At your risk.
Even a Pleiades (R,L) microphone inline microphone filter can reduce the signal substantially since it is a subtractive equalizer.
So this is an option, boosting the mic with a very low noise single stage pre preamplifier.
It operates with just 1.3V (in series with a protective fuse). Current consumption is about 23 microamperes! so theoretically a good AA battery should last for say a decade. Current consumption may be even less than a good quality SONY ECM series battery powered electret condenser microphone.
This mic pre preamplifier is designed to be used with a low output, high quality moving coil or ribbon microphone but other types may much fine too. It is an amplifier operating in class A i.e. (electrons flowing all the time), so distortion tends to 0 as signal tends to 0. At high levels it may gradual distort which may prove to be a good thing (Tubes vs Transistors, is there an audible difference? - Russel O. Hamm). The operating point Vd (quiescent drain potential) should not really be at mid point, so that clipping when it occurs is asymmetric giving even order harmonic distortion too which is euphonic. The very low power supply Vb=1.3V matches the very delicate and low output of a microphone and perhaps aids in getting the most of analog amplification character when input levels start getting hotter. Inspired by Neumann U47, low device current has almost always given impressive low noise results, see for example the Pleiades EF183 pre preamplifier. It was decided to built this preamplifier before building the Pleiades DK96 battery electron tube one stage pre preamplifier operating with 1.3V for heater and say 9V for anode (grid 2 and above, see previous post).
As it turns out after finishing yesterday building the Pleiades K117 1.3V in an diecast aluminum box, it is so low noise that it can read the thermal resistance (electron thermal agitation) of a 200ohm resistor placed at the input, as earlier versions of Pleiades battery powered electron tube or JFET preamplifiers can. When the input is short circuited even less (almost absent hiss is produced) even when the next stage for example SONY TC-D5Pro preamplifier is at a relatively high recording level.
So here is the circuit description:
Neutrik XLR input pin 1 is connected to its next chassis pin. Pins 2,3 (live, return) are connected to input transformer primary. Input transformer is a studio quality Sowter 1:5 in octal base. Secondary pin 8 is connected to ground, pin 6 trough a 82nF capacitor to JfET gate. A metal film 1Mohm resistor is between gate and ground. For bias, a 12Kohm resistor is between source and ground. A good quality ELNA magenta color 10uF electrolytic capacitor is in parallel with said bias resistor to keep gain high (negate negative feedback). A load resistor Rd=24Kohms is used to convert output music signal current to output music signal voltage. 24Kohm was chosen so that if the amplifier output impedance is about 24Kohms then a say 10:1 output transformer will bring this down by (10x10) i.e. to 240 ohms. Coupling capacitor from 24Kohm drain resistor to output transformer primary is 680nF. Output transformer is a Sowter 1;7 octal base transformer connected as 7:1. So pin 6 is connected to ground, pin 8 to said capacitor, and pins 1,3 to Neutrik XLR pins 2,3. XLR pin 1 is connected to the next to it chassis pin. The positive pole of the 1.3V battery is connected to the upper side of Rd and the negative pole to ground. Start grounding pattern is used, so all ground are connected to one point. From this point another thin cable goes off to input XLR chassis connection.
All resistors are good quality metal film, made in Germany. capacitors are blue MKT Philips.
For a preliminary test the following signal path was used:
Soft, male singing voice mic 24in from singer, mic above head, 30 degrees towards nose - Shure SM58 - Pleiades (74ohm,25mH0 gentle slope bass, mid bass correction (subtractive) filter - Pleiades K117 1.3V mic pre preamp - SONY TC-D5 Pro tape recorder used are mic preamp - Sennheiser HD-580 monitoring headphones
Without the Pleiades filter the usually bass, mid heaviness was observed giving an indication that the preamp is flat frequency response down to low frequencies. Sensitivity is tremendous and voices could be heard taking place outside of the building. When the filter is connected the singing voice clears up from bass, mid bass heaviness, and sounds with flat frequency response to listener's brain. Average signal drops but by making up gain by increasing the recording level full modulation could be achieved as should by the VU meter. Without the Pleiades K117 pre preamp, the VU meter needle barely moves although the singing voice can still be clearly heard.
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