This mic isn't about having a 47. Its about looking like you have a 47. I already figured this out a while ago.
Hi Adam, you are exactly right. We have a client that loves our current microphones and carries several with him but asked if we could build ones that looked like the original U47 and M49.
He scores films and time is worth about $50 a minute in the studio.
He works in some very prestigious studios that have a handful of U47, M49 and C12/ELA M251 microphones.
However, these vintage microphones are getting long in the tooth and when you have a dozen of them up on a orchestral session it is not unlikely that one will start to crackle, hiss or fail.
Re-recording a 12 minute score costs about $600 plus you now have the players for 12 minutes less.
Our film client asked if we could build microphones that would look exactly like the original "vintage" ones but would sound as good as the originals or better, are more reliable, impress the client and are affordable enough that they can budget $15K to $20K for 12 full body tube microphones.
The dilemma is which one do you build and try to emulate. With the U47 there were 5 different transformer versions and those made before 1959 had M7 capsule in late 1958 they starting using K47 capsules and by1961 they were using a 6CW4 nuvistor tube instead of the VF14m.
As, you guys have observed you can argue for decades over which is the best transformer. For, example the first 200-BV08 transformer were wound on a power supply transformer core. These are claimed to sound warmer but in my experience their simpler core/lamintation would roll the highs out earlier.
However, Once U47 sales caught on, Neumann designed the first BV08 with a core and laminations designed for audio.
There is a version of the BV08 that would roll out the low end more agressively from 200hz down used by broadcasters. Oliver mentioned that these could be re-assemble to have the "standard" U47 roll-off which means moving the laminations around which will change the inductance of the transformer.
The .5ufd coupling capacitor and the inductance of the BV08 create a 12db/octave roll-off with a 3db down point somewhere around 50hz depending on the BV08 or T47's inductance. The T47 seems to act like the BV08 in my measurements with a .68ufd capacitor.
This roll off will also change with the output impedance of the tube.
For, example the 5751 is probably being operated as a CCDA circuit to have a low enough plate impedance to drive a T47. The 5751 has a plate impedance of 56K or 28K ohms if you parallel both halves.
If you configue it as a CCDA circuit then you can get an output impedance down at 1K ohms.
We use the CCDA circuit in our current tube microphones with a 6072a tube and get and output impedance down at 600 ohms. Our microphone are flat down to < 20hz.
The U47 has a plate impedance from my calculations of about 14K and therefore you really need a tube with a voltage gain of 18 and a plate impedance of 14K ohms to get the output transformer to react the same as it does with a VF14.
Also, it is important that the input impedance of the tube circuit whether a VF14m or replacement is 60 Meg as this with the 78pf to 83pf capacitance of the capsule will roll the low end out 6db/octave again around the 50khz area.
This is one of the design "features" that makes the U47 work well for vocals and be very quiet, considering it had an metal clad VF14m tube.
If you reduce everything below 60hz at 18db per octave then this give you an advantage when measuring signal to noise.
The relationship between the plate impedance, coupling capacitor and inductance of the output transformer will also cause a slight rise in the low frequency response of about 2db around 200hz.
I have had the rare privilege of being lent a perfectly working NOS VF14 tube for a few weeks this summer. This tube operates perfectly and is quiet, so its worth more than a pair of my current tube mics.
This was a spare VF14 found during cleanup in the back of the repair shop of a large recording complex in Europe. I was sent the tube to determine if it works.
I built a regulated dc filament supply and plate supply. When I measured 34v dc on the plate as displayed on the schematic I measure .5ma of current which is also specified on the schematic.
I measured an output level of +12dm <1% distortion with a 10K load on the output of the T47 and +8dbu <1% distortion int a 1k ohm load on the output of the T47
I duplicated the simple class "A" U47 circuit but used a GE/JAN 5654w tube with the same transformer and input impedance resistor and output coupling capacitor.
By tweaking the plate/cathode resistor in the 5654W circuit, I was able to get the gain of the two circuits within < 2db of each other.
The distortion figures with the 5654w being about 1db better when operated at 125v as opposed to the 105v on the VF14m.
The 5654w is a socketed 7 pin and like the VF14 it has to be selected to be quiet enough for an LDC impedance converter. However, we reject much less than Neumann did with the VF14.
The GE/JAN5654w was designed for use in military aircraft so it much less prone microphonics and vibration than the VF14.
The other important design point when trying to duplicate a U47 is that the M7 or K47 type capsule is polarized passively in Cardiod.
When you passively polarize the capsule you get 3db more output than when you match the rear diaphragms polarizaton voltage to the back-plate.
That's why the prototype of our CM47Le has a switch on the microphones as well as a 9 pattern power supply. When switched to the left its passive cardiod and when switched to the right its variable pattern.
The proximity effect also changes and it seems wider in passive cardiod during our recent testing.
Our CM48T microphone also has passive cardiod polarization and a 5654w but with the smaller footprint BV8 pictured below and our AK47 capsule.
You can see the custom BV08 that we plan to use pictured on the left of the T47.
BTW, the WA-47 has a Class A/B Shoep transformerless circuit. Our CM48 and CM87 have a class "A" 414eb circuit having nearly 14db more headroom than a U87 circuit.
The CM48 has a very "hifi" output transformer with dual bobbins and bi-metal laminations its the exact same size as the BV18 pictured above but with a 2.25:1 ratio to work in the emitter follower circuit.
That's what I have learned from measuring the VF14m tube wired into a U47 circuit.
Cheers, Dave
aamicrophones.com
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