The Pleiades EQ

How simpler could it be?


Well in fact there is a simpler and perhaps a better way. Phil Collins can be seen placing his hands in front of his mouth to do acoustic wave cavity EQ. (Horn shape etc as it does acoustic impedance matching too, Phil is intelligent). If memory is correct it can be seen on youtube in a video showing Sir George Martin recording with his friends.


This is about a simple and very effective electronic equalizer. It is done by changing the heater voltage of the electron tube amplifying the sound.


It does great and subtle slope drop at low and high frequencies. This can be very useful to compensate for exaggerated bass or treble frequencies. Or in other words compensating for voice effort curves. Low and high acoustic frequencies are boosted when we sing softly. Also for compensating the Fletcher Manson equal loudness curves. Our brain perceives exaggerated bass and treble frequencies the more the acoustic sound pressure level is.


As the electron tube cathode cools down transconductance decreases, anode resistance increasess and this results to voltage drop at low and high frequencies at the primary of the output transformer. The output impedance of the tube interacts with the reactance of the primary winding, On inductors the reactance increases with frequency. At low frequencies the reactance is low therefore the tube is loaded and there is more voltage drop. As the tube cathode cools down and its output impedance increases we have a parametric low cut filter. A similar effect may be happening at high frequencies by the transformer capacitance or other capacitance interacting  with the electron tube output impedance.


Of course it was not found this way, It was found by chance. The heater was disconnected while the anode voltage was still on. As the temperature from the 6.3V achieved temperature was droppiing initially there was an increase in gain. Possible reasons are explained on other euroelectron posts on the subject of reduced heater voltage, low noise, reduced electron cloud etc.


A convenient heater voltage in terms of low noise was found to be 3.9V. Convenient as the anode voltage of the Pleiades V6 pre preamplifier is also 3.9V. This came about as both anode and heater circuits are currently powered by 3 AA 1.2V rechargeable batteries. On the Pleiades V4 it was found that the of 3 AA batteries must be giving  even lower noise than 4 AA batteries even if the gain with more voltage supply is greater.


The Pleiades V6 electron tube prepreamp is still in breadboard stage. While testing various mics and disconnecting occasionally the heater voltage it was found that the sound to listener's brain improved. More clear, more natural what one would expect from a world class EQ.


The need and success of this type of EQing become apparent when the world class Sennheiser MD21 HN microphone was tested.


Signal path, setup:


microphone - Pleiades V6 - Sony TC-D5 Pro - Sennheiser HD 580 Precision headphones


The input transformer of the Pleiades V6 has a primary inductance 0f 140mH to compensate for proximity effect on directional mics or some subtle and sometimes annoying bassyness due to Fletcher-Munson, voice effort effect etc which is apparent on all mics including pressure ones which do not have a proximity effect.


But 140mH turned to be perhaps a bit too low for the MD211 firstly tested on the Pleiades V6. It sounds great but it is on the border of sounding bass light on some types of singing.


Then other mics where tried. Very impressive were the Electro-Voice RE-15 with its low cut on and the Sennheiser MD21 HN connected at its low Z terminals.


The MD21 sounds fouler in bass than the MD211 under similar conditions. Perhaps too bassy for some type of singing registers.


It was immediately found that by playing with cathode temperature one can tailor the sound by fine tuning until it sounds almost perfect.


When the heaters are connected directly to the batteries, say at 3.9V the anode current of the triode connected EF183 (with 8Mohm anode to grid electron accelerating resistor) is 100 microamperes. This is let us say max bass and treble response, nearly flat from mic out to preamp out.


As heaters are disconnected and the temperature of the cathode gradually drops so does presence increase. Presence being max at about 20 to 30 microamperes.


Between these infinite values of anode current a value can be found where the sound is almost perfect and flat from singer's vocal chords to listener's brain.


It is not easy to adjust as one has to wait for the temperature to drop.


One way of doing it is switching to a series to the heaters 9 ohm resistor. Using 9 ohms the anode current will finally settle at the low extreme of 20 microamps.


While a microamp anode current meter needle is monitored, once the sweet spot is heard one can record the value of the anode current at that instant.


Then this value of anode current can be achieved by adjusting a 10 ohm series wire wound rheostat in series with the electron tube cathode heater circuit.


Of course a variable resistor could be connected between anode and output transformer. Compensating by another ganged resitor for reduced Vb voltage will the same EQ effect be induced? Or are additional  effects into play when electrons get loose?


Pleiades ideas, concepts, schematics are open source.


References:


Pleiades V6 Schematic


Flat frequency Response from Actor's, Singer's Vocal Chords to Listener's Brain, Sound Picture Recording and Reproducing Characteristics - Loye, Morgan - Journal of the Motion Pictures Sound Engineers


Operating Features of the Audion - E.H. Armstrong (for explanation on how an electron tube self biases it self negatively internally. The Pleiades V6 uses an 8 Megohn resistor from anode to grid to counteract externally this internal effect and make the electron tube operate with only 3.9 volts at the anode.)


Pleiades ideas, concepts, schematics are open source.