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BOOKS AND ARTICLES
What is RIAA, MM and MC
The combinations of letters in the title will surely evoke pleasant nostalgic memories from experienced fans associated with the end of the 70s - the time of the peak of popularity of vinyl records and equipment for their playback. Since interest in this technique has recently reappeared, we decided to talk a little about it. So what is hidden behind the message, mysterious for many modern users, that this device has an RIAA equalizer input with the ability to connect MM and MC heads and a RUMBLE or SUBSONIC filter? What do these abbreviations and names mean? Let's start with history. The prototype of modern recording was the invention 120 years ago almost simultaneously by the Frenchman Charles Crome and the American Thomas Edison of the method of mechanical sound recording. Moreover, Krom invented a method of recording, proposing to apply this groove in the form of a spiral on a rotating disk or cylinder, while the recording cutter oscillated along the track. Such a record is called transverse. Edison proposed a method of deep recording, in which the cutter oscillated perpendicular to the surface of the carrier, that is, in depth. Manufacturability, purity of reproduction and ease of replication of transverse recording made it the main method of recording (although later both methods were used to create a stereo record). Later, the German scientist Emil Berliner invented the gramophone. This device had a reproducing needle, a membrane, its holder or tone arm, and an acoustic signal amplifier or horn. A similar principle is used in modern record players. Berliner also invented a way to mass-produce records, proposing to use a galvanic method for making copies. As a material for records, Berliner proposed a composition whose main component was shellac resin, and the first record was made in 1897 by Victor in America. This very successful way of sound recording was constantly improved and reached its peak in the 70s and 80s. But this was already a kind of swan song of mechanical sound recording, since at the same time a new sound carrier, the CD, began its victorious march. In parallel with the improvement of the method of mechanical recording, there was an improvement in playback devices. Both are very closely related and have their own characteristics, determined by each other. Long-playing records are made on the basis of vinylite resins and, with very perfect characteristics, have a number of technological features that determine the required parameters of the preamplifier. In order to limit the large amplitude of vibration of the recording cutter, to prevent loss of contact with the groove, and also in order not to reduce the recording density, the lower frequencies are attenuated. And to overcome the inertia of the cutter and reduce the noise level, high frequencies are amplified. To obtain the original sound, that is, a linear resulting frequency response, the frequency response of the playback channel of a recording, of course, must have an inverse form. The specific levels of rise and fall of these characteristics during recording and playback of a record depend on its rotational speed, of which there have been four throughout history: 77,92; 45,11; 33,33 and 16,66 rpm, but currently only 33,33 is used. Introduced back in 1953, the RIAA standard (by the first letters of the name Record Industry Association of America) set standards for frequency dependence in the range of only 30-15000 Hz. Improving the quality of sound recording and reproducing equipment, as well as the need to normalize the frequency response in the frequency range below 30 Hz, led to the creation in 1978 of a corrected and supplemented version of the standard, called RIAA-78. In 1963, the characteristic was recommended as a standard by the International Electrotechnical Commission (IEC) and entered the national standards of most countries, including the most famous DIN45500 in Germany or GOST 7893-79 in our country. As a rule, modern amplifying devices have a corrector or, as they say, a recording characteristic equalization circuit using RIAA recommendations. How many copies were broken in disputes about corrector circuitry, the best options of which can now only be found in High End devices! In a standard Hi-Fi, if you can find such a part of the circuit, then most likely it will be built around a simple dual opamp, albeit with decent parameters. As can be seen from the RIAA equalizer reproduction frequency response, low frequency signals receive a significant boost compared to mid frequencies. The difference at the frequency boundary reaches 20 dB. If the quality of the playing device is not high, this naturally also leads to increased interference from vibrations of the moving mechanism of rotation of the record, which manifests itself in the form of an unpleasant hum, and if the player’s body is not sufficiently decoupled from the speakers, it can cause acoustic feedback. To prevent all these troubles, a special infra-low frequency filter is included in the corrector circuit or behind it, which provides a decrease in gain at frequencies below 20-30 Hz with a large steepness (12-18 dB / octave) - the so-called RUMBLE or SUBSONIC filter. The improvement of recording also went along the path of improving pickups - converters of the mechanical vibrations of the needle into electrical signals. In a relatively short period of time, this device has turned from a simple needle for a phonograph or gramophone (by the way, made not only from metal, but also from special types of wood) into very complex electronic-mechanical products, many of which can be considered as a kind of work of art. Without going into complicated technical explanations, we can say that modern pickup heads differ in the principle of converting the mechanical vibrations of the needle into an electrical signal and the physics of the conversion process. Those pickups that interest us - magnetic heads - are divided into heads with a moving magnet (moving magnet), or MM-heads, and heads with a moving coil (moving coil), or MC-heads. They have the following advantages: a wide range of reproducible frequencies (up to 45-50 kHz with an unevenness of 1-2 dB), low downforce (less than 15-20 mN), great flexibility of the moving system, good envelope of the record groove, sufficient separation between stereo channels, significantly less non-linear distortion, and most importantly - an incomparably better sound. The existing disadvantages - low sensitivity, the need for a corrective amplifier, sensitivity to magnetic fields - are more than offset by their advantages. The main differences in their device are as follows. In MM-heads, a permanent magnet is mechanically connected to the needle, and the electromechanical transformation is carried out due to the vibrations of the needle and the permanent magnet associated with it relative to the fixed inductors. And in the MC-head, inductors are mechanically connected to the needle and the conversion is carried out when they oscillate relative to a fixed permanent magnet. Technological limitations that arise in the manufacture of these heads determine their comparative advantages and disadvantages. In the MM head, the disadvantage is the need to use a magnet with high magnetic energy and low mass to increase the sensitivity of the head and its upper cutoff frequency, and the resulting non-linear distortion is higher than that of MC heads. But a very important advantage of the MM head is the ease of replacing a worn needle. The advantage of the MC head is the small mass of the moving system (and therefore good flexibility) with a small amount of non-linear distortion, and the disadvantages are low sensitivity (due to the limited number of coil turns) and, more importantly, the impossibility of replacing a worn needle. In addition, the cost of MC heads is significantly higher than MM heads. But in systems where the main thing is sound quality, an MC head is more preferable. The currently observed revival of interest in the forgotten "vinyl" has some objective prerequisites. According to many experts, the digital sound processing in the CD gives it a too "cold", as they say, digital sound, in contrast to the phonograph record, which is considered to sound warmer and more natural. The quality of both the records themselves and the playing devices is also growing. And in the production programs of a number of well-known companies, a new direction has appeared - the exact reproduction of successful models of previous years. For example, Marantz has released an exact copy of the famous Model 7 preamp, which has three correction standards: RIAA, Old Columbia LP and 78 rpm.
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