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.

Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect

And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · · · · Shorter-range wireless (IR, ISM RF) · RFID and NFC · bluetooth · 802.15 (including zigbee) · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

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 ·

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes

Microcontroller and computer platforms Arduino and AVR notes · ESP series notes · STM32 series notes · · · ·

Less sorted: device voltage and impedance, audio and otherwise · electricity and humans · power supply considerations · Common terms, useful basics, soldering · PLL · pulse modulation · signal reflection · resource metering · SDR · Project boxes · Unsorted stuff

See also Category:Electronics.

Digital logic voltage levels


VCC and Gnd tend to refer to BJT transistors and TLL logic
(Vcc seems to have originally meant "the voltage common to all BJT collector pins")
VDD and VSS tend to refer to FET style ICs and boards
V+ and V- are more generic terms, seen on board power connectors

...but many of these terms have become a little fuzzy and people will mix them.

There's also the point that V-, Vss, and Gnd are what in many cases you'd call 0V and/or voltage reference and/or is the lowest voltage around -- but there are exceptions to all.

Some voltage level systems

For visual overview, An image search like this may be the best answer.

I prefer diagrams like this one: http://www.jsykora.info/2014/05/logic-voltage-levels/

...because these summarize and compare many more, and some related conventions, like the voltages in LVDS communication[1].

The ones you'll probably run into most (in home DIY context):

  • 5V TTL: (Vcc is ideally between 4.75V and 5.25V)
    • VOL: 0.4V
    • VIL: 0.8V
    • VIH: 2V
    • VOH: 2.4V (can be slightly higher in some logic families(verify))
    • VCC: 5V

A more intuitive view on that is:

on inputs
low is 0V to 0.8V
high is 2V to 5V
on outputs
low is 0V to 0.4V
high is 2.4V to 5V

The difference between input and output is mainly there so that you can tie such components together, and noise voltages lower than ~0.4V introduced between them won't bother anything.

  • 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%

Where you may want to controlling a 5V Led string from a 3.3V device,

you can control 5V TTL but not 5V CMOS, because 5V TLL is high above ~2.4V and 5V CMOS is high above 70%*Vcc = ~3.5V (all the highness happens above 3.3V so it'l never go high). For completeness:

3.3V CMOS means (OL=0.33V, IL=1V, IH=1.65V, OH=2.3V) and matches 5V TTL well enough
5V CMOS means (OL=0.5V, IL=1.5V, IH=2.5V, OH=3.5V)
5V TTL means (OL=0.4V, IL=0.8V, IH=2V, OH=2.4V)

  • LVTTL: 0 to 3.3V. All threshold levels are identical to the above, and only the VOH to VCC region is smaller.

Note that the above means that 3.3 and 5V doesn't always imply the same thresholds unless you know the logic family.

Simple serial

  • RS232
space between +3..+15V, mark between -3V..-15V
yes, you still see these, mostly on non-PC devices
  • "TTL serial" is RS-232-style in communication,
0 and whatever the IC's Vcc is, usually 5V or 3.3V
common on modern boards and ICs that do serial

Seeing a DE-9 connector, you probably want to use a multimeter to check that it is oldschool RS232, and not connect it directly to the latter. Doing so may work, but will also everntually burn out the 5V side.

  • RS485
  • RS422

See also:


Analog audio voltage levels

...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 input (typically on RCA plugs - which are also used for the higher line level)
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 (often on 3.5mm TS plugs)
on the order of ~10mV signal output (verify)
because that's the order of what ~1cm electret (with FET) will output, which is what most cheaper mics are based on
note that the recording side will often put a DC voltage on that TS plug, required by electret mics's internal preamp
computer sound cards mic inputs 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. You could turn that off, but separate mic and line in is easier to explain to people)
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 (often stereo, often on RCA or 3.5mm TRS plugs)
on the order of ~300mV (~310mV RMS, ~440mV peak, 0.9V peak-to-peak)
also sometimes known as -10dBV (mostly in situations that also do +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.
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 (mono, often on XLR3 plugs)
order of 10mV, but can vary because of varying microphones
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 lower, due to simpler design but this also makes them better for louder things
...so you will need that gain knob
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 (mono, often balanced and on 6.35mm TRS)
is +4dBV, so ~1.2V typical. See notes anove.
  • instrument level (mono)
has no standard, though is generally quite predictable, 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)
instruments will typically be unbalanced, TS
the 6.35mm inputs on mixers will often be balanced (TRS) because this makes sense to use for device interconnects when using these plugs.
They may also accept unbalanced (TS) (will be marked with something like 'bal or unbal'), to allow for a few other things (see also #Plugs.27s_relation_to_balanced.2Funbalanced.2C_voltage_levels.2C_etc.)
output impedance: (verify)

  • guitars are sort of their own story, a more specific variant of instrument level
guitar pickup impedance is often quite high (see notes below on pickup impedance)
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 is a directly wired second port) so that you can both have a guitar amp on stage (mostly as a monitor), and send its result to the mixer
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)

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
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 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)

See also:

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 (the third pin in shield)
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 two different things:

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)
(rarely) unbalanced stereo. This is an exception and will be noted.

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

Effect pedals are typically unbalanced, instrument level
Tip is signal, Sleeve is shield/ground
unpowered instruments may have rather lower voltage levels - but still close enough to gain without much trouble
powered instruments may be somewhere inbetween(verify)

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.

Edge cases

  • TS mic to XLR:
impedance difference - this is what a DI box is for
  • TS instrument to XLR:
a DI bix helps isolate

  • Plugging line level output into instrument level input usually so much stronger that it distorts
there is a very small (but non-zero) chance of some damage
  • plugging TRS outputs into battery powered pedals is potentially harmful - to the output
specifically when they are switched on by plugging it in
they do this by actually using a TRS socket, and completing the power circuit by having the TS's sleeve short Ring to sleeve
but when you use an actual TRS plug, it puts 9V between ring and sleeve.

  • TRS consumer stereo into TS-only input will lose the right channel
because it shorts ring (right) to ground (sleeve)
  • TRS consumer stereo into TRS balanced - depends a little on the case
usually sounds quiet and weird, because it presents left minus right as the signal

  • TRS balanced into a unbalanced-only TS input
shorts one of the pair to ground
might work, might not?(verify)
  • TS output (often instrument) into balanced-only TRS input
the plug will short between sleeve and ring, i.e. tie one of the signal lines to shield.
which on the 'cheaty' single-ended balanced driver design might be fine{{verify{{,
on a halfway proper one can cause damage
while particularly fancy ones may be protected against this case (but don't assume it)
might introduce common mode from the mixer itself. (verify)

Plugging audio where it's not quite intended

Further edge cases, when considering more plugs

Things that won't work / things to avoid

dBV and dBu

Notes on balanced audio

Balanced in electrical terms
Some other terms you see

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, unbalanced signal (often roughly pro line level), usually a 6.35mm TS socket, 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 (cable that ends up at the) mixer.

Impedance-matching adapter / impedance-matching transformer / line matching transformer

Theory: Impedance when connecting two things

Output impedance is larger than the load's input impedance

Impedance matching

Impedance bridging

Impedance mismatches


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 (may be Gnd)
  • VDD - positive supply, FET
  • VSS - negative supply, FET (may be Gnd)

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

In more general use you might use

V+ and V-
VS for supply voltage
Schematics often mention a rail's voltage, e.g. +12V

...though people have their habits