Lighthouse Electric Tutorials

 

Understanding Phono Cartridges & Pre-Amplifiers

 

WHAT IS A PHONO PRE-AMPLIFIER?


Unlike line level audio sources like DVD/CD players, tape decks, FM tuners, mini disc, etc., which have output voltages anywhere from 0.7V to 2.5V, the output voltage from a magnetic cartridge installed in a turntable is very low (2-5mV), and requires additional amplification. This additional amp stage, the phono pre amp, was built into most older receivers and audio amplifiers, allowing direct connection of a turntable. However, since the CD entered the market (early 1980's), the vinyl records and turntables were declared obsolete. Audio equipment manufacturers slowly phased the phono pre amp out. Newer stereo equipment, including most mini-systems and home theater units, have no magnetic cartridge phono input. In order to connect a turntable, you need to first pass the signal thru a phono RIAA equalization pre amp.

WHAT IS RIAA EQUALIZATION?

RIAA equalization is a specification for the correct playback of vinyl records, established 1955 by the Recording Industry Association of America (RIAA). The purpose of the equalization is to permit greater playback times, and improve sound quality. Before that specification, each record company applied its own equalization. Some better known were AES, LP, NAB and FFRR. RIAA equalization is a form of pre emphasis (in the recording process), and deemphasis on playback. A record is cut with the low frequencies attenuated and the high frequencies boosted. On playback the opposite occurs with the help of the equalization pre amp. The result is a flat frequency response with noise reduction, such as hiss and cracks.

WHAT ARE THE DIFFERENCES BETWEEN PHONO CARTRIDGES?

Most common cartridge of the 50's and 60's was the ceramic phono cartridge. Typically, these were supplied as turnover cartridges in early (non Hi-Fi) tube and solid state stereo systems, record changers and children's players. Some were higher quality versions such as the Decca Deram. Ceramic cartridges are high impedance and high output voltage (200-1200mV) devices. Another feature of this type of cartridge is that its response is amplitude based pick-up instead of velocity based like magnetic cartridges. This means that phono equalization is not required. A record player with this type of cartridge may be plugged directly into an high impedance (tube) input. Today, this type of cartridge is best used with old 78rpm records.

Moving magnet (MM) cartridges are the common variety, characterized by high output impedance (typically 47kohms) and a removable, replaceable stylus. In this type of pickup, a tiny permanent magnet attached to the stylus is moving in front of two coils mounted at 45-degree angles. The motion of the magnet's field through the coils induces currents in them proportional to the velocity of the stylus movement in the groove. These are the cartridges most phono pre amps work with.
Moving coil (MC)cartridges are the other type, where the relationship of magnet and coils are reversed. Unfortunately their output voltage is even lower than MM cartridges and require additional amplifying stage. This circumstance makes them more prone to hum and noise. Due to their low impedance (anywhere from 10 to 1000 ohms) a special MC dedicated pre amp is required. However, most high end pre amps are MM/MC switchable.

 

 

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