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Microphones are everywhere and these days a
great many of them are unseen or concealed, and quite possibly listening to
every word that you are saying. Isn’t that right Alexa, Siri and Google? And if
you’ve recently downloaded some malicious software on your laptop could be
earwigging on your private conversations. But not so long ago microphones were
a lot less discrete, and some of them, like this National Panasonic RP-8135 stereo
dynamic mike was definitely meant to be seen. It first appeared in 1974 and
originally marketed as an accessory for home recording enthusiasts. 
It certainly stands out -- no pun intended --
and that brushed aluminium T-shaped holder is an eye-catching combination of
form, function and classic 70’s styling. There’s another clever touch. The
right and left channel mikes are removable modules, either of which can be
inserted into a supplied stick adaptor, for hand-held use or used with a stand
in a two mono microphone desktop setup. Each module has one metre’s worth of
cable terminated in standard 6.3mm (1/4-inch) mono jack plugs. Later versions
had 3.5mm jacks and the outfit included a couple of 3.5mm to 6.6mm jack
adaptors. 
The microphone capsules are of the dynamic or
moving coil type. Inside there is a thin diaphragm attached to a small coil
that fits around a permanent magnet. Sound vibrations cause the diaphragm to
move the coil within the magnetic field that surrounds it. This generates a
tiny current that’s fed, via the cable, to an amplifier or recording device.
Essentially it is a speaker in reverse, and capable of excellent quality.
Dynamic mikes have a Cardoid response, meaning they are most sensitive to sound at the front and sides, and least sensitive at the rear. They are also rugged, reliable and simple to use in that they do not need
fancy wiring or a separate power supply. In short they’re the workhorses of the
microphone world and the type of mike most frequently used on stage, for vocals
and louder instruments. 
The RP-8135 capsules have no markings but
knowing a little about how Panasonic operated in the 70s and 80s, they tended
to avoid cheap components and it is possible they were sourced from one of the
leading manufacturers. That’s born out by contemporary reviews and comments
from users which suggested it was suitable for serious and semi-pro
applications
The RP-8135 you see here came from a local car
boot sale a while ago and was another lucky find. The cardboard box looked a
bit tatty but the National Panasonic branding and logo pointed to what might be
lurking inside could be a good age (National was dropped from the name in the
early 80s). The contents of the box didn’t disappoint and the £1.00 asking
price was too low to haggle over so the deal was done. It had been hardly used
and may have lain undisturbed in the box for at least the last 10 years. The
clue was smears of the polystyrene packing bonded to the vinyl-covered cable.
It’s evidence of the very slow chemical reaction that occurs when the two
materials are in contact with one another over a period of several years.
Fortunately, if caught in time it’s not permanent and the residue scrapes off fairly easily,
leaving the surface of the cable undamaged. All of the metal parts cleaned up
well and with some gentle polishing it now looks like it just rolled off the
production line. Luckily it had been stored in a dry atmosphere and the twin
microphones produced a crisp, clean, well-rounded sound that I’m guessing
wasn’t too far off what it was like when new. 