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Veteran and General Yakker
- Joined
- Jul 6, 2010
- Messages
- 17,704
- Tagline
- Broken beyond repair but highly affable
Does this explain why old black and white films sound so comforting?
Almost all amplifiers including cheap domestic amplifiers have a frequency capability from 0Hz to 100kHz at low power. The gain of an amplifier should be rolled off below 20Hz to stop un-wanted sub-sonic frequencies getting to the speaker. In a previous era (before solid state technology) when only valve amplifiers existed, the BBC in London did extensive research about how high in frequency response an amplifier needs to be to give faithful music reproduction. They found no evidence that an amplifier which was designed to continue amplifying frequencies above 20kHz was musically any different to an amplifier deliberately designed to restrict frequencies above 20kHz. Amplifiers which were capable of amplifying supersonic frequencies above 20Khz were not only un-stable but were also susceptible to Rf (radio frequency) interference. Hence the statement “The larger the window the more sh*t gets inâ€.
In this era of magnetic recording, the limitation of high frequency response was governed by the tape speed and the high frequency bias oscillator. The high frequency bias oscillator acted as a carrier for the audio signal to be magnetised on to the recording tape. The highest audio frequency that could be transferred on to a recording tape was 1/4 of the bias oscillator frequency. Bias oscillators in the majority of professional recording machines in many of the worlds top recording studios were set at 60kHz which restricted the highest audio frequency that could be recorded at 15kHz. Special mastering recording machines had bias frequencies of 80kHz to 100kHz which enabled 20kHz to be recorded on to a tape. But as a general rule, the higher the bias frequency the less audio energy could be recorded and therefore the higher the signal to noise ratio. Only very expensive tape recording machines were not restricted in this way.
The spectral energy of music is flat to approx 1kHz. The highest notes on any musical instrument is approx 2kHz. Above 2kHz are the harmonics which decrease in energy at approx -6dB per octave. At 10kHz the harmonic energy of music is approx -20dB which is 1/100 of the power compared to the middle sector of the voice range. Therefore very little energy exists above 2kHz at these extreme high frequencies.
By combining all the previous information knowing that no audio information is traditionally recorded above 20kHz and the spectral energy of music decreases at -6dB / octave above 1kHz and the slew limitation of many amplifiers only enable a flat power bandwidth to 1/4 full power demonstrates that slew restriction above 10kHz is almost never a factor in limiting an amplifiers ability to reproduce music with normal use. However in a high power 4 way active system the amplifier driving a tweeter from 6kHz up should not have slew restriction.
