Impedance and impedance matching

What is impedance?

Impedance is very technical measurement that is hard to explain without equations and some scientific jargon. When you want to understand impedance, first think about electrical resistance (represented by R), measured in Ohms. In basic battery circuit battery generates a voltage which tries to force a current around the circuit connected between the battery’s two terminals – the higher the value of the resistor, the lower the current will be. This was resistance for Direct Current (DC) circuits.

In AC circuits the situation becomes slightly more complex, because, in addition to the resistance, there are two other fundamental components which affect the current flowing around an AC circuit: capacitance and inductance. In addition to the simple resistance we have already discussed, there is also capacitance and inductance to consider – and how those affect the current flow depends on the AC signal frequency.

To make life slightly easier for ourselves, we often consider the total ‘resistance’ of a complex circuit involving resistors, capacitors and inductors as a composite lump, and that’s what we call the impedance. Impedance has the symbol Z  and is still measured in Ohms. However, the actual value depends to some degree on the frequency of the signal voltages involvedAny device which generates a voltage has what is called an output impedance and any device that takes in signal had input impedance. The output voltage from the source is developed across the input impedance of the destination (often called the load impedance).

Characteristic impedance

Besides inputs and outputs interconnection cables from source to destination have impedance. So, when we say that the input impedance of your TV’s composite video jack is 75 ohms, that’s what we mean. It doesn’t mean is that the resistance of the cable will be 75 ohms. When we say that the characteristic impedance of a cable is 75 ohms–or 50, 110, 300, or what-have-you–what we mean is that if we attach a load of the specified impedance to the other end of the cable, it will look like a load of that impedance regardless of the length of the cable between. The characteristic impedance of a transmission line is the ratio of the voltage and current of a wave travelling along the line.

Wikipedia definition:  The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm. The characteristic impedance of a lossless transmission line is purely real, with no reactive component.

The formula for characteristic impedance is Zo = sqrt(L/C) – this is the impedance for a lossless line.

To achieve this characteristic impedance we need two things, firstly the inductance in the cable, and secondly the capacitance between the cable and ground. These each present a complex impedance of opposite polarity and thus come together to form a real impedance. The impedance of the cable is typically determined by the selection of cable geometry and the used insulation materials. Try a Coaxial calculator if you want to see how different materials and dimenstions affect the impedance.

When an RF voltage and current are transmitted along a wire, the impedance of the cable itself becomes significant, and for any distance that is “significant” – which is to say any distance greater than about 0.1 of the signal’s wavelength – matching is necessary. The RF systems are typically built as matched systems where the signal source, cable and load impedance have the same value (or at least “close enough” for the application).

Most typical impedance seen on cables are 50 Ohm single ended impedance (coaxial cables) and 100Ohm differential impedance (usually twisted pair or twinaxial). If you play with coax, short for coaxial cable, you probably know this it is available in a number of different impedances. Another common is 75 ohm, like video cable or antenna cable. There are also special coaxila products range from 32 ohms up to 124 ohms.

In a PCB layout you have different technologies to get the impedance of traces right, but the predominant one is called “Microstrip”. In PCB / PWB boards we are following 50 ohm impedance for single ended lines and 100 ohm impedance for Differential lines. To get ideas on impedances on transmission lines on circuit boards try Transmission line calculator.

Typical impedance values

4 ohms:  4 ohms is a common speaker impedance. Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms.

8 ohms: Most common nominal speaker impedance. The impedance of most speaker systems is not constant with frequency. A speaker that is rated at 8 ohms may be exactly 8 ohms at only a few frequencies. The rest of the time it may wander above and below this value several times. Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms. Speaker systems are rated for a nominal impedance, which will be typically 8 or 4 Ohms for hi-fi systems. Many of these claimed impedances are very misleading, since the actual impedance varies widely with frequency. At the very worst, a nominal 8 Ohm speaker system may have an impedance at some frequencies (usually at or near the crossover frequencies) of perhaps 4 Ohms – some may be lower than this.

15 ohms: This is the lowest impedance coaxial cable inductance I could find available….

16 ohms: Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms. The physical differences between an 8-ohm and a 16-ohm speaker of the same type generally come down to voice-coil wire size and the number of voice-coil wire turns in the magnetic gap. The 16 ohm impedance is a British thing. There are some guitar speakers with different parameters for the 8ohm vs. 16ohm model.

25 ohms: Advanced users often need non-standard coaxial cable types and 25 ohms seem to be one that pop ups somewhat often.  25 ohm miniature RF cable is extensively used in magnetic core broadband transformers. Typical field of use for these coaxial cables is in various impedance transformers, internal wiring, combiners, couplers, splitters etc.

30 ohms: For high power, the perfect impedance for coaxial cable is 30 ohms.

32 ohms: Common headphone impedance (Typical designs provide impedances in the 8-32 ohms region)

50 ohms: 50 ohms is the most commonly used impedance for RF cables for general use and transmitters. In most commercial RF applications 50 ohms coax cable has been taken as the standard for very many years. Experimentation in the early 20th century determined that the best POWER HANDLING capability could be achieved by using 30 Ohm Coaxial Cable, whereas the lowest signal ATTENUATION (LOSS) could be achieved by using 77 Ohm Coaxial Cable.  However, there are few dielectric materials suitable for use in a coaxial cable to support 30 Ohm impedance.  Thus, 50 Ohm Coaxial Cable was selected as the ideal compromise; offering high power handling AND low attenuation characteristics. According to Wikipedia the arithmetic mean between 30 Ω and 77 Ω is 53.5 Ω; the geometric mean is 48 Ω. The selection of 50 Ω as a compromise between power-handling capability and attenuation is in general cited as the reason for the number. With 50 Ohm Coaxial Cables being the best compromise solution, practically any application that demands high power handling capacity. 50 ohm cables are intended to be used to carry power and voltage, like the output of a transmitter. Also several common antennae are easily matched to 50 ohms cable impedance. The widespread idea that 50 Ω and 75 Ω cable nominal impedances arose in connection with the input impedance of various antennae is a myth.

60 ohms: For high voltage, the perfect impedance is 60 ohms. 60 ohms cable was used in some early CATV systems (later replaced with 75 ohms cable).

75 ohms: 75 ohm coax cable is used almost exclusively for domestic TV and VHF FM receiving applications – The approximate impedance required to match a centre-fed dipole antenna in free space (i.e., a dipole without ground reflections) is 73 Ω, so 75 Ω coax was commonly used for connecting shortwave antennas to receivers. In addition 75 ohms coaxial cable is widely used for all kinds of video applications and in telecomunications systems. If you have a small signal, like video, or receive antenna signals, the graph above shows that the lowest loss or attenuation is 75 ohms. These typically involve such low levels of RF power that power-handling and high-voltage breakdown characteristics are unimportant when compared to attenuation. 75 Ohm Coax is very good widely available coaxial cable offering not only low signal attenuation (loss), but also relatively low capacitance. This combination of low attenuation and capacitance effectively make 75 Ohm Coaxial Cable the cable of choice for practically all types of digital audio, digital video and data signals. So if you are, for instance, connecting a 75 Ohm video camera connection to a studio monitor, the coaxial cable must also be 75 Ohm AND the connectors on the coaxial cable (i.e. BNC connectors) must be 75 Ohm in Impedance. All common home analog video standards use 75 ohm cable, as do coaxial digital audio (S/PDIF) connections.

90 ohms: The USB (1.0 and 2.0) low speed/full speed bus has a characteristic impedance of 90 ohms +/- 15%High speed USB must have both ends of the line terminated with 90 Ohm differential load (e.g. 45 Ohm resistors to the ground on RX side and in series on TX side.

93 ohms: RG-62 is a 93 Ω coaxial cable originally used in mainframe computer networks in the 1970s and early 1980s (it was the cable used to connect IBM 3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET, adopted RG-62 as their coaxial cable standard. The cable has the lowest capacitance per unit-length when compared to other coaxial cables of similar sizeTechnically 93 Ohm Coaxial Cable has the lowest capacitance of any type, but 93 Ohm Coax is rare and expensive.

100 ohms: Local area networks (LANs) commonly use a 100 ohms impedance twisted pair cable, manufactured to tighter tolerances than is necessary for telephony. 100 ohms is the differential impedance of CAT5/6/7 UTP cables (EIA/TIA-568).100 ohms UTP cable is also the most common cable used in computer networking (modern Ethernet). UTP cables are found in many Ethernet networks and telephone systems. UTP cables are typically made with copper wires measured at 22 or 24 American Wire Gauge (AWG), with the colored insulation typically made from an insulator such as polyethylene or FEP and the total package covered in a polyethylene jacket. Hard disk interface SATA uses fully shielded twin-ax conductors and termination resistor is 100 Ohms [+/- 5 Ohms] differential.

110 ohms:  If you have balanced AES/EBU type digital audio lines, you’ll want 110 ohm AES/EBU cable because the system is matched to this impedance.

120 ohms: 120 ohms is the typical nominal impedance of many telecom cables (differential impedance) at the frequencies used by communications systems like ISDN, ADSL, E1 and T1 (cable impedance is somewhat higher at voice frequency range is considerably higher, typically around 600 ohms). 120 ohms is also the impedance level used on many RS-485 systems and suitable for CAN bus. For RS-485 networks twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line

The impedance of telephoen wiring is typically around 100-120 ohms at the frequencies ADSL system uses. The cable impedance is somewhat higher at voice frequency range, considerably higher than 100 ohms, and where where historical 600 ohms impedance comes to picture (cable might not be exactly 600 ohms for voice, but that’s what devices are designed for).Read more at:

150 ohms: An early example of shielded twisted-pair is IBM STP-A, which was a two-pair 150 ohm foil shielded cable defined in 1985 by the IBM Cabling System specifications, and used with token ring or FDDI networks.

300 ohms: 300 ohms is commonly used twin-lead antenna cable impedance. Twin-lead is supplied in several different sizes, with values of 600, 450, 300, and 75 ohms characteristic impedance. The most common, 300 ohm twin-lead, was once widely used to connect television sets and FM radios to their receiving antennas (today it has been largely replaced with 75 ohm coaxial cable feedlines). Twin-lead is still used in amateur radio stations as a transmission line for balanced transmission of radio frequency signals. Half-wavelength folded dipole, commonly seen on television antennae, has a 288 Ω impedance.

600 ohms: 600 ohms is telephone line nominal impedance and old balanced audio standards Matched impedance was once used in professional audio systems, but nowadays it is rare to find any 600 ohms matched-impedance audio equipment outside venerable broadcast institutions.The istory of 600 ohms impedance comes from telephone companies – Telegraph companies used a vast network with a huge installed base of open wire pair transmission lines strung along wooden poles. Typical lines used AWG 6 wire at 12 inch spacing and the characteristic impedance was about 600 ohms. Early telephone companies started to use those, therefore 600 ohms became the standard impedance for these balanced duplex (bi-directional) wire pairs and subsequently most telephone equipment in general. The wiring to the subscriber in telephone networks is generally done in twisted pair cable. Its impedance at audio frequencies, and especially at the more restricted telephone band frequencies, is far from constantThis might be quoted as a nominal 600 Ω impedance at 800 Hz or 1 kHz. Below this frequency the characteristic impedance rapidly rises and becomes more and more dominated by the ohmic resistance of the cable as the frequency falls.At higher frequencies impedance drops due larger effect of capacitance and inductance (at hundreds of kHz to MHz range it typically is in 100-120 ohms range).


What is impedance matching?

In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load.

Power transfer

Whenever a source of power with a fixed output impedance such as an electric signal source, a radio transmitter or a mechanical sound (e.g., a loudspeaker) operates into a load, the maximum possible power is delivered to the load when the impedance of the load (load impedance or input impedance) is equal to the complex conjugate of the impedance of the source (that is, its internal impedance or output impedance). For two impedances to be complex conjugates their resistances must be equal, and their reactances must be equal in magnitude but of opposite signs.

Reflection-less matching

Impedance matching to minimize reflections is achieved by making the load impedance equal to the source impedance. If the source impedance, load impedance and transmission line characteristic impedance are purely resistive, then reflection-less matching is the same as maximum power transfer matching.

In electrical systems involving transmission lines (such as radio and fiber optics)—where the length of the line is long compared to the wavelength of the signal (the signal changes rapidly compared to the time it takes to travel from source to load)— the impedances at each end of the line must be matched to the transmission line’s characteristic impedance (Z_c) to prevent reflections of the signal at the ends of the line.


When impedance matching is needed?

In cases where it is necessary to transfer the maximum power from a source to a destination (power being proportional to both voltage and current), the output impedance of the source and the input impedance of the destination must be equalload).In a matched system like this we have the ideal power transfer arrangement, but the output voltage from the source device is shared equally across both the output and input impedances (assuming negligible cable effects) – this is taken into account in the design of equipment for matched systems.

If you are into RF (radio frequency) design, or perhaps telecommunications, impedance matching becomes an important topic. In systems where high frequencies and/or long cables are used, the input and output impedance should match the impedance of the cable used to connect them (avoids signal losses and reflections at the cable ends). Typical application fields where source impedance, cable impedance and load impedance are matched are radio frequency systems, video systems, high speed digital signal transfer, digital audio connections, industrial automation bus systems, telecommunications circuits and computer networks. Video interfaces normally operate with 75(omega) matched-impedance connections. RF systems operate at 50 ohms and 75 ohms impedance systems depending on application. At modern high frequency electronics sometimes impedance matching needs to be considered on the circuit board signal traces level.

When an RF voltage and current are transmitted along a wire, the impedance of the cable itself becomes significant, and for any distance that is “significant” – which is to say any distance greater than about 0.1 of the signal’s wavelength – matching is necessary. The wavelength is calculated from the speed of light (3 x 10 ^ 8 m/s, or 300,000km/s) multiplied by the “velocity factor” of the cable. Impedance mismatch in RF connections causes power to be reflected back to the source from the boundary between the high and the low impedance. The reflection creates a standing wave if there is reflection at both ends of the transmission line. Tried and true, the Smith chart is still the basic tool for determining transmission-line impedances.

All of the components of a coaxial system should have the same impedance to avoid internal reflections at connections between components. Such reflections may cause signal attenuation and ghosting TV picture display; multiple reflections may cause the original signal to be followed by more than one echo. In analog video or TV systems, this causes ghosting in the image.

In the early part of the 20th century, it was important to match impedance in telecommunications networks. Bell Laboratories found that to achieve maximum power transfer in long distance telephone circuits, the impedances of different devices should be matched. For this application 600 ohms output feeding a 600 ohms  input is perfectly matched. Typical open wire lines used AWG 6 wire at 12 inch spacing and the characteristic impedance was about 600 ohms. Early telephone companies started to use those, therefore 600 ohms became the standard impedance for these balanced duplex (bi-directional) wire pairs and subsequently most telephone equipment in general.

Impedance matching is a requirement in telecommunications networks even today, but in many cases not for any of the reasons you might think. It is actually rare for an analogue phone line to run more than about 5km from the exchange (Central Office in the US) to the user’s location. Impedance matching is required to enable the hybrid – a circuit that allows simultaneous transmit and receive on a single pair without interference – to function properly. If the impedances are not properly matched, you will hear too much of your own voice when you speak, the far end speech will be too soft, and both parties will very likely get lots of echo on the line. The nominal impedance used in normal analogue telephone (PSTN) system is 600 ohms.

Matched impedance (6oo ohms) was once used in professional audio systems, but nowadays it is rare to find any 600 ohms matched-impedance audio equipment outside venerable broadcast institutions

Audio circuits and impedance matching

There are also systems where things are built intentionally so that impedance is not matched. Examples of such systems are found especially in the audio systems. When it comes to low level audio signals, we’re not concerned about transferring lots of energy — we just want to get the signal voltage from equipment A to equipment B with as little loss as possible. The goal of the signal transmission system is to deliver maximum voltage, not maximum power. Sometimes called “voltage matching.”

For the most part, audio systems both professional and domestic, have their components interconnected with low impedance outputs connected to high impedance inputs. These impedances are poorly defined and nominal impedances are not usually assigned for this kind of connection.

Audio cables are NOT transmission lines. Audio systems the signal distances are so small compared to highest signal frequency (20 kHz typically) that we don’t have to worry about impedance matching. In theory, we could send an audio signal 12km without having to worry about impedance matching, although at extreme line lengths matching can reduce high frequency signal losses.

Bill Whitlock: Audio cables are NOT transmission lines. Marketing hype for exotic cables often invokes classic transmission line theory and implies that nano-second response is somehow important. Real physics reminds us that audio cables do not begin to exhibit transmission-line effects in the engineering sense until they reach about 4,000 feet in physical length.

For hi-fi and professional audio, impedance matched system is a meaningless concept and will actually cause an increase in noise. When you use impedance matching, the output will be loaded with the same impedance as it’s output impedance. If the output is loaded, then the available voltage from the source drops, and that in turn means that more amplification is needed to obtain the final voltage needed. The output signal level in impedance matched system is reduced by 6dB, this means that an additional 6dB of gain is required to compensate – therefore the circuit will have 6dB more noise.

For audio applications generally, it is desirable that the output impedance is low, and the destination impedance high, and this is the case with the majority of modern audio equipment. Preamps usually have an output impedance of less than 1k Ohm, and power amps will have an input impedance of at least 10k Ohms, but more commonly 22k or 47k. Line outputs usually present a source impedance of from 100 to 600 ohms. The voltage can reach 2 volts peak-to-peak with levels referenced to −10 dBV (300 mV) at 10 kΩ.

In professional audio devices need low differential (signal) output impedances and high differential (signal) input impedances. This practice is the subject of a 1978 IEC standard requiring output impedances to be 50 ohms or less and input impedances to be 10 k or more.

It has often been claimed that a 600 Ohm microphone should be matched to a 600 Ohm input for best performance – but this is simply wrong. The ideal for a microphone is to use a high impedance input, but this creates other problems, so a compromise is needed. Typically, a good mic preamp (for microphones of up to 600 Ohms) will have an input impedance of between 1.2k and 3k Ohmsdoes not cause any problems for the microphone. There are three general classifications for microphone impedance: Low Impedance (less than 600Ω), Medium Impedance (600Ω – 10,000Ω) and High Impedance (greater than 10,000Ω). Low impedance microphones are usually the preferred choice.

There are some cases where you need to think impedance levels in audio connections (sometimes this is called impedance matching although not any accurate matches are made). Perhaps the most common mismatch is when someone plugs a guitar or other instrument level device direct into a mixer.  Mixers typically only have LoZ and line level inputs, so plugging in the HiZ guitar output is a bad recipe. The Direct box or DI box as it is sometimes called changes the unbalanced HiZ signal to a balanced LoZ signal.

Loudspeakers (4-8 ohms)  and headphones (8-32 ohms) presents a known impedance load to the amplifier that drives to them, but the output impedance of driving amplifier is not matched to load (power amplifier driving speaker typically has very low output impedance). An amplifier will always have a defined (and measurable) output impedance (typically one ohms or less for power amplifiers that drive speakers) – measuring it can be hard and it can vary depending on frequency. The speakers also have varying impedance.  At the very worst, a nominal 8 Ohm speaker system may have an impedance at some frequencies (usually at or near the crossover frequencies) of perhaps 4 Ohms – some may be lower than this. When matching amplifiers to loudspeakers, the rated load impedance of the amplifier match that of the loudspeakers – the actual output impedance of the power amplifier output does not typically match the speaker impedance (on modern solid state hifi amplifiers output impedance is typically considerably less than one ohm, it can be several ohms on some tube amplifiers). Audio signals are carried by voltage, not power high-level amplifier output impedances can be made small compared to the speakers they drive. Maximum power theorem is for matching loads to sources, not matching sources to loads.

Different make and model headphones have widely varying impedances, from a common low of 20-32 Ω to a few hundred ohms. Most headphones with low impedance (less than 25 ohms, approximately) require little power to deliver high audio levels. For example, low impedance headphones will work well with equipment with weak amplification like portable music players, phones, and other portable devices. Headphones with higher impedance (25 ohms and over, approximately) demand more power to deliver high audio levels.

Impedance matching is not used in modern audio electronics:

A mic output might be around 600 Ω, while mic preamp inputs are 1 kΩ or more.
A line output will be something like 100 Ω, while a line input is more like 10 kΩ.
A loudspeaker amplifier will be less than 0.5 Ω, while loudspeakers are more like 4 Ω.
A guitar output might be 100 kΩ, while a guitar amp input is at least 1 MΩ.

In all these cases, the load impedance is significantly larger than the source; they are not matched. This configuration maximizes fidelity.

Impedance matching went out with vacuum tubes. Modern transistor and op-amp stages do not require impedance matching. If done, impedance matching degrades audio performance.

For more details on audio systems inter-connections read Lines and Interconnections study material.


Measuring impedance

Because Impedance is an AC property it cannot be easily measured like resistance. Connecting an Ohm meter across the input or output of an amplifier only indicates the DC resistance. It is quite possible however to measure input and output impedance at any frequency using a signal generator, an oscilloscope (or AC voltmeter) and a decade resistance box or a variable resistor.

The input or output impedance can be determined by measuring the small signal AC currents and voltages. The input or output impedance of a two terminal network can be determined by measuring the small signal AC currents and voltages. The classical measurement for the the input is the following: the voltage is measured across the input terminals and the current measured by inserting the meter in series with the signal generator.

Output impedance of signal generating equipment may also be determined using this simple technique: A load resistor is used to load the signal source. The output voltage is measured first with full load (V1), then without the load (V2). If the load resistance is exactly the same a output resistance, the signal voltage voltage V2 is twice the voltage V1. Other resistance values can be also used. I have made an Output Impedance Measurement Calculator to help to make impedance measurements using this method. NOTE: Keep the load impedance within the allowed load impedance range!

Then talking on the cable characteristic impedance, you need some more tools to measure it. Why not use TDR and measure itTime Domain Reflectometer (TDR) is is an electronic instrument that uses time-domain reflectometry to characterize and locate faults in metallic cables (for example, twisted pair wire or coaxial cable). A TDR measures reflections along a conductor. Simple way to do it is with a scope and a fast pulse. The TDR waveform shows the effect of all the reflections created by all of the impedance discontinuities. You can use TDR and adjustable resistor to determine the impedance of a cable. Usually when talking on cable impedance best is to do first the calculation and then rely on actual measurement in the end.


Main information sources:

Understanding Impedance

The Importance Of Impedance

What Is Headphone Impedance?

How important is impedance matching in audio applications?




  1. Tomi Engdahl says:

    Impedance Matching Basics

    Learn the basics about impedance match and how impedance matching networks works.

    Impedance matching is an important topic in RF and Microwave electronics. In this video, Gregory shows how an LC impedance match network was designed to match a 150ohm load to a 50ohm system.

    The designed network is demonstrated using time domain signals and measurements of S11 with a directional coupler.

  2. Tomi Engdahl says:

    Impedance Matching Basics: Smith Charts
    Sept. 7, 2021
    This article offers an introduction to the Smith chart and how it’s used to make transmission-line calculations and fundamental impedance-matching circuits.

    What you’ll learn:

    Plot complex impedances on a Smith chart.
    Determine SWR from the Smith chart.
    Determine the impedance of a load at the end of a transmission line.
    Identify impedance-matching component values from the Smith chart.

    Most of you have probably heard of the Smith chart. The intimidating graph, developed by Philip Smith in 1939, is just about as bad as it looks. How he came up with this is an untold story, but he provided a solution to the complex calculations on transmission lines. And as you will find out, it’s useful for working out transmission-line problems and in designing impedance-matching circuits. If you have avoided the Smith chart in the past, here’s a primer on how to take advantage of it.

  3. Tomi Engdahl says:

    What does “600 : 600 Ω” mean for an audio transformer?

    I found audio transformers online having the following specification: 600 : 600 ohms.

    My question is what does it mean? Does it mean that it is simply an isolation transformer? Or is it in some way used for impedance matching?

    Impedance is just the ratio between voltage and current, like a resistor.

    A transformer can change the ratio between in- and output voltage (and current as well) for AC signals.

    So a 600 ohms to 4 ohms transformer lowers the voltage (and increases the current) so that 4 ohms at the output behaves as 600 ohms at the input.

    That is useful when you want to connect a 4 ohms speaker to an amplifier which can only handle 600 ohm loads.

    A 600 to 600 ohm transformer can indeed be an isolation transformer for an audio distribution system or a telephone line. The transformer is 1 : 1 meaning in- and output voltage stay the same (and the current as well).

    It means this transformer is intended to isolate a audio signal between two different common mode voltages. It otherwise is intended to alter the audio signal as little as possible. This transformer is not intended to have a speaker connected to one side.

    600 Ω is the official impedance of “line” audio, although line audio drivers are often lower impedance. The 600 Ω spec is giving you a clue that the secondary should be loaded with that resistance. That is the load resistance the frequency response and other specs are valid at. Anything else will probably yield a less flat frequency response over the audio range.

    One use for such a transformer is at the receiving end of long cables. The driving and receiving equipment can easily have ground offsets between them, which would be added directly to the audio if it was a single-ended signal with ground as the common. At the end of the cable, both the signal and common are applied to the primary of the transformer. The common mode voltage then is largely cancelled out (only a little capacitive coupling between the transformer windings remains). The receiving equipment can then turn the result into a ground-referenced signal, if that’s what it wants, just by grounding one side of the secondary.

    It is an isolation transformer. It is for isolating signal lines with a characteristic impedance of 600 Ω. This may include audio and telephony circuits.

  4. Tomi Engdahl says:


    The standard for audio was born out of the radio industries need to set a standard, and has always been related to the 600 Ohm audio standard impedance with the level measured in dBm which refers to Decibel milli-Watts of power delivered to the “load”. One milli-Watt = 0dBm. However in recent years the audio industry has changed it method of delivering audio power.

    Most audio systems no longer adhere to the 600 Ohm standard and yet they still measure the audio level in terms of dBm as though it was still 600 Ohms.

    Early on, engineers discovered that that in order to deliver the maximum amount of audio power to the receiving terminal at all frequencies, it was necessary to match the end of line “load” to the characteristic impedance of a twisted pair wires and also match that impedance to the audio source driving impedance.

    Long twisted pair 16 gauge audio transmission wires were found to exhibit impedances in the vicinity of 600 Ohms at voice and musical frequencies up to 15 KHz, so of course the source of the audio and the load at the receiving site must also be 600 Ohms in order to achieve maximum power transfer to the receiving equipment. Since at that time there was power loss on the twisted wire cable it was necessary to apply a vacuum tube amplifier at each end of the transmission wires. Yes, that’s right the 600 Ohm standard originated in the days of vacuum tubes.

    As the radio studios became more complicated, with dozens of microphones and considerable recording equipment and audio mixing panels the coupling between these various components became much more complicated. Large studios like CBS “Black Rock” in New York had to do something to simplify how large networks with long wire runs could be connected while at the same time being able to keep audio levels and frequency response correct.

    They discovered that if the originating source impedance was kept very low (as close to zero as possible) and the end of line load impedance was made very high an audio signal could be transmitted over long distances without degrading the frequency response. Also because the end of line termination is very high impedance almost no power is absorbed at the receiver with miniscule loss.

    Of course, now the concept of one milli-Watt of audio power delivery to a very high receiving impedance was completely “out the window”, so it was decided to “pretend” that one milli-Watt was being delivered to the load, even though essentially zero power was being delivered. Basically you take the measurement as though it was terminated with 600 Ohms.

    When the 600 Ohm impedance became zero at the sending end and infinity at the receiving location the 600 Ohm standard disappeared. The only remnant of the original standard is the Voltage that can be measured the same as though there was an actual 600 Ohm Source and Load. Thus even though no actual power is delivered to the load a level of 0.7746 Volts for 0dBm is present at the load. A zero dBm tone on a modern audio circuit only means that a Voltage of 0.7746 exists at that point, and not any particular power level as the dBm would indicate.

  5. Tomi Engdahl says:

    Why 600 Ohms?

    I’m getting ready to build a trafo box to run line level audio through for some coloration, but one thing that’s been bothering me is everyone’s advice to use a 600:600 transformer.

    The question is: why?

    That ’600 ohm’ rating has to do partially with the transformer’s frequency response, that is the impedance that it is supposed to be loaded with (in theory) in order to have a reasonably flat frequency response at the design frequency range. Some transformers with low leakage inductances and interwinding capacitance are quite tolerant of this one – many line output transformers can drive a bridging input, unterminated, with hardly any difference in sound (or for that matter, measurements) when compared to the terminated case. Some, though, ring rather badly.

    At the same time, the ’600 ohm’ rating also defines the maximum voltage that can be applied to the transformer (core saturation), and the maximum current that can go through it (heat buildup), or basically the power rating.

    “Thought you might be interested to know a little history behind the venerable “600 ohms”. The first telephone lines that linked cities miles apart were actually existing telegraph lines. The wise engineers at Bell knew transmission line theory and realized that, even at audio frequencies, the lines were long enough to be true transmission lines (a pair of conductors behave as a transmission line when their physical length becomes more than about 1/10 wavelength at the highest frequency of interest). Therefore, they needed to know the characteristic impedance of the existing telegraph line pairs. They were typically #6 or #8 AWG wires spaced about 1 foot apart. If you do the calculations, an average value is about 600 ohms! To eliminate reflections (echoes), any transmission line must be driven from and loaded by a resistance equal to its characteristic impedance. So all telephone filters, transformers, etc. were designed for a 600 ohm system. And this hardware found its way into the first radio stations, and later into the first recording studios. Today, it is rarely necessary (or desirable) to “terminate” an audio cable unless it is a vintage passive filter or tube gear with transformers. Incidentally, audio cables begin to exhibit slight transmission line effects only when they are over about 4,000 feet long!”…-question.html

  6. Tomi Engdahl says:

    Digital audio coax is specified to be 75 ohms. USB 3.0 specification defines a 90-ohm nominal characteristic impedance. For digital signals right impedance copper cable is always better than wrong impedance cable built with exotic materials and/or construction.

  7. Tomi Engdahl says:

    #138: How to Measure Output Impedance

    This video shows a few methods for measuring the output impedance or resistance of a function generator, amplifier, or other circuits. These methods generally apply to relatively low frequency use cases, and won’t necessarily apply to all circuit types. The methods shown are all variations on measuring the response of the circuit to different load impedances, and calculating the device or circuit’s output impedance from those readings. A low frequency sinusoid (sinewave) is used as a test signal to prevent problems from reflections on misterminated transmission lines, etc. Notes from the video can be found here:

  8. Tomi Engdahl says:

    Effect of cable capacitance to low frequency analog voltage signal transmission

    A cable is a transmission line and it has four notable parameters: -

    Capacitance per unit length
    Inductance per unit length
    Resistance per unit length
    Conductance per unit length

    These four parameters are used in t-line analysis to predict the characterisic impedance of the cable

    At mid audio the two parameters that dominate are R and C

  9. Tomi Engdahl says:

    Understanding Skin Effect and Frequency

    You will notice that even for largest wire size, the difference between the inside and outside of a conductor is a few percentage points. Note that this is based on frequency not on the length of the cable, as mentioned in the quote above. You can see this effect very clearly if you look at the impedance of cables at low frequencies. Figure 2 shows the impedance of a 75 ohm video cable from a frequency of 100 MHz (right margin) down to 10 Hz (left margin).You will see that this 75 ohm cable is really only 75 ohm after around 100 kHz and above. Below that it is way higher than 75 ohms. In fact, down at 10 Hz, the impedance of the cable is around 4,000 ohms.

    That high-frequency value (75 ohms) is called the “characteristic impedance” of the cable and will stay at 75 ohms (or whatever it was designed to be) out to much higher frequencies. If you compare the low frequency formula to the high-frequency formula, there is one huge difference.R (the resistance of the wire) is a major factor at low frequencies. But in the high frequency formula, there is no R, no resistance. What happened to the resistance? And the answer is “skin effect”. As the frequencies got higher and higher, less and less of that conductor is being used, until, around 100 kHz, only the skin is actually carrying the signal.

    This is one reason why we can’t build an audio cable to a specific impedance. That number will only apply to one frequency. At a different frequency, above or below, the impedance will be a different value. That’s why we don’t list the impedance of most audio cables and, if we do, that impedance is measured at some high frequency, like 1 MHz, and that cable might be used for some non-audio application. But perhaps you are thinking, “If the resistance of the wire makes no difference, then why won’t a small cable go as far as a big cable? “And the answer is equally simple: the big wire has more skin than the small wire.

    This is why, when we make cables for high frequencies, we spend a lot of time on the surface of the wire. That’s the skin. And at high frequencies, that’s the only thing working. So we do a lot of things (many of which are “trade secrets”) to make sure the surface of that wire is as perfect as we can possibly make it. Our digital video cables, for instance, are sweep tested and measured out to 4.5 GHz. Signals at these highest frequencies use only micro inches of the outside of the conductor. If all you were carrying was high frequencies, you could use a copper tube as a conductor with no additional loss compared to a solid conductor.

    This is why our broadband cables are most often copper clad steel (called “CCS” in our catalog). There’s only a thin layer of copper on a steel wire. This means such a cable will only work at high frequencies/And that’s OK because TV stations start at Channel 2 which is 54 MHz, well into the skin effect range.

    Our digital video cables are all-copper, but that’s so you can use them for analog or digital video, analog or digital audio, satellite dishes or pretty much any signal at any frequency from DC to 4.5 GHz. Of course copper-clad steel is a lot stronger than bare copper, something that has saved many a CATV/broadband installer who was less than gentle when installing such a cable. So the next time the salesperson is telling you about the “skin effect” in his speaker cable, well, you know the truth!

  10. Tomi Engdahl says:

    Measuring Impedance Virtually

    We always enjoy a [FesZ] video and we wonder if the “Z” stands for impedance? That’s the topic of his latest video series: measuring impedance with LTSpice. Of course, he also does his usual thorough job of mapping the virtual world to the real one. You can see the video below.

    LTspice tutorial – Measuring Impedance (part 1/2)

  11. Tomi Engdahl says:

    Coax Impedance: Coaxial Cable Characteristic Impedance

    The characteristic impedance of any coaxial cable is key to the selection of the required type. It is often the first consideration.

  12. Tomi Engdahl says:

    A common joke in electronics is that every piece of wire and PCB trace is an antenna, with the only difference being whether this was intentional or not. In practical terms, low-frequency wiring is generally considered to be ‘safe’, while higher frequency circuits require special considerations, including impedance (Z) matching. Where the cut-off is between these two types of circuits is not entirely clear, however, with various rules-of-thumb in existence, as ……


    A popular rule is that no impedance matching between the trace and load is necessary if the critical length of a PCB trace (lcrit) is 1/10th of the wavelength (λ). Yet is this rule of thumb correct? Running through a number of calculations it’s obvious that the only case where the length of the PCB trace doesn’t matter is when trace and load impedance are matched.

    According to these calculations, the 1/10 rule is not a great pick if your target is a mismatch loss of less than 0.1 dB, with 1/16 being a better rule. Making traces wider on the PCB can be advisable here, but ultimately you have to know what is best for your design, as each project has its own requirements. Even when the calculations look good, that’s no excuse to skip the measurement on the physical board, especially with how variable the dielectric constant of FR4 PCB material can be between different manufacturers and batches.


Leave a Comment

Your email address will not be published. Required fields are marked *