Digital Amplifiers

To understand how a digital amplifier works, it is first important to understand how a classic linear amplifier works. Basically, an audio amplifier is an electronic device that takes a small signal audio inputs, usually in millivolt range and amplifies them into several watts strong enough to drive a speaker load.

Most audiophiles and enthusiasts have grown up with at least a basic understanding of what an amplifier does. It takes a tiny alternating electrical signal that represents the moment-to-moment variations of musical frequencies and their amplitudes (volume levels), and increases their strength many times so they're powerful enough to drive the cones and domes of speakers back and forth to generate air pressure variations (waves), which replicate the original sound waves.

Musical tones vary as slowly as 16 times per second (16 Hz)--a very low pipe-organ note--to as fast as 15,000 times per second (15 kHz) or more--the highest harmonics of a cymbal or a violin, for example.

The audio amplifier was invented in 1909 by Lee De Forest when he invented the triode vacuum tube. The triode was a three terminal device with a control grid that can modulate the flow of electrons from the filament to the plate. The triode vacuum amplifier was used to make the first AM radio.



Early audio amplifiers were based on vacuum tubes (also known as valves), and some of these achieved notably high quality (e.g., the Williamson amplifier of 1947-9). Most modern audio amplifiers are based on solid state devices (transistors such as BJTs, FETs and MOSFETs), but there are still some who prefer tube-based amplifiers, due to a perceived 'warmer' valve sound. Audio amplifiers based on transistors became practical with the wide availability of inexpensive transistors.

Shown below is my QUAD II valve amplifier which I treasure so much, it's been my pleasure to own such a great amplifier.





Most modern audio amplifier components are transistor based, shown below is the Marantz PM7001 Integrated amplifier.



The true measure of amplifier output is based on its RMS power and not in PMPO, PMPO rating tends to fool consumers who doesnt know anything about electronics. I won't go into details about that, just remember that when buying amplifiers, ask for the RMS rating and not the advertised PMPO rating.

[sinusoidal signal]



Amplifiers discussed above are classified as 'analog' amplifiers, that is, they accept audio signal in sinusoidal form. The input signal is amplified at the output to produce the same signal, only amplified. Deviation of the output signal from the input signal is called distortion, most amplifier manuals specify THD on the specs measured in %THD. For high fidelity, this is usually expected to be < 1% .

In Short, What In The World Is A Digital Amplifier ?

Unlike analog amplifiers, digital amplifiers accepts digital audio signals for processing directly from the CD or DVD player's laser pick-up. Most digital amplifiers has a build in PWM decoders and digital to analog converter integrated circuits.



What's inside the Optical Disc

Optical discs contains pits and bumps representing digital signals. These signals are read by a laser beam and fed into the digital output terminals. Modern optical disc players usually has a digital output, either optical or coaxial. Coaxial digital outputs uses the standard RCA jack to connect it to a digital amplifier, whereas optical digital outputs uses S/PDIF fiber optics links, or TOSLINK.

[I'm using an Audio-Pro fiber optic cable]




[The back panel of my Pioneer DV-300 DVD/CD player with both optical and coaxial digital output shown]



I am using Pioneer VSX 518K as a digital amplifier, its has both coaxial and optical digital input.




[back of VSX 518K showing coaxial digital input, assignable for CD/DTS digital input signal]



[The optical digital input, also assignable for CD and DTS]



[My DV-300 coupled to VSX518 K via fiber optic link]

The MiniDisc

A MiniDisc (MD) is a magneto-optical disc-based data storage device initially intended for storage of up to 80 minutes of digitized audio. It's basically similar to a CD but smaller and enclosed in a shell. Shown below is the MZ1, the first minidisc player manufactured by Sony. See IEEE Specs here.




A recordable MiniDisc is capable of storing 140 megabytes of information (it records in ATRAC format). Music can be scattered all over the disk and the player can "put it together" correctly when playing the disk, just like those data in your hard drive. This means that you can erase and re-record songs on a MiniDisc without having to worry about how they fit together. This is tremendously convenient compared to a cassette tape, where you have to basically re-record the entire tape if you want to change any of the songs on it. There are also 4-track MiniDisc recorders for musicians, which are great for recording songs as they are performed and then mixing the tracks.

Even though both the MiniDisc format and the ATRAC codec were Sony inventions, the company was more than happy to license the technology to other manufacturers. This resulted in a plethora of MiniDisc hardware from all the big names in consumer electronics, although the level of choice in Europe was still tiny compared to the model line-up available in the Far East.

Unfortunately for Sony, the market had moved on by this time and MP3 players had already pretty much put MiniDisc in its grave. Sad, I really loved this format until today.


My Minidisc Player/Recorder (SONY MZ-R37)

One of my favorite audio gadget is my Minidisc player/recorder. One good thing is that you can still buy blank disc, there's a specialty store in Harrison Plaza that sells professional audio equipments and peripherals. Shown below is a blank disc and on the foreground is sample disc which I have removed from the shell.



The front side contains all the controls such as play, fast forward, rewind, AMS, record, volume controls and the eject switch (rightmost).



The upper lid accepts the disc, you can buy blank MDs and record all your favorite selections or the pre recorded ones but I doubt if it's available locally.



The left side of the player contains the optical digital input jack (white), this jack automatically detects whether the input signal is digital or analog. Next to it is the analog line-out (black) which you can directly hookup into a pre-amp or an audio amplifier. The elongated terminal is for the remote controller RM-MZ37. The small button on the right is the hold switch, it disables all operations when switched to "hold".



The player with the remote controller RM-MZ37.



The underside (shown here, still without scratches) contains the MEGABASS button, the edit/display button, mode (record/play) and the T-mark if in case you want to place labels on the recorded materials. It also has a synchro record switch that automatically buffers the input signal once it is detected and start recording.

Phonographs, phonographs

The Phonograph

On December 4, 1877 Thomas Edison became the first person to ever record and play back the human voice.

The technology that led to the phonograph came from developments that Edison made in the telegraph and telephone. Edison at the time was experimenting with how a moving diaphragm linked to a coil, could produce a voice modulated signal. Meanwhile, he was also experimenting with a telegraph repeater which was simply a device that used a needle to indent paper with the dots and dashes of the Morse code.

Out of these two ideas, came the concept of attaching the stylus from a telegraph repeater to the diaphragm in the mouthpiece of a telephone. The first test in July of 1877, involved a sheet of paper pulled under the needle mechanically coupled to a diaphragm, as he shouted into the mouthpiece..... Sadly, It didn't work..... though it did produce an unrecognizable sound which was luckily, just enough to prove the concept and spark intense interest in developing a solution to the problem of recording the human voice. (Had he only imagined at the time what it would all lead to !)

For the next year Edison and his staff worked on the solution. Tin foil replaced paper, and thus tin foil became the first viable recording media. A band of tin foil was mounted on a cylinder, and the cylinder was turned via a hand crank during the recording and the playback. Edison turned the crank and spoke the first recorded words. "Mary had a little lamb, its fleece was white as snow, and everywhere that Mary went, the lamb was sure to go." Of course, the rest is history !

How Modern Phonographs Works

Phonographs play records made by analog disc recordings. An analog (which means likeness) of the original sound waves were stored as jagged waves in a spiral groove on the surface of a plastic disc. As the disc went around on the phonograph, a needle called a stylus moved along the groove. The waves in the groove caused the stylus to vibrate. These vibrations then were turned into electric signals that were changed back into sound by speakers.



Cartridge Types

In early high-fidelity systems, crystal and ceramic pickups were used. They have been replaced by the magnetic cartridge, using either a moving magnet or a moving coil. Magnetic cartridges provide much lower tracking forces (and thus damage the record much less). They also have a much lower output voltage than a crystal or ceramic pickup, in the range of only a few millivolts, thus requiring greater amplification. At this point in the hi-fi stereo system it is considered pre-amplification.




a) Moving Magnet (MM) cartridges


In a moving magnet cartridge, the stylus cantilever carries a tiny permanent magnet, which is positioned between two sets of fixed coils (in a stereophonic cartridge), forming a tiny electromagnetic generator. As the magnet vibrates in response to the stylus following the record groove, it induces a tiny current in the coils. Because the magnet is small and has little mass, and is not coupled mechanically to the generator (as in a ceramic cartridge), a properly adjusted stylus follows the groove far more gently and faithfully, requiring less tracking force.


b) Moving Coil (MC) cartridges


Moving coil systems are extremely small precision instruments and are generally expensive. The MC design is again a tiny electromagnetic generator, but (unlike a MM design) with the magnet and coils reversed: the coils are attached to the stylus, and move within the field of a permanent magnet. The coils are tiny and made from very fine wire, so are even lighter than the small magnet used in an MM cartridge, thus improving the tracking ability of the cartridge. This can give extended frequency response as well as greater fidelity. A disadvantage however is that moving-coil cartridges generate an even smaller signal than an MM cartridge, because the moving coil cannot be large enough (too heavy) to generate equivalent voltage levels. The resulting signal is only a few hundred microvolts, and thus more easily influenced by noise, induced hum, etc. It is challenging to design a preamplifier with the extremely low noise inputs needed for MC working, so a "step up transformer" is often used instead.



c) Crystal and Ceramic cartridges

Crystal and Ceramic cartridges work on the piezoelectric effect principle. Huh? It is what is known as a transducer, basically turning mechanical motion into voltage. The piezoelectric effect was discovered by Pierre Curie in 1883. When mechanical pressure is applied to certain types of crystals, the properties of the crystals causes them to produce voltage.Certain crystals, ceramic materials, Rochelle salt, and quartz exhibit this effect.The stylus is attached to a cantilever which is attached to these crystals, ceramic materials,or salts inside the cartridge body. When it vibrates it applies the mechanical pressure or motion which in turn produces the voltage. With that said, at the other end of your stereo is the cartridge in reverse. The loudspeaker is a transducer in that it converts voltage into mechanical motion. So we have the cartridge converting mechanical motion (the stylus being drug around in the groove of the record) into voltage, the pre-amplifier and amplifier processing the voltage so the loudspeaker can understand it and reproduce this whole mess into those high fidelity sounds that we all love to hear.

Digital Turntable, Analog Vinyl Records

During the 1980s, I have this huge collections of vinyl records, several dozens of them. The 80s is quite a radical year when the latest advancement in audio equipments sky rocketted. When CDs dominated the audio software storage in the mid 1990s, those precious vinyls have been collecting dusts in my store room.



Until one day, 19 years later, I have spotted this usb digital turntable on the internet. This turntable is quite unique for it has a USB output that you can hook up to your laptop or PC (or Mac).



It has a bundled software that allows you to convert your analog audio into the digital realm in the mp3 (oh my!) format, that is, in real time. I am not really into mp3 because it's a lossy compression system, and therefore not considered by many audiophiles as a high-end materiel. The reason why I bought this digital turntable is for me to savor that good ole analog audio sound, it's warmth and all of its hiss and rumbles as well (hahahaha). But hey, all my records are in near mint conditions, I take good care of them.

Out of the box, the unit contains the turntable itself, the bundled software/driver (green CD) and the USB cable shown on the top of the dust cover.



Further examining the unit, the electrical plug is quite of Australian standard, so I went to some local hardware stores and tried to look for an adaptor



The back of the unit contains a USB plug for connecting this unit to the PC



The complete setup is shown below, laptop is also connected to the VSX series Altec Lansing speaker systems (subwoofer under the table). Or if you want, you can just simply insert a stereo headphone on the headphone jack of the PC. I am using Audicity software for running the unit, with Audacity, you can convert to MP3 or WAV.



The Digital Turntable in action:



The verdict, if I could find a USB turntable with a magnetic cartridges, either MC (Moving Coil) or MM (Moving Magnet), I would be more than glad to try and buy it. This phonograph uses a crystal ceramic cartridge common to cheap priced turntables. It has one advantage though, it doesnt need an RIAA preamp, unlike the magnetic cartridges. Nevertheless, sound is still good. More about phonographs and cartridges here.