Why do Pleiades filters act as electromagnetic pop filters too

This is explained in some detail in the next post on Grampian GR1/L.

Why does the Grampian GR1/L ribbon mic needs so low terminating inductance to sound correct at small distance?

The inductance needed in parallel with this brilliant mic is ridiculously low, typically 3mH to 14mH.


The mic sounds correct without any EQ needed when recording at large distances, eg greater than 1-2 meters.


When we sing close to them the sounds gets extremely bass heavy and we need to compensate for the proximity and some psychoacoustic effects.


The fact that a so small inductance is needed makes input transformer design and construction a paradise. Especially if a Magnetec Nanoperm ring core is used the turns are so low the primary winding may consist of just say 7 turns.


Why such a low inductance needs to be connected in parallel?


It is best to find the right inductance exoerimentaly by what sounds correct to listener's brain. Increasing or subtracting turns until the sweet spot is reached for the particular voice, distance, song etc.


Here are some reasons of the very low inductance needed.


They are 25ohms so they need an inductance of 10 times less compared to a 250ohm mic for the same effect.


They are ribbon. So the bass increases the more we come close. At a few inches even the mid range would have increased.


The more softly or intimately we sing close to a mic the more bass or treble is produced by our vocal apparatus. This is described by the voice effort curves.


The more loud the acoustic reproduction is, the more our brain becomes sensitive to bass and treble. This is described by the Fletcher, Munson or equal loudness curves.


A proposed signal path for excellent signal to noise ratio:


Grampian GR1/L - Pleiades V6 head amplifier with a low inductance input transformer - a mic preamplifier


Since at very low frequencies the impedance of an inductor is very low, mich current is flowing through the ribbon. So there is a magnetic force to it as on ribbon loudspeakers. This force opposes it's cause which is our voice. If it did not oppose we would have energy from nothing which is against the law of conservation of energy. This is called Lenz's law. So the ribbon is damped and the pops are greatly reduced. Nevertheless you act at your own risk and always protect those precious excellent sounding mics.


References:


Microphones - Borwick


Flat frequency response form singer'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


Conceptual Physics - Hewitt


The Feynman Lectures on Physics - Feynman, Leighton, Sands

How to make a ribbon mic sound more detailed and bright than a condenser

There is no reason why they should sound only dark.


There is no reason why they shouldn't be at least as detailed as the best condensers or moving coils. Their diaphragm, the few micron thin aluminum ribbon itself is of extremely small mass. From Newton's law. Acceleration = force : mass . The lower the mass for given force the greater the acceleration.


They are tuned to a very low frequency.


So as the frequency is increased above this resonant frequency displacement drops as in all resonant curves. They are mass controlled.


But the velocity of the ribbon remains the same because the frequency is higher.


And since the emf or voltage out is proportional to velocity of a moving conductor in a magnetic field by the law of magnetic induction the voltage output with respect to frequency is constant.


This is the reason they are so good at high frequencies. Displacement of the ribbon is very small so there is no distortion. And they can handle the great sound of cymbals as for example the STC 4038s used by Alan Parsons on Dark Side of the Moon - Pink Floyd for drum overheads.


But when we sing close to them proximity effect increases the velocity at low, low mid, mid frequencies and output increases making them sounding dark.


An inductor in parallel with the mic does the trick of inverse compensating this effect (Pleiades filter).


And the filter can be chosen or adjusted until they sound flat for any particular distance.


See next post on how to equalize the Grampian GR1/L.


An inductor in parallel with the mic output is used on many world class directional microphones for low cut or high pass (making the resultant frequency response flat) such as the RCA 77DX, Electro-Voice RE15,...


References:


Microphones - Borwick


RCA 77DX manual


Electro-Voice RE15 manual

What is the mic used on Castle in the Snow - Kadebostany?

Could it be a Beyer M88, is this the mic seen live?


Or could it be a Beyer ribbon handheld mic?


Shouldn't these things be written on credits?

A great advantage of 25 ohm microphones

Is that we have direct access to a low thermal noise low resistance voice coil.


State of the art electron tube (or JFET too?) pre preamplifiers and nanocrystalinne cores for input transformers permit a direct connection to the grid through just one low turn ratio transformer. For example a 1:10 ratio transferring 25ohms to 2500ohms. (The impedance transformation ratio of transformers is the turn ratio squared).


Whereas with 200ohm microphones (assuming they do not have an internal transformer to step up 200ohms) it is again almost impossible to connect directly to the grid. Again a transformer should be used although it can be of smaller ratio if the pre preamp has a very low equivalent input noise resistance.


Some microphones, some very successful ones, have an internal transformer to get from 25ohms? to 200ohms. So another external transformer may then be used making the count of transformers before the grid 2. This may even be a good thing. Signal transformers are miraculous devices. They may make the sound better to listener's brain. Is this because of gentle roll offs that make the frequency response flat from producer's brain to listener's brain?


Would it be better or worse if the voice coil of an SM57, Unidyne III etc assuming it is 25ohms is accessed directly? Then an external (one only) transformer can step up to 2500ohms. This 2500ohms output impedance can be connected to the 2500ohm equivalent input noise resistance of the grid of the EF183 battery operated of a Pleiades low noise pre preamplifier. The input impedance of the EF183 at the Pleiades V6 conditions is 100Kohms so the microphone will not be loaded for worse or better. The primary turns of the input transformer will adjust for proximity compensation. How would this setup sound like?


Of course the Unidyne III has an additional high Z internal transformer winding which can be connected directly to grid. It is a high ratio which may roll off some HF which may be a good thing. Bass compensation can be made by adjusting the coupling capacitor to grid or connecting an inductor (Pleiades filter) to the 200ohm winding or a much larger one to the hi Z winding.


References:


Flat frequency responce form 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


Pleiades V6 schematic

A very interesting looking microphone book, Witnesses of Words - Hoeven

https://www.witnessesofwords.com/en/

Is the Quality of the prepreamp that dictates the quality of the whole system?

Great voice coil and ribbon microphones exist. How about prepreamps?


Classic condenser microphones almost get away with it as they usually have a superb quality internal prepreamplifier.


Is the quality of the prepreampifier, the quality of the rest of the system?


To listener's brain.


Songs that get air played are few. And most of them if not all have top quality vocals.


There are many U47, M88, 77DX etc.


But few hit songs.


Apart from the song and singer's ability could it be that the secret was in the pre preamp?


It is perhaps the only equipment that can be relatively easily handmade and of top quality.


The open source Pleiades V6 pre preamplifier is such an attempt. It is based on U47 but the hassle of the power supply is eliminated by a few Mohm resistor from anode to grid making operation of the electron tube possible with just a battery. The transformers are relatively easily wound on Magnetec 070 Nanoperm magnetic ring cores.


A great prepreamp may not be a commercial equipment. If it were perhaps thousands thousands of producers would produce top vocal sound.


But only a few do on the whole planet.


What is it that makes the vocal so good on Eternal Flame - Bangles ? Apart from the top intune singing and varispeed (a help for intune singing should be low acoustic monitoring level of the song on the singer's ears, as pitch perception depends on acoustic intensity, S.S. Stevens).


References:


Sound and Hearing - S.S. Stevens


Neumann U47 schematic


Pleiades V6 schematic


For varispeed see Michael Tretow interviews on how he recorded ABBA, or Mike Oldfield on his visit to the BBC Radiophonic Workshop etc.