U47fet Microphone versus Affordable Alternative Posted on November 02, 2019

IF YOU HAVE FOUND THIS PAGE THEN YOU MOST LIKELY ARE LOOKING FOR HISTORY OF THE NEUMANN U47fet :

The U47 FET was developed and designed circa1969 as the source of tubes for the now famous and still popular U47 & U48 microphones started drying up.

During this time period, the “Cold War” and resulting “Space Race” had caused rapid advancement in electronic technology. During the late 60’s, now reliable and high quality silicon transistors were starting to be manufactured very economically compared to more costly tubes. Very “HiFi” and quiet audio circuits could now be manufactured with transistors instead of the older tube technology.

Considering this, along with the new trend of placing microphones very close to instruments and voices in Cardiod…Neumann decided to re–engineer the U47fet as a solid–state, single-pattern microphone, designed around a “state of the art” Field Effect transistor (FET) with 5 additional silicon transistors.

This new FET microphone circuit will now do the work of the tube in previous designs; matching the very high impedance capsule to the final low impedance balanced 150-200 ohm output of modern recording microphones.

The newly designed U47fet was in full production by early 1972 and it provided a welcome alternative the use of dynamic microphones for close recording of instruments that can deliver high SPL’s.

Although, the U47fet could be used easily to record an acoustic guitar, producers often preferred the brighter U47, U87 or AKG 414eb to the U47fet. However, It was lovely sounding on upright bass or in front of the guitar or bass amp.

FET CIRCUITS VERSUS TUBE CIRCUITS

A tube’s input and output impedances are much higher than we see in early transistor circuits. The output impedance at the transformer is typically 200 ohms.

However, with the development of the field effect transistor in the early 60’s; which has a much higher input impedance compared to early silicon transistors it was possible to built professional condenser broadcast and recording microphones with solid state devices. Modern field effect transistors could now offer better noise and gain figures than a single vacuum tube and were easier to power.

The typical FET “transconductance” or impedance to gain ratio is many times higher for a FET compared to a tube. This means that a FET can yield 94db more gain than an equivalent single tube circuit and provide a better signal to noise ratio than an equivalent tube circuit.

However, the tube is still used today for condenser microphone circuits because impedance matching is more important than obtaining maximum gain in typical condenser microphone circuits; plus tubes can take and incredible amount of “abuse” from excessive input levels or excessive power supply voltage variations compared to any solid-state device.

In a way the FET circuits are more detailed and with a slightly faster transient response than tube circuits while the TUBE circuits are more “forgiving” with a overload characteristic that is more “compressor” like than hard IC clipping.

Solid State circuits must be able to provide more “headroom” or maximum output before distortion compared to a tube amplifier to be preferred in listening tests.

Here is a reference to an AES Paper from 1973 comparing tube circuit to solid- state circuits in audio gear. This is a fascinating study and the full study is well worth reading for any enterprising recording engineer. All the findings are verified scientifically with double blindfold testing.

THE PAPER ASKED:

TUBES VS TRANSISTORS: IS THERE AN AUDIBLE DIFFERENCE?

Engineers and musicians have long debated the question of tube sound versus transistor sound. Previous attempts to measure this difference have always assumed linear operation of the test amplifier. This conventional method of frequency response, distortion, and noise measurement has shown that no significant difference exists. This paper, however, points out that amplifiers are often severely overloaded by signal transients (THD 30%). Under this condition there is a major difference in the harmonic distortion components of the amplified signal, with tubes, transistors, and operational amplifiers separating into distinct groups.

THE CONCLUSION OF THE STUDIO STATED:

The U47 FET’s circuit was far more complex than any other Neumann fet microphones designed in this post tube era including the U87 and KM84.

The later U89 microphone featured a similar circuit to the U47fet but with a different capsule than either the K67 or K47.

However, in the U87 and the SDC, KM84 circuits a single FET impedance converter directly feeds an output transformer with a 10:1 ratio. But, in the U47 FET/U89 circuit the FET is buffered from the output transformer by 5 silicon transistors configured like a discrete OP amp to provide a class A/B output stage.
A sixth transistor serves as a voltage regulator to ensure the circuits working voltage and capsules polarization stays consistent at 43V allowing the 48v phantom supply a 10% working tolerance.

The U47fet circuit uses negative feedback to reduce the HF response of the K47 capsule much like a “HF limiter” giving the U47fet a tighter more controlled sound.

Although the FET model incorporated exactly the same K47 capsule as the original U47, the revised internal electronics, contributed to a darker sound signature in the Cardiod pattern compared either to the U47, M49 or the U87. Nevertheless, the U47 FET became a reliable “go to” workhorse microphone and it was highly regarded in high SPL settings whether in front of a kick drum, over the Timpani’s or in front of the bell on a French horn.

It was a favourite of mine in front of the kick, on trumpet, trombone, upright bass and the low end of a grand piano, or on a vocal where a U47 or C12 is a bit bright, or you have an overly sibilant singer and don’t want to resort to an even darker dynamic or ribbon microphone.
The U47fet was well regarded in its ability to handle extremely high sound-pressure levels. It can handle an SPL level of 147dB with the 10dB pad switched in. For, comparison a U47’s distortion will start to increase with levels above 120db. Although never as popular on vocals as the original U47…it sounds a little more “compressed” and has a slightly flatter response than the original 47 (which has a much broader 5dB presence boost) — the FET 47 was still quite usable in this role too, particularly for male vocals and speech. Its modest 2dB of presence boost between about 2 and 5 kHz is enough to aid clarity and intelligibility in a mix, while the gentle proximity boost adds a degree of body and warmth. (SOS July 2015).
Here is the schematic of the U47fet courtesy of RecordingHacks

Note: the 5 transistors are all dc coupled. C10 protects the output transformer from passing DC and R13 is providing negative feedback to the transistor circuit.
T4 & T5 provide a class A/B NPN-PNP transistor output pair. A similar circuit is used in the output stage of the Neve 1081 modules.

You can still buy a true Neumann U47fet (collectors edition) for a sale or street price of $3999. However, today it is possible to build a FET/LDC microphone that will handle high SPL’s with a similar frequency response and sound on dynamic instruments for much less money than the new U47fet.

Having used and serviced U47fet, U47’s, U87’s and the AKG 414eb microphones in a professional studio setting during the 70’s and 80’s I was familiar with the overall functions, use and circuitry of these microphones.

The U47 were the GOTO microphones and probably responsible for over 75% of the recording duties during my tenure at Ocean Studios in Vancouver thru the 70’s & 80’s.

The U87’s were relegated to voice work and string instruments. The 414’s were able to handle the level of drums percussion and being placed close to the hammers of a Yamaha C7 grand piano.

The U47fet was and great sounding work horse in front of the kick drum, in front of loud brass instruments, on an upright bass or on the bass amp and for the low end of a grand piano. It was also used for vocals and if the vocalist was overly sibilant or harsh for the U47 then it was the U47fet next and then a SM7.

The U47fet was not as open as the U47, U87 or 414eb. Plus, it could only be used in Cardiod and it could have proved useful in OMNI.

The CM47fet is not a direct clone or copy of the U47fet.

But it was designed to exhibit the same type of headroom necessary for extreme “close miking” situations and the recording of loud sources.

It has a switchable OMNI option plus a -10db pad and HP filter.

The stock CM47fet actually uses our AK89 (original k67/k87 curve). The CM47fet uses a de-emphasis circuit similar to the U87 to tame the extreme rise in this capsules response above 10khz.

The stock CM47fet, even though slightly brighter than the original U47fet works as well as the U47fet in front of the kick drum. The CM47fet can be used in very close proximity to toms, snare drums, hand drums, percussion instruments and in front of any instrument speakers.

It uses an original AKG 414 2-stage class “A” type transformer coupled circuit with K67 de-emphasis added and has nearly 14db more headroom than a U87 circuit.

The CM47fet also comes as a CM47fetCE. This features our AK47 capsule with the de-emphasis circuit and has a response curve nearly identical to the original U47fet but with a true class “A” circuit.

The CM47fet uses a 414eb class “A” type circuit, which reduces the amount of semi-conductors from a FET and 5 silicon transistors to a FET and a single silicon transistor configured as an emitter follower. This keeps the CM47fet circuit class “A” but with a lower output impedance.

Because the emitter follower circuit has such a low output impedance and no gain it can more than adequately drive a 2:1 ratio output transformer instead of the 10:1 used in the U87 and the 9:1 used in the U47fet.

When a level of 0dbu is measured on the input side of the 2:1 transformer then a level of -6dbv is measured on the output.

When a level of 0dbu is measured on the input side of the 9:1 transformer then a level of -19db is measured on the output.

The 2:1 transformer has just over13db less loss than the 9:1. This allows us to decrease the gain of the first stage by 14db and allows the use of a medium mu FET to be used instead of the high gain 2N3819 in the U87.
This increases the headroom and reduces the signal to noise ratio.

The U47fet can deliver a final output from the microphone of -5.5dbv before the onset of distortion.
The CM47fet can deliver a final output from the microphone of 0dbv before the onset of distortion. This is nearly 6db better than the U47fet.


       
The shootout mic positioning (above)


MICS INCLUDED IN THE SHOOTOUT:

A) Vintage U87 pre-1988 which is a lovely sounding specimen ($3500 new)
B) Advanced Audio CM47fet which sells for $295 with mount & case
C) Advanced Audio CM87se which sells for $595 with mount & case
D) Vintage AKG 414eb P48 which is in lovely condition ($1000 new)

                         
The microphone set-up was lowered and placed in front of our Tinker acoustic guitar.(above)

These microphones were patched directly into the Jensen transformer coupled preamps in the MCI/Sony MXP3036 console.Channel 1 is the AA CM47fet, Channel 2 is the Neumann U87, Channel 3 is the AA CM87 and Channel 4 is the AKG 414eb P48.The output gain of the CM47fet and the older U87 set to 200 ohms are nearly identical. A new U87Ai will have the same output level as the AKG414eb while the CM87se has 3db more output level than the original U87 but 3db less output than the AKG 414eb P48.

The outputs from the microphone preamplifiers appear in the patchbay of the MCI/Sony 3036 console and they were patched directly into the multi-track inputs 1-24 of the RADAR 24.



                     

HERE ARE THE RAW, UNTREATED AUDIO CLIPS FROM THE SHOOTOUT