Electronics project notes/Device voltage and impedance, audio and otherwise
Contents
- 1 Theory: Impedance when connecting two things
- 2 Impedance mismatches
- 3 Audio
- 3.1 Analog audio voltage levels
- 3.2 dBV and dBu
- 3.3 Audio device differences
- 3.4 Balanced audio / pro audio
- 3.5 Plugs's relation to balanced/unbalanced, voltage levels, etc.
- 3.6 On pickup impedance
- 3.7 Things that are't pure bridging
- 3.8 Other notes
- 3.9 DI
- 3.10 Impedance-matching adapter / impedance-matching transformer / line matching transformer
- 4 Digital logic voltage levels
Theory: Impedance when connecting two things
Output impedance is larger than the load's input impedance
Impedance matching
Impedance bridging
Impedance mismatches
Audio
Analog audio voltage levels
...and impedances.
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me) |
Most audio levels are not so well standardized,
and some have changed over time, somewhat quietly.
So assume each can be a factor of two off and can require a little knob twiddling at least.
- phono input
- on the order of a milliVolt, and seems to often be the not-yet-amplified output of phono cartridges
- There are two common types:
- 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.
- phono pre-amps will amplify this to (typically) consumer line level (and impedance)
- phono directly on line-level (or mic level) input as-is will be very too quiet (or if you manage to amplify it, very noisy)
- avoid connecting non-phono and in particular line-level outputs to phono inputs, it may be possible to blow the preamp
- impedance
- cartridge output: varies, order of 500 Ohm or lower (verify)
- phono amp imput: 47k Ohm
- consumer microphone level
- on the order of ~10mV (verify)
- because that's the order of what ~1cm electret (FET output(verify))s give, which is what most of these are
- computer sound cards have
- a voltage bias for electret mics - as electrets are the typical mic you plug in
- internal amplication to get it to the same level as line level. (Relevant mainly in that plugging line level into mic in will distort)
- 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)
- consumer line level
- on the order of ~300mV (~310mV RMS, ~440mV peak, 0.9V peak-to-peak)
- also sometimes known as -10dBu (mostly in situations that also do +4dBV 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
- line in impedance is often ~100 Ohm. Possibly higher, up to 1 kOhm
- line out impedance is often ~10 kOhm. Possibly higher, up to 1 MOhm
- professional microphone level
- order of 10mV, but can vary:
- Can be ~1mV, can be ~200mV (in theory more but this is atypical)
- more varied designs, and possible amplification at the mic, means more variation with design and per use
- (e.g. dynamic mics are somewhat intentionally low, to be able to deal with the louder things)
- ...so you will need that gain knob
- impedance:
- mic output: most are (higher) in the 50..200 Ohm range, with deviations (see more notes around here)
- mic preamp/mixer input: order of 1..2kOhm
- professional sound line level
- the standard also known as +4dBV means 1.2V RMS (1.7V peak, 3.4V peak-to-peak) (verify)
- with some variants a little higher and lower, so think 1V order of magnitude
- instrument level has no standard, though is generally quite predictable:
- voltage is often somewhere between mic and line level
- output impedance
- pickup impedance is often quite high (see also notes below on pickup impedance)
- input to a mixer will typically need a direct box (a.k.a. DI) to convert most things to typical XLR
- output to guitar amps is unchanged - they expect high impedance from a directly connected pickup (DI boxes tend to have a thru on the input side so that you could do both)
Less standard / more varied:
- headphone level
- roughly commercial line level, but less of a standard - can be higher.
- headphone amps tend to aim to power a ~30-60Ohm headphone (with a few milliamps(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)
- so a headphone amp can be a good cheat to connect consumer-level things to this
- consumer speaker wires
- The voltages are proportional to the amplifier's/speaker/s ability (and relate to be).
- For ~100W speakers you'll see up to a few dozen volts
- a tiny desktop speaker may be <1W (verify)
- impedance
- speaker load is often 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
- 6.3mm TS: already-amplified signal to a passive speaker (Sometimes avoided to avoid smoky mixups)
- Note these cables are different from TS instrument cables, basically in that instrument cables use a thinner core-and-shield and these are beefer and not shielded (just 2-lead stranded(verify))
See also:
dBV and dBu
Audio device differences
Balanced audio / pro audio
Balanced in electrical terms
Some other terms you see
Plugs's relation to balanced/unbalanced, voltage levels, etc.
Connectorwise:
- XLR3 is pro mic level, always balanced/differential, always mono.
- mono, because one signal requires a differential pair.
- if you want to carry stereo over XLR, use two cables (in practice, stereo is often about inter-device, and may well be two balanced 6.3mm TRS instead).
- 6.3 mm is (typically) pro line level (6.35mm but people are lazy typers)
- 6.3mm TRS is balanced mono, see notes on that above.
- or, sometimes, unbalanced stereo. This is an exception and will be noted.
- 6.3mm TR is unbalanced mono, often instruments, which is also often lower voltage levels (but close enough(verify))
- when a device instead uses 6.3mm these for mic in, aux, or controllers like pedals, or stereo, they will be marked as such (or switchable)
- mixer outputs are often two 6.3mm jacks (balanced, TRS)
- RCA on a mixer are typically only used for phono in, or consumer in (aux)
- so common mode. Ideally it's isolated (to avoid conductive ground loops)
On 6.3 TS versus TRS
- 6.3mm (1/4") TRS is most typically mono and balanced/differential. A mixer input will often mark this as "balanced"
- Tip and Ring is the pair, Sleeve is shield.
- No, it's not stereo. And using a 3.5mm-to-6.3mm converter to plug in consumer line level will do Weird Things.
- 6.3mm (1/4") TS - instrument cable
- is mono, and not differential
- Tip is signal, Sleeve is shield,
- mixers tend to accept both TRS balanced and TS unbalanced. If on the same socket they usually mark it (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.
- contrast with...
- 6.35mm (1/4") TS - speaker cable - that is, amplifier-to-passive-speaker
- basically a pair of thicker wires than instrument cable would use, and no shielding (it doesn't have to care at all because it's higher voltage, low-impedance load so coupling falls into nothing)
Things to avoid
On pickup impedance
Things that are't pure bridging
Other notes
On microphone impedance
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me) |
On 600 Ohms, and impedance matching
DI
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me) |
Direct box, DI box, DI unit, DI. (people argue over whether it stands for direct input, direct injection, direct induction, or direct interface).
Functionally
Takes a high-impedance, possibly-unbalanced signal (often roughly pro line level), e.g. from a passive instrument - probably most frequently electric guitars and electric basses.
Outputs a low-impedance pro-mic-level balanced signal, usually on an XLR plug.
In other words, usually plugs a high-impedance instrument into a (line that ends up at the) mixer.
Impedance-matching adapter / impedance-matching transformer / line matching transformer
Digital logic voltage levels
In the context of logic levels: (sorted from higher to lower voltage spans)
- (VCC)
- VOH - maximum output high
- VIH - minimum input high
- VT - theshold level, used in a few definitions, and applies to devices that transition at a given level(verify)
- VIL - maximum input low
- VOL - minimum output low
- (Gnd)
See also a few notes on margins below the summary
Some voltage level systems
An image search like this may be the simplest answer.
- TTL: O to 5V. Boolean levels: 0V~0.8V should be low, and 2V to Vcc should be high (where Vcc is ideally between 4.75V and 5.25V). More specifically:
- VOL: 0.4V
- VIL: 0.8V
- VIH: 2V
- VOH: 2.4V
- VCC: 5V
- LVTTL: 0 to 3.3V. Threshold levels identical to 5V TTL. (so only the VOH - VCC interval is smaller which generally affects nothing)
- VOL: 0.4V
- VIL: 0.8V
- VIH: 2V
- VOH: 2.4V
- VCC: 3.3V
- CMOS defines levels as a percentag of VCC (which can itself be 5, 2.5, 1.8, 1.5, 1.2V).
- VOL: 10% ()
- VIL: 30%
- VIH: 50%
- VOH: 70%
- ETL
- BTL
- LVDS
- PECL
- RS232
- RS485, RS422
Notes:
A given IC may deviate a little, so when in doubt check the datasheet.
Usually/ideally, boolean levels should be in VCC-VOH for high, and VOL-Gnd for low.
The VIH-to-VOH difference is the high noise margin (sometimes NMH),
the VIL-to-VOL difference is the low noise margin (sometimes NML).
These two mean that noise (or voltage drop) of about this magnitude won't disturb the boolean interpretation.
The VIL-VIH interval is usually not defined as either logic level (devices could choose one or the other).
When, power-supply-wise, a distinctin is made between e.g. Vcc and Vdd, it's BJT and FET:
- VCC - positive supply, BJT
- VEE - negative supply, BJT
- VSS - negative supply, FET
- VDD - positive supply, FET
Which note, is referring to collector, emitter, source, and gate
V+ and V- is not specific.
https://en.wikipedia.org/wiki/IC_power-supply_pin
See also: