I've written several articles discussing system noise, but this well written paper by Jim Hagerman zeroes in on noise problems encountered in vinyl playback systems. With his permission we have re-posted the article on our website.
Through his company, Hagerman Audio Labs, Jim offers a wide range of excellent products and accessories geared toward vinyl reproduction. Enjoy the article and head on over to Hagerman Audio Labs and check out his products! - Galen Carol
Reducing or eliminating background noise to acceptable levels in vinyl playback systems can often be an arduous and difficult task. Even the quietest of phonostages can buzz like a bee if not installed properly. Fortunately, such sources of noise can be found and removed via the implementation of good practices.
The first thing to keep in mind is that signal levels from a cartridge (turntable) are typically a thousand times smaller than that from a CD player or DAC. This is why noise problems are so widespread. A typical Moving-Magnet phonostage will have a gain of 40dB at 1kHz. In layman terms, that’s a gain of 100. However, due to RIAA equalization, the gain down at 60Hz (power line frequency) is ten times greater, or 1000. An input disturbance of 0.000005V becomes audible. With Moving Coil cartridges the problem is again ten times worse. So what can we do? Simple, identify the possible culprits and suggest possible corrections. In many systems there is more than one issue, so eliminating noise is like peeling an onion.
There are many sources of noise, the most common being power line disturbances. Noise can be injected either via magnetic induction or from electric fields. And not just into the phonostage; noise can be coupled into cables, tonearm, or the cartridge itself.
The following table lists various noise issues and what generally causes them. It only covers amplified electrical noises – not for example, the audible buzzing coming from a power transformer (loose screw or DC on primary) or the high-pitched tone emanating from a wall wart.
|Hiss||Normal. This is what we are trying to achieve. In a good system the level of electrical hiss is lower than that of groove noise.|
|Tic||Record scratch, of course. Phonostages employing feedback tend to accentuate this, as the excessive slew rate will force an open loop condition, from which the amplifier must then recover.|
|Crackle||Bad electrical connection. This can be caused by a loose-fitting connector, cable, or socket (especially tube). Can also be a broken solder joint.|
|Thump||An impulse signal caused by power-up or power-down of electronics.
Mechanically either footsteps or dust cover rumble.
|Hum||Magnetic induction. Typically caused by a nearby power transformer, with low frequency fields coupling to wire loops that effectively act as secondary windings. Surprisingly this is caused by too many grounds. There are many invisible loops created via ground wires, interconnect returns, and power cords. Hum can also be caused by a missing ground, but that situation tends to manifest as buzz.|
|Buzz||Missing ground wire. Common in audio systems that have no connection to Earth ground (usually via a 3-prong plug).|
|Pulsing||WiFi, cell phone. Modern digital wireless communications can couple into audio systems, as they transmit in low frequency bursts.|
|Whine||Nearby motor. Refrigerators or other motorized appliances can couple noise when plugged into the same power branch.|
|Sizzle||Insulation breakdown, often within a tube. Typically associated with higher voltages.|
|Voices||RF, radio interference. This is more common with solid-state phonostages, as parasitic semiconductor rectification of radio signals at the input stage performs demodulation. Metallic shielding can help, especially with input cables, where coaxial types are superior.|
|Hollowness||One channel inverted. Soundstage disappears.|
|Popcorn||Static discharge damage to semiconductors. Usually on inputs to operational amplifiers|
|Howl||Acoustic feedback. Speaker too close to turntable or low quality shelf.|
|Muddiness||Too much resistive loading. MM cartridges need loading from 47k Ohm to 100k Ohm.|
Too much capacitive loading on cartridge. It is an old rule of thumb to add capacitance resonating with cartridge inductance, thereby causing a peak in treble frequencies in an attempt to increase bandwidth. Unfortunately, smearing and sonic degradation caused by the capacitor itself outweigh any benefit. It is a far better approach to reduce total
Trouble shooting can be difficult, especially if there is more than one problem to uncover. Fortunately there are a number of tricks you can employ to ferret out perpetrators.
Adding a turntable to an audio system should not be an afterthought. It may be prudent to reassemble everything from scratch, giving primary consideration to the analog front end. If not, you may be forever chasing noise.
Start by placing the turntable and phonostage far away from power amplifiers and speakers. Give them a solid platform. Plug all source electronics into the same outlet using a power strip or line filter. Keep turntable interconnects (and ground wire) short and separated from other wiring (do not bundle). Continue building up the system in this manner, following signal path from start to finish. Distance is your friend. Keep noise sources as far away from front-end analog electronics as possible.
- Jim Hagerman, Hagerman Audio Labs