmps into a ~30-60Ohm headphone. There isn't a lot of distinction between consumer and pro headphone.
phono (record player) unbalanced	~2.5mV for Moving Magnet (MM) pickups ~0.2mV for Moving Coil (MC)	varies, but 500 ohm or lower	47k Ohm	typically on RCA plugs. Older amplifiers have a 'phono' in meaning an integrated pre-amp with this input impedance. Newer vinyl people may need an external one.
STAGE	voltage	output impedance (of sound source)	input impedance (e.g. mixer, amp)	notes
professional mic (XLR) balanced	up to ~1.2V ("+4dBu"), though most mics output something 1mV and not often as much as 200mV only XLR used as interconnects might see anything near 1.2V	most are in the 50..200 Ohm range (with some deviations)	order of 1..2kOhm	note that XLR as an interconnect is nominally the same as pro line but does not have the same leeway:
professional line (6.35mm TRS, or XLR) balanced	"+4dBu" as an aim but input stages often have roughly 20dB of headroom above that(verify)	order of 50..150Ohm	order of 10-20kOhm	Seems to exist as a smallish-but-sturdy interconnect between rack gear, broadly compatible with XLR in that ~1V levels are valid in both, BUT line level input stages may have a lot of headroom above +4dBu (and allow ~10V or so)(verify) actual mics's level is practically almost always lower than +4dBu (maybe order of 100mV)
guitar unbalanced	may be 200mV, often weaker, or somewhat stronger for humbucker	easily 10 kΩ to 100 kΩ (and frequency-varying)	order of 1MOhm	guitar pedals work at similar levels and impedances
"instrument"	a mess. See below.
LESS COMMON	voltage	output impedance (of sound source)	input impedance (e.g. mixer, amp)	notes
mic on grosstuchel, kleintuchel balanced	similar to XLR mics	same as XLR, or somewhat higher
old consumer mic (6.35mm mono TS)	electret (same as newer), or dynamic	often higher impedance (a few kOhm)
car audio	may be 1 or 2V, sometimes higher

Notes:

• "+4dBu" and "-10dBV" are in quotes because these nay be best treated as names -- see also Music_-_studio_and_stage_notes_-_technical_side#On_+4dBu_and_-10dBV

• between consumer and pro, impedances have broadly converged over time, and voltages are not too dissimilar - it is primarily the balancing that makes them not directly compatible(verify)

"Instrument level"

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Guitar levels

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Guitars are distinct from roughly everything else around.

(It's arguable whether you should even consider them part of 'instrument level' - they are at best a specific flavour of it - it depends on who you ask.)

A passive guitar's

output impedance is easily 10 kΩ to 50 kΩ (due to it being a coil)

and increasing with frequency (due to being a coil)

which will not work great on typical audio inputs because it's the same, even more, so won't be impedance bridging -- the result is a very thin sound

output voltage may be low to moderate (...from a +4dBu perspective)

(see also notes below on pickup impedance)

The input impedance of what you want to plug that into is often on the megaohm range, sometimes more,

which is why guitar cabs are good primarily for guitars.

Notes:

• that high an impedance is generally bad at carrying something very far (so)
• You can go to a mixer instead of a guitar cab, but only via a direct box (a.k.a. DI), not only because XLR carries it further,

but also to change the impedance and the voltage.

(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 go to both cab and mixer- though in many music styles people prefer to mic their cabs)

a few guitar amps have a built-in DI, mostly to save on parts to haul

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

See also:

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

On +4dBu and -10dBV

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More on headphones

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Connectors on stage (mixers and instruments)

XLR3 can almost always be assumed to be pro mic level, balanced/differential, and 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

(in practice, though, two balanced 6.35mm TRS instead may what you actually use. Same electrical interconnect, but there instead of XLR when space matters)

6.35 mm is two or three different things:

6.35mm TS is unbalanced, mono, often instruments and often called instrument level.

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 TS is sometimes guitar

Ideally you keep this separate in all senses -- guitar cables are different -- but it'll broadly work if you mix them

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.

Pinouts

TS:

• T: signal
• S: common

XLR

1: shield

2: +/hot

3: -/cold

TRS

T: +/hot

R: -/cold

S: Ground

The cables between the connectors

Broad cable types in audio

If you cut cables open, you will find a handful of different variations, mostly:

· Braided-shield with two wires inside (outer isolation removed),
· wrapped-shield with two wires inside (outer isolation removed),
· individually (wrapped) shields,
· individual unshielded (here 3-wire),
· let's pretend that's guitar and not regular coax
·speaker/lamp cord

"Can I safely connect X to Y (with sufficient adapters as necessary)?"

Insert tricks

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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.