At your own risk. When 2 or more microphones are connecte together the vibration of the membrane of one makes the vibration of the membrane of the others too unless hybrid transformers are used. Experiments have been done with moving coil microphones so far.
It consists of many XLR female input connectors joined electrically in series parallel combinations. Everything including each mic level adjustment, filtering etc is done with inductors, resistors in parsllel etc before feeding the resultant output to a microphone preamplifier. More on how this is done towards to end of this post.
On BBC or EMI consoles processing of the level of the microphone signal was done before the tube preamplifiers. For example each mic on the simplified block schematic of the REDD.37 or .51used for Beatles etc., the attenuators and LF boost circuits are before the V72 preamplifiers.
And clearly on BBC or EMI productions we can listen to the low hiss or noise or dynamic sounding miracles. And these consoles were low hiss dynamic sounding miracles too.
A similar process happens by our ears. The tympani (outer ear) is mechanically coupled to the middle ear containing the 3 tiny leverage bones. They perform automatic gain control and impedance matching. The inner ear then takes on electronically. Electrons are involved of course and current theories may involve both analog and digital transmission by nerves to brain.
Of course electron forces (electrons repel each other) do the job in the middle and outer ear too. Bones do not touch, it is electrons that repel. Molecules of air striking the tympani do not touch. It is again electrons that repel. More on this on the nearby euroelectron post.
Back to the console.
It is a generalization of the concept described on connecting many Beyer M55 microphones together, on a previous euroelectron post.
The female XLRs or 3 pin DIN or Tuchel may be cable connectors of chassis connectors.
Assuming use of Neutrik XLR chassis connectors.
Many female connectors are connected in parallel series combinations.
Many male XLR cable connectors are made such a way that some are:
0Ω links ie pin 2 internaly connected to 3.
Resistance or variable resistance links between 2 and 3.
An inductor connected between pins 2 and 3 ie a Pleiades filter.
The console uses the following properties, given in these examples:
A mic of 500Ω connected in parallel to a mic of 500Ω make a combined signal output impedance of 250Ω. Ie a new mic with 2 feeds whose output impedance is 250Ω.
A mic of say 200Ω connected in series to a mic of 250Ω make a mic with combined output of 550Ω.
Higher impedance does not mean higher noise as the signal is higher too since signals, varying voltages, or instatenious voltages are added.
In general series connections may be preferable as modern preamplifiers may be noisy and higher voltages at the input help riding above their noise floor. The good thing about modern preamps is that they may have an input impedance of even 5KΩ making a joy to connect microphones in series ,Christmas tree bulb way.
If the impedance gets too high, it may be easily reduced by connections 2 or more high Z groups together in parallel.
A mic connected to to a Pleiades filter ie inductor is compensated for the proximity or other Fletcher Munson or voice effort effects.
A Pleiades filter of cutoff frequency f1 connected in parallel with another Pleiades filter of f2 cutoff frequency creates a resultant filter of f=f1+f2 cutoff frequency. The slope is still gentle 6dB per octave and this addition property is proved at an earlier euroelectron post.
Microphones can be attenuated by a resistor in parallel with them and omnidirectional microphones are not affected in frequency responce. Cardioid ones are affected in bass responce due to their impedance being larger at low frequencies and this may be used to advantage compensating the proximity effect.
So the:
0 Ω XLR male links are used to complete the circuit in the absence of a microphone.
The Pleiadss Male XLRs (inductor inside them) are used to compensate for the proximity effect. In fact they increase mid or highs by decreasing unwanted lows. So they are substantive filters that can be used as EQs without reducing the wanted frequency component of the signal. Pleiadss filters can be connected to create any desired cutoff frequency or microphone focus distance. A combination just 3 filters of 100Hz, 200Hz, 400Hz can create all the binary combinations of 100, 200, 300, 400, 500, 600, 700, 800 Hz.
There is not way to overload this system as mic balancing is done before any active device. And this was done on those legendary EMI consoles, the first gain adjustment was a mic attenuator for each mic.
Of course microphone output quality was top and hot at these days by using Neumann U47 microphones that have one stage of electron tube amplification inside them.
If more sensitivity from a mic is needed a Pleiades electron tube amplifier can be connected between a mic's voice coil output. They are one stage too with minimum number of components too and 3V battery operated including heaters and anode. This is made possible by a 4Mohm resistor from anode to control grid to neutralize it and free electrons to travel to the anode of low potential.
Pleiades schematics and concepts are open source.
References:
Online simplified block diagram for EMI REDD.37 or .51
High Quality Sound Production and Reproduction - Haden - BBC Sound recording and reproduction - VVC
Microphones - Lou Burroughs
On preserving transconductance of electron tubes at anode potential as low as 3 volts - euroelectron blogspot
Other Euroelectron posts on Pleiades filters or amplifiers including the post on connecting many M55 microphones together
Acknowledgement to Sean Davies for letting known his admiration of the BBC techniques of combining signals before the amplification taking place as his making of condencer capsule amplifiers with electrometer tubes. This played its importance while experimenting with them in coming up with the Pleiades preamplifier.
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