Equalization preamplifier
An equalization preamplifier , also known as an equalization preamplifier , is an electronic circuit or electronic device that is required to play back records . It is the combination of an equalizer and a preamplifier to form an active low-pass filter with a specific frequency response .
When cutting a record , low and high frequencies are cut into the record with differently modified amplitudes . According to the cutting characteristic , the depths are weakened and the heights are increased. This results from a careful coordination of the playing time of the record, the background noise and the level control / modulation, i.e. the signal-to-noise ratio . The playback volume depends on the speed of the needle movement (fast). If this equalization were not used, the slower movement of the basses would require a more powerful deflection and thus a larger groove spacing. That would result in a shortened playing time. Furthermore, the acceleration would become extreme in the heights and the pickup would skid, i.e. not reproduce it faithfully.
In order to linearize the frequency response ( emphasis ) of the record that was changed during recording , the method must be used in mirror image as deemphasis during playback . It is said that the frequency response is equalized . Very different equalizations have been used. The one used today was standardized by the RIAA in the mid-1950s . In the case of a pure equalization process, the signal that is fed into the equalizer by the pickup with about 0.1 to 5 mV would be too small for a high-level / line input on the amplifier. It is raised to a level of around 500 mV by the amplifier section of the equalization preamplifier. The mean gain factor is usually 40 dB (factor 100).
Most integrated amplifiers have built-in an equalization preamplifier with a characteristic curve according to RIAA . This can be recognized by the fact that they have one or more phono inputs (old name: "TA Magn. "). There are few record players that already have an equalization preamplifier built into them; the output of a separate equalizer preamplifier should be connected directly to one of the "Line" inputs (high level inputs) of an amplifier. Connecting two equalizers in series is wrong, the sound becomes dull and distorted.
Shellac records as well as many old mono and partly early stereo recordings were not recorded according to the RIAA characteristic. These recordings sound bass-heavy and somewhat muffled with a RIAA standardized equalization preamplifier. There are special equalization preamplifiers with adjustable characteristics for this purpose . Also software for post-processing has sometimes functions for curve fitting. In many cases, a good sound control or, better still, an elaborate equalizer helps .
In principle, crystal scanning systems do not require any significant frequency response correction ; Their high output impedance, however, requires a high input resistance of 1 MΩ and higher, which today is hardly found in transistor amplifiers (old name "TA Krist. "). The former German record player manufacturer DUAL had published a circuit proposal in the transition period that adapted the crystal system to the MM connection. Because of the high contact pressure as well as the high needle and plate wear, crystal systems have been practically meaningless for decades.
Equalizer preamplifiers are required for dynamic (electromagnetic) scanning systems that convert the speed of the groove deflection. One distinguishes
- MM - systems with fixed coils, which are combined in the group MM ( Moving Magnet ), whereby either the magnet actually induces the signal with the needle movement or a moving iron part controls the magnetic flux and the magnet is firmly connected to the coil. MM systems deliver 1 to 5 mV signal.
- MC systems with moving coils ( moving coil , formerly called "dynamic pickup") with signal voltages of 0.1 to 0.5 mV.
The difference between the system types MM and MC can be compensated for by a 1:10 transformer matched to the system, or the phono preamplifier (equalizer preamplifier), which is actually designed for MM systems, can increase the voltage with an additional amplifier stage. With an input impedance of 100 to 1000 Ω, the MC pickup system is individually terminated, while the standard 47 kΩ terminating resistor is common with the MM systems. Since MM systems have a significantly higher number of turns and inductance, the capacitive termination plays an important role for the transmission range above 10 kHz.
This is why high-quality phono preamplifiers have a choice of terminating resistor (50 to 1000 Ω) in the input switch position for MC and different capacitance values (between 100 and 500 pF) in the MM position. If the connection is reversed, an MC sounds too quiet in practice because there is no amplification, and an MM too dull because it is loaded incorrectly.
literature
- Roland Enders: The home recording manual. 3rd edition, Carstensen Verlag, Munich 2003, ISBN 3-910098-25-8
- Siegfried Wirsum: NF tricks for the audio freak. Franzis Verlag GmbH, Munich 1990, ISBN 3-7723-3321-4
- Gustav Büscher, A. Wiegemann: Little ABC of electroacoustics. 6th edition, Franzis Verlag, Munich 1972, ISBN 3-7723-0296-3
- Fritz Kühne: Low-frequency amplifier with tubes and transistors. 13th edition, Franzis Verlag, Munich 1970