It is not even in fairytales that all we have to do is just connect pin 2 to pin 2 and pin 3 to pin 3.
Some parts of this elementary article assumes knowledge of Fletcher-Munson equal loudness curves and voice effort curves. It is 2017, more than 100 years since it all started and there is no excuse of not knowing or ignoring. A Hollywood paper as reference is added at the end.
As a summary to the above we may state that the goal is not flat frequency responce from microphone to loudspeaker or headphones. It is flat frequency responce from the vocal chords of the singer to the brain of the listener. Or to further generalize, form the brain of the musician to the brain of the listener.
Attempting to write this elementary article in as few words as possible there may be many omissions. Please also refer to other euroelectron posts for details and open source Pleiades prepreamp schematics.
A stricking example of the difficulty of the subject is as follows. Connecting a world class microphone to a world class preamplifier to world class hi impedance studio headphones and the sound may even be one of the worst one might have ever heard.
It may sound bassy, unnatural, with hiss, exposed low frequency reverberation etc.
The annoying low frequency reverberation can be eliminated by elastomer panel absorbers as done by BBC, EMI etc, (see other euroelectron posts on EMI, BBC, Pleiades absorbers). Or recording outdoors, the best broadband absorber is the open window, or outdoor space.
After correcting this there is still hiss, exagerated bass perception, unnaturalness.
Dealing secondly with hiss, and assuming a quiet space, if we have connected a condenser microphone hiss may be lower.
This does not mean that a condenser microphone is more sensitive or more quiet than a moving coil dynamic microphone for example.
A condenser microphone has already inside it next to the capsule an ultra low noise prepreamp, a very sensitive preamplifier which can be just a JFET. So the signal is hotter, in terms of hiss quieter and dominates or covers the noise produced by the preamplifier that follows.
Correct comparison therefore will be connecting a moving coil mic or capsule to a similar quality low noise prepreamp. The impedance of the voice coil will normally need to be stepped up by a transformer. (The impedance of a condenser capsule is already very high and the signal voltage is taken by a very large dropping resistor).
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This will make a dynamic microphone compete or exceed a condenser one. Some moving coil microphones have an amazing equivalent noise level of 14dB. This for example means they can easily hear the acoustic hiss of a room of noise of only 20dB. A quiet room of 40dB of acoustic noise has already an acoustic noise power 100 times that of the 20dB room.
Solving the problem of hiss, the sound will still be bass heavy.
A part of this bass prominence comes from using directional microphones which have the bass proximity effect. The closer we use the mic to the acoustic source the more exagerated the effect becomes.
This can be compensated by making and using Pleiadss filters and then the microphone can be used even at a very close distance. This further increases the signal to noise ratio even in noisy recording spaces.
Or we can use an omnidirectional microphone for avoiding the proximity effect. Some microphones have adjustable directional patterns. A famous example is Karen Carpenter using a Neumann U87 stuck to omni, near the control room window.
Even after using an omnidirectional microphone although we may now have flat frequency responce from mic to speaker, it is not flat to the listener's brain.
It would be, perhaps if the recording is of an opera singer where the acoustic intensity of recording and reproduction might be the same.
It rarely is so in pop music.
In pop music where singers usually use the great Speech Level Singing technique. At the same time the requirement might be of the voice or a single instrument sounding as loud as a full symphony orchestra or a rock band together with a symphony orchestra in the backing track. The acoustic intensity of the reproduced voice (power in acoustic watts) needs to be much higher that the acoustic intensity when the singer was performing in front of the microphone.
The psychoacoustic Flether Munson curves and the voice production voice effort curves explain the prominence of bass to our brain, this has to compensated too and in many cases not only low cut would be needed but high cut too. Until it sounds flat or correct to the listener's brain.
Pleiades filters can again be used to compensate this. Pleiades filters are usually an inductor in parallel with the mic output. They are open source and can be easily made by winding the turns needed for the desired result on a ring core such as the state of the art Nanocrystalinne Nanoperm Magnetec cores. The same cores can be used for winding world class input or output transformers such as the Pleiades transformers. They are fairly easy to wind as usually the number of turns needed is not to large, especially when excessive bass to listene's brain needs to be compensated.
Below are a few examples of solutions that can be applied to the above problems. They are not really problems. It is how nature works. It is hoped that you find your own better and simplest solutions or improve on them.
A few solutions not nessasarily in importance order:
1. A Pleiades filter. It not only compensates for exagerated perceived bass. It also damps the moving coil membrane eliminating pops and wind noise. It is just an inductor connected to the output of the microphone. The idea comes from using inductors in parallel with the signal on RCA ribbon mics or Electro-Voice RE15 or RE16 microphones etc. The Pleiades filters can be built inside an XLR female to male adapter and can pass phantom power if needed.
2. Pleiades 1:2 or other low ratio transformer. For example your mic is 200Ω or even a ribbon 50Ω and you want to improve the signal to noise ratio of a preamp accepting 600Ω mics. A low ratio Pleiades transformer can be wound on a Magnetec Nanocrystalinne core. A 1:2 ratio will increase the impedance 4 times. The bass can be reduced by reducing the number of the primary turns making the construction very easy even at home.
3. Building a Pleiades K117 JFET prepreamp with just a JFET, input and output transformer, a source to ground resistor, battery or phantom powered. It can be housed inside female XLR Neutrik connectors or modules for example. Different inline Pleiades transfomers can be connected according to the step up ratio or bass compensation needed. A transformer may be housed in an XLR female to make adapter or Neutrik modules.
4. Building a similar Pleiades K117 JFET prepreamp cable. It does not nessasarily need an output transformer. The principle is using a JFET in a way similar to the electret capsules prepreamp. The JFET can be housed inside the female XLR or mini Tuchel DIN or Amphenol mic connector etc. It can be phantom powered or even directly connected to a tablet or smart phone such as iPad or iPhone. In fact any portable device that can power an ordinary electret condenser mic.
5 Building a state of the art battery powered electron tube Pleiadss V series prepreamp. Perhaps the lowest noise prepreamp ever existed. The schematic is very similar to the Neumann U47 V14 prepreamp inside the mic. A 3.6V battery supplies both heaters and anode of a triode connected EF183. With 6-12V an ECC82 is suitable too. The operation is made possible by a high Megohm electron liberating resistor from anode to control grid. The reason of its quietness may be that temperature and potentials are even lower than the U47 almost eliminating any secondary electron emission effect. The Pleiades V6 is a version with both input and output transformer.
Most of these solutions require the use of input and output transformers.
There is no excuse of not using an input trasformer. They are used always when the least noise and cleanest studio quality sound is desired.
And by adjusting the number of primary turns bass compensation for flat frequency responce to listener's brain can be approximated as closely as we can.
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Some microphones already have an input transformer inside them that gives a high Z output option to the outside world. This makes life easier as they can be directly connected to grid through a coupling capacitor or to gate. In this case a Pleiades V0 or V4 may be used.
Such mics are the MD421 HN or HL, MD21 HN or HL, M55 HN or HL, Shure Unidyne III etc.
The Beyer M55 has the advantage of sounding very close to flat to listener's brain directly as it is.
So for example a Beyer M55 HL or HN can be directly connected to a Pleiades K117 prepreamp cable the other side of it connected to iPad. The Pleiades prepreamp can be inside the 3 pin DIN female connector. This can give perhaps the best state of the art no compromise class A operation (electrons flowing all the time) sound. With the added advantage of almost portability. If for example an iPhone with GarageBand is used, the user may create music that exceeds studio quality, amazing quality samples from outdoor sounds, superb vocal tracking at quiet outdoor places etc.
In these solutions an attempt was made to keep the signal the simplest ie preserving most of the quality of the signal source. Avoiding spending thousands or millions. And not compromising.
One may build or buy anything that may be suitable. It is not the color of the cat that matters but whether it can catch mics.
References:
(Flat frequency response from singer's vocal chords to listener's brain) - Loye, Morgan, Journal of tue Society of Motion Pictures Engineers
Jensen's superb website for the importance of signal transformers.
Tubes vs Transistors, is there an audible difference - Russel O. Hamm - JAES
Operating features of the Audion - E. Armstrong
On preserving electron tube transconductance at potentials as low as 3.6 volts - euroelectron blogspot
Pleiades schematics - V series pre preamplifiers - euroelectron blogspot - open source - public domain
Feynman Lectures is Physics - Feynman, Leighton, Sands - Addison Wesley
Pleiades broadband absorbers - euroelectron blogspot
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