Features
The type of pickups ("heads") for which the phono stage is designed
— MM. Moving Magnet Pickups: As the stylus moves through the tracks, it vibrates a permanent magnet, which induces current in the fixed coils. Among the main advantages of such devices are simplicity and relatively low price. They provide a fairly high output level, which, in turn, simplifies the design and reduces the cost of phono stages. However MM-cartridges in general are somewhat inferior to MC in terms of quality and sound fidelity; however, these moments are largely subjective, moreover, they directly depend on the price category of the “head”. Note that some types of MC pickups (models with a high output signal level) can also be connected to phono stages of this purpose.
— MC. Pickups using a moving coil circuit — it is connected to a needle and during operation it oscillates relative to fixed magnets, due to which a signal current is created. Such pickups are said to be superior to MM cartridges in sound quality; at the same time, they are noticeably more complex and expensive. Moreover, the latter is also relevant for phono stages: they are difficult to design and require a high gain, since the output signal level of
MC pickups is usually low — less than 0.5 mV (there are exceptions, but very rarely).
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MM/MC. Phono stages that can work with both MM and MC heads; the selection is usually made manual
...ly by means of a switch. For details about the features of both types of pickups, see the corresponding paragraphs. Also note here that such universal correctors are convenient, but they are often inferior to specialized models in terms of performance.ADC bit depth
The bit depth of the analogue-to-digital converter installed in the phono stage.
As the name suggests, the ADC is responsible for converting the analogue audio signal coming from the pickup into a digital data stream — for example, for transmission via S/P-DIF (see "Outputs"). The principle of such a conversion is that the original sinusoid of the analogue signal is divided into separate fragments — "steps", and the data on each "step" is encoded in digital form. At the same time, each fragment has two main parameters — duration and volume level.
The ADC bit capacity determines how accurately the volume of each individual digital “step” will correspond to the volume of the original analogue signal. For modern phono stages, the minimum bit depth is 16 bits — this (combined with the appropriate sampling rate, see below) is enough to get digital sound in the quality of an Audio CD or even DVD-Audio. However, there are also higher values — for example, 24 bits.
ADC sampling rate
Sampling frequency of the analogue-to-digital converter installed in the phono stage.
As the name suggests, the ADC is responsible for converting the analogue audio signal coming from the pickup into a digital signal, for example, for transmission via S/P-DIF (see "Outputs"). The principle of such a conversion is that the original sinusoid of the analogue signal is divided into separate fragments — "steps", and the data on each "step" is encoded in digital form. The sampling rate describes the number of such fragments per second of the original sound. The higher it is, the smaller the width of each individual “step” will be and the more accurately the digital signal will correspond to the original sinusoid.
In modern phono preamps, the sampling rate is usually at least 48 kHz — this is slightly higher than in the popular Audio CD standard. In the most advanced models, this figure reaches 192 kHz, which is comparable to high-end DVD-Audio.
Output level
The highest root mean square (RMS) output level that a phono stage can provide. Actual level may be lower depending on input signal level and gain
Recall that a phono preamplifier usually operates in the form of a preamplifier, boosting a weak signal from a pickup to a line level for transmission to a power amplifier. The minimum signal level sufficient to feed the power amplifier is about 150 mV; however, phono preamps usually provide higher values. This is done on the basis that the higher the intrinsic signal, the less it is affected by external electromagnetic interference. The maximum for modern phono preamps is actually 2 volts on the RCA outputs, higher values can be found on the XLR outputs.
In addition, some nuances of matching the phono stage and an external amplifier are associated with the output level; detailed information about this can be found in special sources.
Signal to noise ratio (MM)
The signal-to-noise ratio provided by the phono stage when working with an MM-type pickup (see "Intended use")
This parameter describes the relationship between the average output level and the average noise floor of the device. The higher it is, the clearer the sound is, the less extraneous interference it has. On the other hand, it is worth remembering that the overall sound quality provided by a phono stage depends on a number of other parameters. As a result, an advanced model with high sound quality may have a lower signal-to-noise ratio than a low-cost device.
Note that in universal phono stages (see "Intended use") the signal-to-noise ratio for MM cartridges is usually higher than for MC ones; this is due to the technical features of both.
Harmonic distortion (MM)
The coefficient of harmonic distortion provided by the phono stage when working with an MM type pickup (see "Intended use").
The lower this indicator, the less distortion the phono stage gives, the more reliable the output signal is. It is impossible to completely avoid such distortions, but it is possible to reduce them to a level that is not perceived by a person. So, it is believed that the level of harmonics of 0.5% and below is no longer audible even to a “trained ear”. However, it should also be borne in mind that the quality of the audible sound is also affected by distortion from other components of the audio system. Therefore, the harmonic distortion of the phono stage should not just be below 0.5%, but as low as possible. Values in hundredths of a percent are considered a good indicator, values in thousandths and below are considered excellent.
Gain (MM)
The gain provided by the phono stage when working with an MM type pickup (see "Intended use").
This parameter describes how much the signal level at the output of the phono stage increases relative to the level at the input. It is traditionally written in decibels; you can convert decibels into "times" using special formulas or tables. Knowing the gain, you can estimate the actual level of the output signal when using a particular pickup. For example, if the characteristics indicate 35 dB (56.2x), and the “head” outputs 5 mV, then as a result, the signal level at the output of the phono stage without additional processing will be 5 * 56.2 = 281 mV. Accordingly, the input sensitivity of the power amplifier with which this model is planned to be used should be less than 281 mV (however, 150 mV is considered the standard minimum for such amplifiers).
Note that the output level of MM cartridges is noticeably higher than that of MC cartridges, so the gains for them are generally lower (although there are exceptions).
Input load capacity (MM)
The input capacitance of the phono stage when working with an MM type pickup (see "Intended use").
This parameter determines compatibility with a specific pickup. The selection rule in this case is as follows: the total capacitance of the phono stage and connecting wires must correspond to the capacitance of the “head”, in extreme cases, at least not exceed it (otherwise the sound will deteriorate noticeably). In order to simplify this matching, it may be possible to adjust the
input capacitance(see "Adjustments").
Input sensitivity (MM)
Input sensitivity of the phono stage when working with an MM type pickup (see "Intended use").
Input sensitivity is the lowest root mean square (RMS) signal level at the input, at which the phono stage is able to “perceive” this signal normally, process it correctly and amplify it. Accordingly, compatibility with the pickup directly depends on this indicator: the signal level from the “head” must not be lower than the input sensitivity of the phono stage.
Note that
MM pickups are noticeably superior to MC pickups in terms of signal power, so the sensitivity threshold for correctors for them is relatively high — usually about 4 – 5 mV. At the same time, there are correctors with a higher "threshold of perception" — more than 100 mW; they must be connected through special step-up transformers or other similar equipment.
