Method of variably changing the frequency responce of a microphone amplifier for making responce flat to listener's brain

By connecting a variable resistor across the secondary of the output transfomer. (Later note), in practice (experiment) it was found to affect much more the frequency responce at low frequencies.
But it was found that reducing the heater voltage affected both side in an interesting and euphonic way, see nearby posts on Pleiades V6 prepreamp.


Description:


The frequency responce changes at both extremes of the audible frequency spectrum.


Just by connecting a variable resistor at the output. (It may also be possible to do this just by connecting a variable resistor across the output of a moving coil microphone).


Example:


A Pleiades V6 pre preamplifier may be made with an output transformer which results in -3dB at 100Hz and 10KHz when the transformer is unterminated or terminated with a high following input impedance. See nearby in time post on Pleiades V6 output transformer comparison).


By terminating the transformer with 300 ohms a substancialy flat frequency responce may be obtained from say 40Hz to 18KHz at a sacrifice of gain. Now, wide response does not nesesarily mean correct sound to the listener's brain, it all depends on the application).


(Later addition: For some reason it was found that when V6 was terminated with 330Ω the gain could not be increased by reducing cathode temperature or heater voltage) Is this an electron tube effect or transformer effect?


If a variable resistor is connected at the output of the the V6 preamplifier all in between values of bandwidth are possible.


So one could adjust for any mic, source to mic distance, application, thereby approaching making the end sound result correct to the listener's brain.


For example reducing bass or treble to compensate for proximity effect, Fletcher-Munson curves, voice effort curves, presure doubling effect at high frequencies etc.


Usually the effort effect requires a reduction at both sides of the spectrum so that voice sounds back flat to the listener's brain.


For example the more softly a singer decides to interprete a song the more low and high end frequencies are produced compared to midrange.


So the output loading may be decreased by an increased terminating loading resistor to compensate for the above voice effort effect.


Usually the input impedance of modern preamplifiers following the Pleiades V6 front end may be as much as 20KΩ. So the output of the V6 is further increased compensating for the soft singing voice both in terms of gain and frequency responce.


Conclusion:


Correct adjustment to listener's brain may be possible in real time by rotating the output loading potentiometer connected to the (output transformer) output of the pre preamplifier. Later addition. It was found in practice that reducing the heater voltage affected the high frequency frequency responce too in an euphonic way.


Important note: In practice this method did not quite work on the high frequencies. See actual measurements on the nearby euroelectron post. It affects more the bass side of the spectrum.


References:


Flat frequency response from singer's, actor's vocal chords to listener's brain, Sound picture recording and reproducing characteristics - Loye, Morgan - Journal of the motion pictures sound engineers


Pleiades V6 schematic