Beogram 4000c Technical Sound Guide Bang & Olufsen A/S This manual is for information purposes only and is not legally binding. May 21, 2021 Contents 1 History 4 1.1 The mechanical phonograph . 4 1.2 Magnets and Coils . 4 2 The physics 5 2.1 Amplitude vs. Velocity . 5 2.2 Surface noise . 5 2.3 Mono to Stereo . 6 3 The cartridge, stylus, and tonearm 8 3.1 MMC: Micro Moving Cross . 8 3.2 Signal Levels . 9 3.3 Tip shape . 11 3.4 Bonded vs. Nude . 13 3.5 Tracking force . 13 3.6 Effective Tip Mass . 14 3.7 Compliance . 14 3.8 Soundsmith SMMC20CL . 15 3.9 Tangential Tracking . 15 4 Audio Specifications 17 4.0.1 Magnitude Response . 17 4.0.2 Rumble . 17 4.0.3 Rotational speed . 17 4.0.3.1 Accuracy . 17 4.0.3.2 Stability . 17 4.0.3.3 Measurement and Weighting . 18 4.0.3.4 Expressing the result . 19 5 Reading the measurement datasheet 20 5.1 Measurements from Vinyl . 20 5.1.1 Magnitude Response . 20 5.1.2 Channel Matching . 21 5.1.3 Crosstalk . 21 5.1.4 Rumble . 21 5.1.5 Wow and Flutter . 21 5.1.6 Frequency Drift . 22 5.2 RIAA Preamplifier . 22 5.2.1 Magnitude Response Deviation . 22 5.2.2 Channel Matching . 22 6 Beogram 4000c Specifications 24 7 Further Reading 25 2 3 History 1.1 The mechanical the groove became smoothed.2 bi-directional. When current is sent through a wire, a magnetic field is phonograph It was ten years later, in 1887, that the generated around it. The greater the German-American inventor Emil current, the stronger the magnetic In 1856, Édouard-Léon Scott de Berliner was awarded a patent for a field, and the more it extends outwards Martinville invented a device based on sound recording and reproducing around the wire. However, it is also the basic anatomy of the human ear. It system that was based on a groove in true that moving a wire through a consisted of a wooden funnel ending at a rotating disc (rather than Edison’s magnetic field will generate a current a flexible membrane to emulate the cylinder); the original version of the that is proportional to its velocity. In ear canal and eardrum. Connected to system that we know of today as the other words, the faster the wire moves the membrane was a pig bristle that “Long Playing” or “LP” Record. through the magnetic field (or the moved with it, scratching a thin line magnetic field goes past the wire), the into soot on a piece of paper wrapped greater the current. around a rotating cylinder. He called this new invention a “phonautograph” or “self-writer of sound”. Figure 1.2: An Edison “Blue Amberol” record with a Danish 78 RPM “His Mas- ter’s Voice” disc recording X8071 of Den Blaa Anemone. Early phonographs or “gramophones” Figure 1.1: The phonautograph. were purely mechanical devices. The disc (or cylinder) was rotated by a This device was conceived to record spring-driven clockwork mechanism sounds in the air without any intention and the needle or stylus rested in the of playing them back, so it can be passing groove. The vibrations of the considered to be the precursor to the needle were transmitted to a flexible modern oscilloscope.1 However, in the membrane that was situated at the late 1870’s, Charles Cros realised that narrow end of a horn that amplified the if the lines drawn by the resulting sound to audible levels. phonoautograph were photo-engraved onto the surface of a metal cylinder, 1.2 Magnets and Coils then it could be used to vibrate a needle placed in the resulting groove. Unfortunately, rather than actually In 1820, more than 30 years before de build such a device, he only wrote Martinville’s invention, the Danish about the idea in a document that was physicist and chemist, Hans Christian filed at the Académie des Sciences and Ørsted announced the first link made sealed. Within 6 months of this, in between electricity and magnetism: he 1877, Thomas Edison asked his had discovered that a compass needle assistant, John Kruesi, to build a device would change direction when placed that could not only record sound (as an near a wire that was carrying an indentation in tin foil on a cylinder) but electrical current. Nowadays, it is reproduce it, if only a few times before well-known that this link is 1It should be said that some “recordings” made on a phonoautograph were finally played in 2008. See www.firstsounds.org for more information. 2see “Reproduction of Sound in High-fidelity and Stereo Phonographs” (1962) by Edgar Villchur 4 The physics curves when they pass Time = 0 ms, you’ll see that the higher the 1 0.8 frequency, the higher the slope of the 0.6 2.1 Amplitude vs. Velocity line, and therefore the higher the 0.4 0.2 velocity of the stylus. 0 It is this second interaction that is at -0.2 Displacement (mm) -0.4 the heart of almost every modern -0.6 1 1 -0.8 turntable. As the stylus (or “needle” ) 0.8 -1 0.6 is pulled through the grove in the vinyl -10.0 -5.0 0 5.0 10.0 0.4 Time (ms) surface, it moves from side-to-side at a 0.2 varying speed called the modulation 0 Figure 2.3: Three sine waves of different velocity velocity -0.2 or just the . An Displacement (mm) frequencies (from low to high: blue, red -0.4 example of this wavy groove can be -0.6 and orange curves), but with the same -0.8 modulation velocity. seen in the photo in Figure 2.1. Inside -1 the housing of most cartridges are -10.0 -5.0 0 5.0 10.0 Time (ms) small magnets and coils of wire, either The solution for this limitation was to of which is being moved by the stylus Figure 2.2: Three sine waves of different use low-frequency audio compensation frequencies (from low to high: blue, red as it vibrates. That movement filters, both at the recording and the and orange curves), but with the same generates an electrical current that is amplitude. playback stages. When a recording is analogous to the shape of the groove: mastered to be cut on a disc, the low the higher the velocity of the stylus, frequency level is decreased; the lower the higher the electrical signal from In order to achieve a naturally flat the frequency, the lower the level. This the cartridge. frequency response from the cartridge, results in a signal recorded on disc with where all frequencies have the same a constant amplitude for signals below electrical output level, it is necessary approximately 1 kHz. to ensure that they have the same modulation velocity, as shown in Figure Of course, if this signal were played 2.3. In that plot, it can be seen that the back directly, there would be an slopes of the three waves are the same increasing loss of level at lower and at Time = 0 ms. However, it is also lower frequencies. So, to counteract evident that, when this is true, they this, a filter is applied to the output have very different amplitudes: in fact, signal of the turntable that boosts the the amplitude would have to double for low frequencies signals to their original Figure 2.1: The groove in a late-1980’s levels. pop tune on a 33 1/3 RPM stereo LP. The every halving of frequency (a drop of 1 white dots in the groove are dirt that octave). This is not feasible, since it should be removed before playing the would mean that the stylus would have disc. 2.2 Surface noise to move left and right by (relatively) huge distances in order to deliver the A second problem that exists with vinyl However, this introduces a problem desired output. For example, if the records is that of dust and dirt. If you because if the amplitude remains the stylus were moving sideways by 0.05 mm at 1,000 Hz to deliver a look again at the photo in Figure 2.1, same at all frequencies the modulation ± you can see white specks lodged in the velocity of the stylus decreases with signal, then it would have to move 0.5 mm at 100 Hz, and 5 mm at groove. These look very small to us, frequency; in other words, the lower ± ± however, to the stylus, they are very the note, the lower the output level, 10 Hz to deliver the same output level. large bumps that cause the tip to move and therefore the less bass. This is This is not possible (or at least it’s very abruptly, and therefore quickly. Since illustrated in the graph in Figure 2.2 in impractical). the output signal is proportional to the which three sine waves are shown with modulation velocity, then this makes different frequencies. The blue line the resulting cracks and pops quite shows the lowest frequency and the loud in relation to the audio signal. orange line is the highest. Notice that all three have the same amplitude (the In order to overcome this problem, a same maximum “height”). However, if second filter is used, this time for you look at the slopes of the three higher frequencies. Upon playback, the 1Some authors reserve the term “stylus” for the device that is used to cut the groove during mastering, and the term “needle” for the device used to play a phonographic record.