An amazing booster microphone preamplifier by RCA and Pleiades amplifiers

The RCA type BA-2C microphone booster or pre preamplifier or head amplifier.


So many years ahead of its time.


It is a front end microphone preamplifier so that a ribbon or moving coil microphone can be connected. And it gives fair play to all the low noise and sensitive ribbon and moving coil microphones.


Fair play by giving them a low noise head amp like condenser capsules have inside the microphone itself.


Many details of the RCA microphone booster can be found on the excellent courant website.
http://www.coutant.org/ba2c/index.html


The Pleiades V5, V6 head amplifiers can be thought of as a more modern implementation of such type of amplifiers as they operate with battery and just 4V to 8V typically, including anodes.


It is known that battery operation reduces many aspects of noise in preamps.


On the Pleiades very low noise operation is further helped by the very low anode and heater voltage. Minimising secondary electron emission effects etc.


Operation at such low voltage is possible by the anode to grid few Megohms biasing resistor.


A 2 stage stage with potentiometer in between can be derived.


Or perhaps a 3 stage with a saturating inter stage transformer for instanteniously peak limiting. Thereby simulating analog magnetic tape recording. See other euroelectron posts.


A three stage head or front end amp could be arranged as follows.


The mic drives through the input transformer the first tube.


A big interstage transformer feeds the second electron tube.


A small interstage transformer driving the 3rd electron tube provides recording preemphasis or peak saturation limiting.


The 3rd tube with high cut implemented (mathematical integration), like a reel to reel tape playback amplifier.


An output transformer creates low output impedance and the adjustable low level but loud sounding output of the booster preamp can then feed an ordinary mic preamp.


This may be called the Pleiades V7.


Pleiades concepts and schematics are open source.


References:

RCA BA2C on www.coutant.org
http://www.coutant.org/ba2c/index.html


Pleiadws V5 or V6 schematic


Neumann U47 schematic