Input impedance, output impedance, terminating impedance, matched impedance, and characteristic impedance are all fairly common terms in the tech specs, but what do they all mean and why are they relevant? In this article I will try to answer these questions and to explain what you need to know about impedance in practical terms, without too much maths and science. So any electronics students reading this can stop right now and go and do their homework instead Okay, let's start with a basic definition of impedance.
Imagine a simple circuit consisting of a battery and a resistor. The battery generates a voltage which tries to force a current around the circuit connected between the battery's two terminals. The resistor resists that current — the higher the value of the resistor, the lower the current will be, and vice versa. In resisting the current, a voltage difference is developed across the resistor. This important phenomenon is defined mathematically in Ohm's Law, where the battery voltage represented by V and measured in Volts equals the current represented by I and measured in Amps multiplied by the resistor's resistance value.
This simple example is of a Direct Current DC circuit — the battery voltage is steady and unchanging ignoring the effect of the battery losing energy over time. However, when we are dealing with audio electronics, the signal voltage changes amplitude continuously to represent the changing amplitude of the audio signal, and it alternates between positive and negative cycles. The currents that flow therefore have varying amplitudes and alternate in direction as well, and we have what is known generically as an Alternating Current AC circuit.
This is where things become slightly more complex, because, in addition to the resistance, there are two other fundamental components which affect the current flowing around an AC circuit. In addition to the simple resistance we have already discussed, there is also capacitance and inductance to consider. In simplistic terms these also act like resistors, except that their resistance to current changes in proportion to the frequency of the signal voltage fluctuations — You Cant Deny It - Various - The Sound Of Quality: Music System Reference Standard (Cassette) rate at which the current flowing through the circuit is made to change direction by the audio signal voltage, in this case.
All audio electronics have combinations of resistors, You Cant Deny It - Various - The Sound Of Quality: Music System Reference Standard (Cassette) and inductors connected in circuits, along with 'active' components like transistors or valves which provide amplification or act as switches.
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 — hence references to high-Z inputs, for example — and is still measured in Ohms. However, the actual value depends to some degree on the frequency of the signal voltages involved. In audio input and output circuits the impedances are principally resistive to make interconnection easier — the impedance won't change too much over the range of audio frequencies.
However, the impedance to radio frequency RF signals will often be very different to that at audio frequencies in order to keep RF interference out. Any device which generates a voltage has what is called an output impedance — the impedance value of its own internal circuitry as 'seen' from the outside ie.
Similarly, any device which expects to receive a voltage input has an input impedance — the impedance 'seen' by any equipment connected to its inputs ie. The output voltage from the source is developed across the input impedance of the destination often called the load impedance, or simply the loadand therefore the signal voltage is passed from source to destination.
However, the input and output impedances will also affect the current that flows around the circuit too. Figure 1. Input and output impedances, also called source and load impedances.
In cases where it is necessary to transfer the maximum power from a source to a destination power being proportional to both voltage and currentthe output impedance of the source and the input impedance of the destination must be equal; a situation referred to as having matched, or balanced, impedances. Strictly speaking, the input impedance should be the conjugate of the source impedance, but I only mention this in case those pesky electronics students are still reading!
If the source and destination are physically separated by a large distance in relation to the wavelengths of the signal frequencies being passedthen the connecting cable should also share the same impedance as both source and destination. 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 not a problem, as it is taken into account in the design of equipment for matched systems, but is worth bearing in mind, because it has some implications which I will return to in a moment. Now let's have a look at what happens if the source and destination impedances are unmatched.
Well, basically, some of the energy being transferred from source to destination is reflected back from the destination or wherever there is an impedance mismatch in the connecting circuit towards the source — not a good thing, in general. Theoretically such reflections could manifest as echoes, or cause signals at certain frequencies to be reduced through cancellation. The telephone industry discovered the practical ramifications of impedance matching almost a hundred years ago.
The wavelength of an audio-frequency signal travelling down a cable as an alternating voltage can be anything from km at 20Hz to about 15km at 20kHz wavelength reduces as signal frequency increasesso telephone cables used to carry conversations between people living in different cities can be considered to be of significant length compared to the wavelength of the signals they carry.
Since cable lengths between towns were comparable to the wavelength of the audio signals carried, it was important that the impedances of the sending and receiving telephone exchange equipment, along with the characteristic impedance of the cables see 'Characteristic Impedance' boxwere properly matched.
If the impedances weren't matched correctly then reflections would occur heard as echoes and colorationsand little energy from the source would reach the destination, resulting in faint signals coming out of the earpieces of the two telephones. These kinds of effects are rare these days, because the majority of telecoms systems are now digital — the basic problems are the same, but the technology has been developed to get around them.
Figure 2. In a matched-impedance system working to the ohm standard, connecting two tape machine inputs to the same console output would cause a level drop of 6dB, because each of the two parallel ohm loads only receives half the signal power. The broadcasting industry, and later the recording industry, grew up directly from the technology of the telecoms industry — the VU meter being a prime example of a telecoms measurement system which has survived unchanged in the recording industry to this day.
However, the idea of matching impedances is not particularly relevant or practical in a recording studio, for several reasons. For a start, we are not really interested in the transfer of power between source and destination — it's the signal voltage fluctuations which carry the information we're interested in — and it is extremely unlikely that any studio cable is going to be 15km long!
For these reasons, there is no technical requirement for impedance matching. Secondly, it is common in studios to want to distribute one output signal to several device inputs say, one mixer output to several tape recorder inputsand there are problems with doing this within matched-impedance systems. For the difference between dBm and dBu, see the 'Signal Levels' box. The tape recorder input meter will show a signal level of 0dBm as well — so far so good.
Without getting too far into the physics here, this is because the two inputs are wired in parallel. The result is a reduction in the signal level at each tape recorder input, as the same source signal current now has to be shared between the two destinations, therefore developing half the voltage across each input impedance.
A halving of voltage is a 6dB reduction in signal level and, You Cant Deny It - Various - The Sound Of Quality: Music System Reference Standard (Cassette), consequently, the tape recorder meters show an input level of about -6dBm instead of 0dBm. This is clearly not a good situation, and is very restrictive in You Cant Deny It - Various - The Sound Of Quality: Music System Reference Standard (Cassette) of what can be connected to what.
The solution to this problem is to dispense with the idea of matched impedances completely, and use what is called voltage matching instead. The idea here is to engineer the equipment to have the lowest possible output impedance and a relatively high input impedance — the difference between them must be at least a factor of ten, and is often much more.
With the minuscule output impedance and relatively high input impedance, the cable impedance can be disregarded completely in comparison You Cant Deny It - Various - The Sound Of Quality: Music System Reference Standard (Cassette) full output voltage should be developed across the input impedance. It's what engineers generally do! This can be an issue with some Revox machines, for example. For this test, both were providing just under 58dB of gain. The AEA preamp distortion percentage continues to fall linearly and virtually identically to the GML preamp mentioned in the 'How Transformers Distort' boxbut it starts to rise dramatically in the Dbx.
Not surprisingly, the AEA TRP sounds supremely clean and transparent at all signal levels, while the Dbx becomes progressively richer and thicker sounding as the input level rises, or the drive control is advanced. If you were asked to choose which sounded warmer and more analogue, the Dbx would win hands down! In both cases, the amount of distortion increases as the signal level falls, due to hysteresis, so quieter signals end up sounding slightly richer and denser, thanks to the additional harmonics.
At higher signal levels, the distortion starts to rise again, but far more gradually. Transformers are inherently inductive, so their frequency response depends on the input and output impedances of the circuit surrounding them.
That may well be yet another element of the 'analogue warmth' associated with vintage equipment, where coupling transformers rather than coupling capacitors was the norm. The distortion ratio for the GML lower brown trace is significantly less than that of the ISA, and that difference continues to fall as the signal level rises, tracking in parallel with the GML preamp. Although the difference in distortion isn't huge on graph paper, when comparing the two units audibly, most would say that the Focusrite ISA sounded warmer than the GML I've also compared the frequency responses of these two preamps in the graph below, and what that reveals is that the ISA's frequency response is 0.
How much current flows from the cathode to the anode is determined by the voltage applied to one or more grids. Triodes have only three terminals: cathode, control grid and anode hence the 'tri' in triode.
The tetrode has four terminals cathode, control grid, screen grid and anodewhile the pentode adds yet another grid, called the suppressor grid. The extra grids in the tetrode and pentode valves are there for complicated scientific reasons, but basically enable higher gains to be achieved and operation at higher radio frequencies than is possible with simple triodes. There's no magic-bullet 'Warmification' plug-in.
Fortunately, there are plenty of good software tools to help you create believable warmth. Another option, of course, is to use the valve gain-stages of emulations of classic hardware devices of which more in a moment.
However, there are plenty that offer control over things like tape drive, saturation and hysteresis, and they range from the subtle to the obvious, the almost free to the expensive.
Both have their uses, depending on the effect you seek. DUY's offering includes models of four different tape machines, as well as different noise-reduction systems, while Reel Tape offers control over wow and flutter characteristics not quite like the real thing, but a very useful addition. There are now more such emulations than you can easily count. They vary enormously in quality and price, and, of course, some You Cant Deny It - Various - The Sound Of Quality: Music System Reference Standard (Cassette) extra features such as tube emulation.
Some of the best are those for the Universal Audio UAD platform: their Neve, Pultec and Fairchild models in particular, are useful, even when the plug-in's bypass mode not to be confused with your DAW's plug-in bypass button is engaged, as well as their Plate reverb.
The output side of the device conveys power from the power supply, under the direct control of the input signal applied to the grid valve or base transistor. If the input signal gets bigger, the valve or transistor lets through more power from the power supply. Ideally, the relationship between the input and output signals would be perfectly linear, but it never quite works that way. It will always suffer some small distortions see 'Understanding Harmonic Distortion' box.
Hopefully, those small distortions will be only 0. The head is designed to force this magnetic field to radiate across the magnetic tape. As the tape passes, the individual magnetic particles are influenced by that varying field, retaining their state of magnetisation as they leave its sphere of influence.
This is called 'hysteresis'. One thing you should know first, this Hi-res Audio Player can't directly play high resolution music on your Windows without any output device. It also has the Mac version. Onkyo HF Player is a Hi-Res audio player app which can get greater control of how music sounds on your Android device with its touch-adjustable linear-phase FIR equalizer which offers 16, discrete bands of HD equalization with zero loss in audio quality.
You can enjoy the high resolution audio with this free Hi-Res audio player app. This app also has iOS version. Getting the best hi-res music player is the getting the container, putting the hi-res audio files into the container is perfect. Where to get the hi-res music files? Here the 5 sites are where you can listen to hi-res music files. For playing the online hi-res audio files, you could use the audio player for better sound experience.
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