Comparison Pro-Ject Phono Box USB vs Cambridge Azur 651P

The range of sound frequencies reproduced by the phono stage. When evaluating this parameter, it is worth keeping in mind several important points. Firstly, in this case, not the entire bandwidth is taken into account, but the range in which the uneven frequency response of the phono stage (according to the RIAA standard) does not exceed the value specified by the manufacturer. Secondly, the human ear is only able to perceive sound within the range of 16-20,000 Hz (in some people, up to 22,000 Hz). Ideally, the audio system should cover this entire range — otherwise, some of the low and/or high frequencies may be cut off; but to provide a wider range from a practical point of view does not make sense. Nevertheless, there are phono stages with quite solid performance, for example, 5 – 35,000 Hz. These specifications are usually a "side effect" of high quality audio circuits and are advertised by the manufacturer for promotional purposes.

Another point to consider when choosing is the frequency range of other components of the audio system. For example, it is hardly worth looking for a phono stage with a lower threshold of 16 Hz, if the power amplifier only supports frequencies from 40 Hz and higher.

As a note to this clause, the RIAA ripple present in the frequency range may be indicated. In most models, it is from ±1 to ±6 dB; the lower the value, the smoother the frequency response and the better the sound.

The dynamic range of a phono stage is the difference between the maximum signal level it can produce and the maximum level of self-noise (which inevitably occurs in any electronic audio device). In a sense, this parameter is similar to the signal-to-noise ratio (see below), but in this case we are talking about the difference between the maximum signal and noise levels, and not between the average ones.

The greater the dynamic range, the more advanced the phono stage is considered (ceteris paribus). In top models, this figure can exceed 90 dB and even 100 dB. We only note that in universal devices (see "Intended use") the ranges for MM and MC may differ — the first is usually higher, and it is given in the characteristics.

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.

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.

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.

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.

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).

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 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.