4. Head room refers to the amount by which a signal can be amplified before clipping occurs. One might imagine the amplifier as having a tunnel through which the signal travels from the amp’sinput to its output. The tunnel’s diameter is more than adequate compared to the small size of the signal present at the input. But as the signal moves through the amp and is amplified, it may grow too large for the tunnel and bump into the tunnel walls. When that happens, hard clipping occurs. The tops of signal waves are cut flat, and the speaker makes a harsh, unpleasant sound. Just before hard clipping, as the tops of the signal waves approach the tunnel walls, the waves are compressed causing a dramatic increase in distortion and loss of the dynamics of the music.
Ironically, even though the input signals from the cartridge are very small, phono amps may lack ample headroom. The possibility that the musical signal will clip is a real one because phono ampsgreatly amplify the level of the incoming signals and because the use of RIAA equalization means the signal comes off the LP record already boosted 20 dB in the high frequencies. The problem is particularly acute in the first amplification stage. A good design will include adequate of headroom toavoid these problems and to reduce the extent to which tics and pops caused by dust or scratches on the record interfere with the musical experience. These annoying artifacts often have much higher voltages than do the musical signals. The nastiness of their sound is magnified if the electrical signals they generate hit those tunnel walls and clip. They become much less annoying and distracting if the amplifier has enough headroom to reproduce them without clipping. A good measure of an amp’s headroom to avoid clipping on loud musical signals and recordpops is its “overload margin”, the amount of room it has between the average signal level and the point at which clipping occurs. The greater the room, the louder the transients it can handle without clipping. Overload margin is usually measured in decibels with reference to a particular input level and gain setting. For example, a phono amp might be said to have an overload margin of 20dB at 5mV input with a gain setting of 40dB. Overload margin can be increased by raising the voltage levels of the power supply rails andadjusting the amplifying circuitry commensurately. But even with high supply rail voltages, the first stage of a phono preamp can be susceptible to overload because that stage amplifies the signal beforethe boosted high frequencies from the LP record are attenuated by the RIAA network. When we say the average signal level from a typical moving magnet cartridge is 5mV, we are actually speaking of the average level at 1 KHz, the midpoint of the RIAA curve. The signal coming off the LP record at 20 KHz(the high end of the audio spectrum) is actually 50mV, which is 20dB or 10 times greater than the 5mV we usually talk about. If that 50 mV signal is amplified 100 times (40dB) in the first stage it will rise to a level of 5 volts. And, remember that 5 volt level is only an average. Transient peaks can easily be 2 or 3 or more times that average. Even a circuit with up to 15 volts headroom in the first stage may not be sufficient to avoid clipping on signal peaks if that first stage has as much as 40 dB of gain. Because there are practical limits to how high the power supply voltages can be raised, it is incumbent on the designer to set the first stage gain to a level low enough to provide adequate overload margin
A good measure of an amp’s headroom to avoid clipping on loud musical signals and recordpops is its “overload margin”, the amount of room it has between the average signal level and the point at which clipping occurs.2 The greater the room, the louder the transients it can handle without clipping. Overload margin is usually measured in decibels with reference to a particular input level and gain setting. For example, a phono amp might be said to have an overload margin of 20dB at 5mV input with a gain setting of 40dB.
Overload margin can be increased by raising the voltage levels of the power supply rails and adjusting the amplifying circuitry commensurately. But even with high supply rail voltages, the first stage of a phono preamp can be susceptible to overload because that stage amplifies the signal before the boosted high frequencies from the LP record are attenuated by the RIAA network. When we say theaverage signal level from a typical moving magnet cartridge is 5mV, we are actually speaking of the average level at 1 KHz, the midpoint of the RIAA curve. The signal coming off the LP record at 20 KHz(the high end of the audio spectrum) is actually 50mV, which is 20dB or 10 times greater than the 5mVwe usually talk about. If that 50 mV signal is amplified 100 times (40dB) in the first stage it will rise to a level of 5 volts. And, remember that 5 volt level is only an average. Transient peaks can easily be 2 or 3or more times that average. Even a circuit with up to 15 volts headroom in the first stage may not be sufficient to avoid clipping on signal peaks if that first stage has as much as 40 dB of gain. Because thereare practical limits to how high the power supply voltages can be raised, it is incumbent on the designer to set the first stage gain to a level low enough to provide adequate overload margin.
Trodde du visste jeg, du har jo holdt på med dette i 70 år.
