Successful music recording may be said to be flat frequency responce from the brain of music producer to the brain of the music listener. (Generalization from Loye, Morgan statement).
This objective does not nessesarily mean flat frequency responce from microphone to loudspeaker. The reasons are practical one. How our brain's frequency responce depends on acoustic intensity, how the spectrum of a singers voice for example depends on voice effort. How amplifiers have not an infinite dynamic range. How the usual actual listening environments rule out a reproduced dynamic range greater than 20dB?!
In fact on successful and great sounding commercial recordings one can observe a VU meter moving even less. It most of the time shows around 0 dB, occasionally exceeding it a little and dipping rarely below -10dB. As an example the reader is referred to listening while VU meter observing on Frozen - Madonna.
Yet while a music recording is being produced the microphone may encounter a tremendous acoustic dynamic range. From the faintest sound of a clavichord to gunshots , thunders etc.
In the same way our ear does. But our ear (middle ear) has those 3 tiny bones that change pivot and do acoustic automatic gain control. Otherwise life would be very uncomfortable. This mechanism is like an input impedance maching transformer of varying step up ratio according to how loud the signsl is. Or a variable impedance transformer according to how intense in energy the acoustic stimulus is.
For wise reasons nature does the gain control right at the beginning of the chain. Just after tympani. Cats even do a bit before the tympani by stretching and closing their ear entrance! Nature does not use a Faichild compressor at the end of the chain! Nor a plug to a lesser signal that could have been much better had it not passed through so many active devices that may have introduced anything including transient intermodulation distortion. Here by lesser we mean an electric signal compromised from the glory of the live acoustic signal.
Ripping off nature's idea it seems fit to do the same right at the beginning of the signal path. Right after the mic element capsule. Some good singers or musicians or engineers may do manual gain control before the mic by changing their distance from it in real time.
This post is about what may be done just after the mic capsule, membrane. Of course an option should be provided to leave the signal dynamic range intact if needed.
Moving coil microphones seem very fit candidates.
1. They can handle practically any signal from the least to the highest powered without distortion. (Sennheiser micro revue 70-71)
2. They have excellent sound quality. And it is easiest and cheaper for the pressure or omnidirectional type of moving coil microphones to have excellent quality. (Lou Burroughs book, Electro-Voice). In our internet days it is easy to verify this by reading the sound engineering forums of professionals rediscovering or discovering the great sound quality of moving coil mics. Comments are not unusual of microphones sounding to their brain at least as good to microphones with many more zeros in their ebay price. For example the Sennheiser MD211 studio microphone or MD 211 N. (Gearslutz forum referenced)
Moving coil mics are normally connected to an input trasformer. Even after this usual impedance step up input transformer, the impedance is not tremendous. So a potentiometer attenuator, (variable voltage divider) could be connected after the secondary. If constant impedance attenuators are available such as those used by BBC and EMI they could be connected directly to the microphone's 200Ω output. With no problems of hiss!
Here we will concentrate on the simple method of a potentiometer after the high impedance output.
A proposed signal path, setup:
Moving coil mic, typically 200Ω -
Step up input transformer, typically 1:10 or 1:20 etc-
A secondary terminating resistor. Here conveniently a potentiometer of typically 1 Megohm -
A JFET or electron type class A (electrons flowing all the time) front end amplifier. For example a Neumann U47 VF14, a Pleiades K117 JFET, or Pleiades V6 battery electron tube, or Pleiades K117 driving a V4,... All these preamplifiers are minimal component count designs that can be hand made in the lab by anyone skilled in the art.
An attenuator can be connected at the output of the preamplifier, after the output transformer so as to get as much of the dynamic processing of the electron tube or output transformer. All these analog devices like nature are very linear to small signals and gradually saturate at higher peaks. So the juice of music is kept intact while peaks are instantaneously limited and at the same time gracefully introduced overtones fool the brain to think that nothing has happened to the signal. (Russel O. Hamm)
To summerise. The signal can be attenuated at the input no matter how intense it is. Option of controllably peak limiting by the electron tube or JFET stage.
Therefore a tremendous live acoustic dynamic range is fitted to a low objective dynamic range.
With the advantage that the subjective dynamic range, punch and edge is preserved due to the controlled non linearity of the electron tube processing. Or by over driving the transformer magnetic analog simulation of magnetic tape recording recording processing.
Then digitisation is possible and will sound loud. Exploiting most of the bits (binary digits). Hopefully as loud and fresh as 60's recordings!
(An alternative setup is directly connecting (through a capacitor) the secondary of the input transformer to an electron tube (see U47 or Pleiades V6 schematics for example) or JFET and then attenuating the output).
References:
Tubes vs Transistors, is there an audible difference? - Russel O. Hamm- JAES
Sennheiser micro revue 70-71
http://lcweb2.loc.gov/master/mbrs/recording_preservation/manuals/Sennheiser%20Micro-Revue%2070-71.pdf
Microphones: Design and Application - Lou Burroughs
https://www.gearslutz.com/board/reviews/988835-sennheiser-md-211-u.html
Neumann U47 schematic, Pleiades V6 schematic
On preserving transconuctane of electron tubes at an anode potential as low as 3 volts - euroelectron
Analog simulation of analog magnetic tape recording - euroelectron blogspot
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