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.

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Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer

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

Less sorted: Common terms, useful basics, soldering · Microcontroller and computer platforms · Arduino and AVR notes · ESP series notes · Electronics notes/Phase Locked Loop notes · mounts, chip carriers, packages, connectors · signal reflection · pulse modulation · electricity and humans · 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

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 percentage 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
space between +3..+15V, mark between -3V..-15V
yes, you still see these on non-PC devices
  • TTL serial (otherwise RS-232-style)
0 and 5V
generally used on

  • RS485
  • RS422

If on a circuit board schematics - and occasionally around power/switch pins on the actual board - you see labels like:

  • VCC - positive supply, BJT
  • VEE - negative supply, BJT
  • VSS - negative supply, FET
  • VDD - positive supply, FET

See also https://en.wikipedia.org/wiki/IC_power-supply_pin

In more general use you might use V+ and V- (though people have their habits). Schematics will also often refer to the specific voltage rail, e.g. +12V

See also:


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
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)
a direct box (a.k.a. DI) to make it XLR is the easiest to put it into a mixer
guitar amps expect high impedance from a directly connected pickup
DI boxes tend to have a thru on the input side (which is a directly wired second port) so that you can also have a guitar amp on stage

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)
(a headphone amp is sometimes 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 6.35mm TRS instead).
  • 6.35 mm is (typically) pro line level (6.35mm but people are lazy typers)
6.35mm TRS is balanced mono, see notes on that above.
or, sometimes, unbalanced stereo. This is an exception and will be noted.
6.35mm TR is unbalanced mono, often instruments, which is also often lower voltage levels (but close enough(verify))
when a device instead uses 6.35mm these for mic in, aux, or controllers like pedals, or stereo, they will be marked as such (or switchable)
  • mixer outputs are often two 6.35mm 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 6.35 TS versus TRS:

  • 6.35mm (1/4") TRS is most typically balanced/differential mono. 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-6.35mm converter to plug in consumer line level will easily do weird things (depending on the case).
  • 6.35mm (1/4") TS - instrument cable
is mono, and not differential
from actual instruments it won't
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 that won't work / 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