Difference between revisions of "Turntable (Vinyl)"
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Latest revision as of 18:10, 8 September 2006
Turntables vary widely in price, construction and age. Audiophiles will categorize the subjective qualities of a turntable on the following metrics:
The plinth is the base of the turntable, on which the platter (and usually the motor) rests. The basic function is to isolate the components mechanically from the base of the turntable and from each other.
Construction styles vary:
- No isolation whatsoever (solid box). For very cheap turntables.
- A box, often wooden, holding a metal plinth supported by springs. Usually found only on vintage turntables.
- A plinth of multiple levels, each level isolated from the other by some rubber-like substance. In some form or another most audiophile turntables use this method.
- Physical separation of the turntable components from each other, and individual isolation of each component from the ground. Certain very expensive turntables use this method.
There are few objective criteria for plinth design. Isolation issues can be resolved through the use of additional isolation placed underneath the turntable. Build quality is important and may be lacking, even for audiophile turntables.
The stick-shaped part of the turntable.
- They hold the cartridge on top of the record, keeping it properly aligned, and carry the wires down to the plinth.
- A counterweight is placed on the back to reduce the force of the needle on the record to very low levels, usually equivalent to an effective mass of roughly one gram.
- A force, the anti-skate force, is applied to the tonearm, pulling it outwards from the center of the disk, to counteract forces during playback that pull it towards the center.
- The tonearm must be lowered slowly onto the disk to prevent damage to the disk or the stylus. All quality turntables include a mechanism, usually hydraulic, to slowly lower the tonearm.
Key differences between tonearms include:
- Build material
- Shape (S-shaped or straight)
- Linear tracking or angular tracking
- Degrees of freedom on the mounting
- Antiskate force mechanism
There are several objective differences between tonearms:
- The mass of the tonearm affects the resonances of the playback system. The mass can sometimes be incompatible with the mass of the cartridge and the compliance of the stylus, leading to an excessively high or low resonant frequency that induces skipping or rumble, or can negatively affect low frequency performance.
- Tracking errors, representing angular deviations between the stylus and the record groove, are present with every tonearm, and different designs use different approaches. Linear tracking arms, in theory, have zero tracking error. In practice they require a motor to move the tonearm, with some known error in correct positioning, as well as some vibration. S-shaped tonearms are shaped to reduce tracking error but (XXXX list issues).
- XXX high frequency effects?
- Counterweight markers are notoriously unreliable and a scale is often employed to get a correct measurement of the counterweight setting.
The platter is the rotating part which supports the vinyl record while it is played. The platter should ideally be acoustically dead, ie. it should not "ring" like a bell. Platters that ring store energy and release it slowly back into the record, causing loss of fine detail.
Belt drive turntables use a small motor turning fairly rapidly, and a belt then transfers the power from the motor spindle to the platter. On some models the motor sits outside the platter and the belt runs around the outer edge of the platter; on others the platter is in two parts with an inner spigot for the belt, with the motor sitting between the spigot and outer edge of the platter. There is no significant advantage to either arrangement, with the possible exception that when the motor is external to the platter it can be sited a long way from the path of the pickup cartridge, and therefore potentially reduce the danger of hum pickup by the cartridge from the motor.
The belt (usually some form of rubber or similar material) provides excellent decoupling so that motor bearing noise is not transferred into the platter (and thence to the LP).
The motors used are usually synchronous AC motors whose speed is determined by the frequency of the mains supply. Some more expensive belt drive turntables include a sophisticated power supply which generates an AC signal even cleaner than that provided by the mains.
Since the motors on belt drive turntables do not have much torque, and because the AC frequency might vary slightly, stable rotation of the platter is achieved by the use of high mass platters that provide a "flywheel" effect to smooth out any variations. A consequence of using a high mass platter and a low torque motor is that start-up speeds are rather slow, and hence belt drive turntables are not really suitable for broadcast or DJ use.
In a direct drive turntable, the central spindle in the platter is an extension of the axle. So the motor itself sits directly underneath the platter, and rotates slowly at the required vinyl record playback speed (ie. 33.3/45/78 rpm). Such motors are servo-controlled DC motors, and the turntable includes some means of measuring the rotation speed of the platter so that it can adjust the motor's speed to keep it steady. In order that any speed inaccuracies can be adjusted quickly enough, the motor must have high-torque; this also means that startup speeds are very rapid, hence the preference for direct drive in broadcast and DJ applications.
Since the platter is directly attached to the motor, it is not well decoupled from any bearing noise. However, since the motor turns at a slow speed, the bearing noise is very low frequency and therefore will not cause audible degradation.
Idler Wheel Drive
This type of drive mechanism was traditionally used on cheap autochangers, and involves a small motor running at fairly high speed (like in a belt drive turntable). But unlike a belt drive turntable, the power is transferred to the platter by means of a wheel (usually rubber) sitting between the motor spindle and the platter edge. The wheel offers far less decoupling, and so motor noise is transferred to the platter. What's more, because the motor rotates at high speed, this noise can affect the audible frequency spectrum.
That said, there have historically been some successful high quality idler wheel drive turntables. Perhaps the most famous is the Garrard 301.
Internal Wiring and Power
The cartridge electrical system is entirely passive; no power is required for the electrical signal to be sent to the preamp. The signal output may be vulnerable to electrical interference because of the high impedance of the cartridge and the long length of wire to the preamp. The motor usually receives the AC wall power directly and converts to its own required power internally. Some higher end turntables use external power supplies.
Power wiring can be a cause of increased interference with interconnects, so try to keep power cables separate from other cables. If they must cross, cross them at 90 degree angles.
All prices are for new turntables. The used market is highly irregular but you can usually expect a 20-50% discount. All prices are sampled from online vendors and may not be realistic for purchase at local stores.
- $200 or less: Inexpensive belt-drive or direct drive turntables. Tonearm, cartridge and preamp are usually included. You'll find turntables in this price range at Best Buy/Circuit City with vintage stylings. Because these are the mass-market items, these will quite often have features like automatic loading, 78rpm operation, and strobe marks that are absent from more expensive turntables. The quality of these turntables is highly variable, and because most audiophiles do not spend much time in this range, reviews are hard to compare against more expensive models.
- $400 or less: Entry level audiophile/DJ turntables, and used mid-range turntables. Tonearm and cartridge are usually included; preamp is almost always not. Most $200-$400 turntables have related models that cost under $200 so they are grouped together here. Brands in this price range include Audio Technica, Technics, Teac, Numark, Stanton, Music Hall, Pro-Ject.
- $400-$1000: Most introductory-level audiophile turntables are at this price, as well as flagship DJ turntables such as the Technics SL1200. Other brands with models in this range include most of the brands in the $400 range and also Rega, Thorens, Sota and Goldring. Tonearms are usually included; cartridges are sometimes included; preamps are never included.
- $1000-$10000: The meat of the audiophile market. Most high-end turntables will be somewhere in this range. Most brands have something here, but brands that operate exclusively in this price range include Clearaudio, Bluenote, JA Mitchell, Origin, Roksan and SME. Cartridges, tonearms and preamps are usually priced separately.
- Over $10000: Esoteric audiophile models, including audiophile linear trackers and laser pickups.