Comparing 2 Fostex M88RP membrane ribbon microphones of different vintage

The newer, (the first coming to the Pleiades lab), has serial number 604020. Both mics look identical but the mic body of this is printed:
Fostex M88RP Bidirectional Printed Ribbon Imp.Low Japan
The mic storage case says:
Fostex Printed Ribbon Microphone



The older vintage has serial number 107038.
Mic body says:
Fostex M88RP Bidirectional Flatcoil Dynamic 600Ω Japan
The case says:
Fostex (with much smaller letters) Regular Phase Dynamic Microphone.


They sound very different, both world class.


The newer is also 600Ω. This had been measured? on an ac signal tone emmited by an impedance meter. DC will damage any ribbon mic.
It sounds as if the bass cutoff is much higher.
Not known if this is from a differently tuned diaphragm resonant frequency, or internal input transformer of lower input inductance.
This mic gives a mid heaviness for vocal use at 7in distance if the mic is used without a Pleiades (R,L) gentle slope high pass filter.


The older mic has a much lower frequency response. This can be heard in ambient noise capture. It can also be confirmed by gently tapping the mic body producing a bass sound.
At 7in vocals without a Pleiades filter the sound is bass and mid heavy.


With the Pleiades (130Ω,40mH) inline XLR filter used inside a Neutrik module adaptor with painted blue stripe, the sound of both mics is natural approaching flat frequency response from singer's brain to listener's brain.


With the Pleiades filter removing unwanted mid or unwanted (bass, mid) in the latter case both mics sound beautifully clear and natural. The lower bandwidth mic sounds a bit more reasonant which can be a good or bad thing depending on application. It may give a wanted edge detail to voice.
The higher bandwidth mic sounds more relaxed with also great treble, bass and midrange.


Both mics have amazing treble with no hint of s problem.


These mics with said filter can possibly be used for producing a sound quality such as:
When I Fall in Love - Nat King Cole
The Power of Love - Frankie Goes to Hollywood


Signal Path, Setup:


male singing voice - Fostex M88RP at (7-9)inch - Pleiades (130Ω,40mH) - Sony TC-D5 Pro - Sennheiser HD580


The Pleiades (130Ω,40mH) EQ is just 65Ω in series with 40mH in series with 65Ω and the total connected to pins 2,3 if mic transmision line cable.