Syncron Fairchild AU7A microphones

Hi!

I had a good google but there doesn't seem to be a schematic around for the Syncron Fairchild mics.

http://www.coutant.org/syncron/index.html

So... I had the opportunity to trace it out, and here is...



EDIT 21/9/2011: There was a mistake in an earlier version of this. Please reload the thread to make sure you're seeing the right one.

Really very very simple. It runs on 4 obsolete mercury batteries (two pairs) which give the 8.4 and 42V supplies. Tranny uses gold plated jumpers to give circa 200 or 50 ohm output.

Construction is pretty interesting, using long thick silver plated brass (?) rods as both the structure and conductors - sort of reminds me old NMR spectrometers I used to work with in my university days! I'll post up some photos later.

I suspect there are quite a few of these kicking around unused because the batteries are unavailable. I'm just working on a mod to run off phantom... any other cunning mods or upgrade ideas greatly appreciated, as always.

:thumb:

Nice , thanks for that

Some photos showing the construction - it's a bit different from other mics, and I rather like it.



The internal assembly has 7 or 8 pins on the bottom that plug into sockets in the bottom of the mic body - these look to be silver plated, and these connect to the weird output socket.

All the active circuit is enclused under the capsule mounting. 5 rods run down from the amplifier circuit past the battery holder. These carry +8.4V, 42V, audio +&-, and ground.



Looks like the circuit was built around available batteries. I wonder if there are better operating voltages?

Since the 2n2497 is a P-Channel FET, circuit is a common source (as is the KM84), but the resistor values seem low.

Blue, Red, Blue is 62M Ohms.

Thanks - I agree. The gate resistor was measured in-circuit and clearly does not match the colour stripes. Blue-red-blue = 62 Meg? There may be a parallel leakage path somewhere.

Edit - Burdij - I was typing whilst you were posting.

I'm just working on a mod to run off phantom..

Click to expand...

Speaking of KM84, that circuit seems like a good fit for this hardware..

Jakob E.

http://www.xaudia.com/omnip/Mics/MicSchLib/NeumannKm8345sch.jpg

:thumb:

I got a couple of the capsules, they sound pretty nice to me - think of a dark, rolled-off, CK12. The diaphragm material is quite thick.

-Dale

Thanks Dale. Capsule looks to be in very good condition. Did you get any feeling for optimum polarisation voltage?

I've been building a little power supply to give variable capsule and amp voltages, but didn't get it finished before the weekend. Sadly the mic came complete with 1960s batteries, so I spent quite some time removing the mercury salts from the chassis. :evil:

I suspect it'll be right around the 48 volts that the mercury cells supplied, although in my noodling around with the things I've put 60 on them and they sounded fine.

The headshell is a real ringin' pig though, gives a strong characteristic honk to the mic. The capsule itself is pretty nice, like Dale says, or Sony C-38'ish.

The connections to all those rods get real iffy and loose over time causing noise gremlins.

I got my first one back in about 1980, and have worked on maybe 5 more since then. They're nice, but they're a $200 mic, not $500.

I've always been interested in these mics.

Looks like it could be pretty usefull if you get those rods and the grill firmly fixed like Dan suggests.

I wasn't aware that solid-state circuits could be so simple too.

[quote author="rodabod"]Looks like it could be pretty usefull if you get those rods and the grill firmly fixed like Dan suggests..[/quote]

I always shake my head when I see new production microphones with that cheese-grater style headbasket. Are they trying to create nasty midrange standing waves? Maybe it distracts from the hyped China-highs. :wink:

[quote author="rodabod"]I wasn't aware that solid-state circuits could be so simple too.[/quote]

Have you seen the simply FET mic circuit that Gus designed? Just as simple with a few more components and no batteries. Same FET as the km84. maybe the difference in operating points has mostly to do with the FET type. I recall discussions of the issues inherent to using members of the 2N24-- family for a microphone head amp.

[quote author="skipwave"]Have you seen the simply FET mic circuit that Gus designed?[/quote]

is it possible for someone to provide the link to this? i couldn't find anything from searching.

The link to the Gus design is broken. I will ask his permission to host the schematic and post link here.

Something was nagging at my brain about the KM84, and think this quote from Gus clears it up:

[quote author="Gus"] It's a inverting follower charge amp, feedback cap/ capsule cap ~= gain. The KM84 is a drain follower charge amp in the same type of way less openloop gain (a 2n3819 has a gain of about 60). The u87 controls it gain via modulation of the capsule voltage supply the cap from the drain move the capsules supply voltage around because the supply voltage has an output resistance of ~= 330k. One side of the r is not bypassed to ground.

The KM84 is a clever circuit a fet does not have the linear gain of a triode. So I am guessing neumann designer(s) controlled the gain and linearized it via the feedback network[/quote]

From this thread: http://www.groupdiy.com/index.php?topic=435

hi skipwave, any word on this? is it possible for you to email me the schem?

I have some more information about these mics.  It started when I found this site:
www.coutant.org/syncron/index.html

The missing link is John Frechette, a friend of mine, who hand built ALL of the original Suncron AU-7a mics.  He estimates that he built about 150-200 of them before he made the deal with Fairchild who took over production.

I met John about 8 years ago and he told me the whole Syncron story.  He even showed me all of his test materials he used in the development of the mic.

I am a video producer so I will be interviewing John in a few weeks to document the Syncron story as well as the many other interesting things that he has been involved in.

Sean Thomas
www.Star-Studios.com

zebra50 said:

http://www.coutant.org/syncron/index.html

Click to expand...

Looks too small polarisation resistor, may be bass equalisation.
(sensitivity of velocity section is higher and frequency dependent)
May be some kind of noise optimalisation, not known from Ne*manns.