Electronics project notes/Device voltage and impedance, audio and otherwise

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This is for beginners and very much by a beginner.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.

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Audio notes: microphones · device voltage and impedance, audio and otherwise · amps and speakers · basic audio hacks · digital audio ·

Less sorted: Common terms, useful basics, soldering · Arduino and AVR notes · ESP series notes · PLL · signal reflection · pulse modulation · electricity and humans · resource metering · Microcontroller and computer platforms · SDR · Unsorted stuff

See also Category:Electronics.

Theory: Impedance when connecting two things

Output impedance is larger than the load's input impedance

Impedance matching

Impedance bridging

Impedance mismatches


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
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)
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.
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
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)
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.


  • 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


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).


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


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.


See also: