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U47/U48 Tube/Valve Microphones vs economical replacements Posted on February 16, 2017

History of the Neumann U47/U48 Tube/Valve Microphones


The Neumann U47 is a microphone that has a prestigious place in the history of the recording industry.  The legendary Neumann U47 was introduced at the 1947 Berlin trade show and was immediately accepted based on its forward looking design and innovative technological standards. The U47 came to North America in 1949 by the distributor Telefunken and quickly eclipsed the RCA 44bx as the first choice studio vocal microphone. The U47 was immediately embraced by artists like Frank Sinatra, Ella Fitzgerald and the Beatles along with recording engineers Tom Dowd, Bill Putnam and George Martin.
frank sinatra

How did the U47 design come about?

The condenser element was invented somewhere between 1916-1919 by E.C. Wente, an engineer at Bell Labs in the United States. Wente, developed the first working model of what he called the “Condenser Transmitter”.  This new device consisted of a tightly stretched metal membrane placed over a stationary back-plate with just a few microns of clearance.  After several experiments with varying dimensions, tensions and grooving of the back plate, he eventually filed for a patent for the "Telephone Transmitter".  The United States Patent and Trademark Office granted patent #1,333,744 on March 16th 1920 for Wente's invention.

Here is the Western Electric 47a amplifier with the #394 condenser capsule was produced in 1928.  The 47a weighed 12lbs


Western Electric held all of the patents on the condenser element and several companies including AEG in Germany built versions of the 394 under license from Western Electric.  AEG was a Company where the young George Neumann was working.  During the mid 1930's ribbon microphones where very popular in the US, condenser microphone production in the US dramatically declined due to lack of demand.   Georg Neumann became obsessed with the possibilities of manufacturing a commercial condenser microphone with a frequency response that would surpass that of those microphones he was developing at AEG. In 1928 George left AEG with Erich Richmann and started the Neumann Company.

The first tube microphones were built using the Neumann M7 capsule specifically designed and developed to get around the patents held by Western Electric on condenser capsules. 

The M7 capsule used a much thinner diaphragm than the 394 (12 microns vs 25 microns) and instead of solid aluminum foil the first M7 used PVC with a gold dusted layer that was later changed to a sputtered “disc” on PVC.
The other major difference was that the M7 featured a diaphragm on both sides of the back-plate enabling the capsule to pick up sound 360 degrees around the mic.  Thanks to the dual diaphragm of the M7, the U47 was the first microphone to feature a switchable polarity pattern (cardioid and omni) on the microphone as opposed to changing the capsule assembly.

The 1936 Olympics in Berlin provided the “testing ground” for the classic bottle microphone circuit and M7 capsule designed by George Neumann. The  “Neumann bottle” could be equipped with different capsules in order to change patterns. Hitler’s propaganda machinery immediately capitalized on the “Bottle Microphones” ability to capture Hitler’s speech dynamics.  The “Neumann Bottle” was the sound of Hitler on News Reels and can be seen in many pictures of Hitler addressing German crowds.

After the war, the Allies traded the Thueringen region to the Soviets in exchange for West Berlin.  The Soviets & East Germans seized all businesses in the territory and prevented any money from being exported out of the country.  Telefunken, who had been the distributor for Neumann microphones before the war, helped Georg Neumann set up a new company in West Berlin after the war in 1945 and became 25% owners of the new Neumann Company as well as continuing the distribution for their products.  The U47 was developed from roughly 1945 to 1947 when it was introduced at the Berliner Funkausstellung  electronic exhibition in 1947.

It is quite likely that Telefunken had some hand in the development of the U47 given the fact that the VF14 tube was specifically developed and produced by Telefunken for use in the U47 and had no other previous commercial use.

VF14M Tube

VF14M Tube


Before 1947, there is no mention of the VF14 tube in any design documents or any catalogs by Telefunken.  The original design documents for the VF14 have no dates on them, but it does specifically state that the VF14 was designed for use in microphones.  The very first microphone that it was used in was the U47 and later in the U48. Telefunken records show that they continued to produce the VF14 tubes up until 1958.

It has been over 60 years since the introduction of the U47 tube microphone and has certainly passed the test of time.

In 1958 Neumann starting fitting the K47 in the U47, M49 and introduced the new 3-pattern U48 microphone built with the same circuitry,M7 capsule and body as the U47 but with a FIG 8 option.

By 1958 Neumann believed the M7s capsules were no longer an option for serious recordings. Even, disregarding the early deterioration of M7 diaphragms, the new K47 capsule had a noise floor 4dB lower than that of an M7. This of course is due to the PVC materials that were used in the M7. The audible changes and drying-up tendencies of the M7 pretty much started as soon as it was poured, and never stopped until it had deteriorated beyond usability.

With its high quality components, “modern” design, head basket, good looks and the distribution might of the prestigious Telefunken Company; the U47 quickly became the microphone to use in nearly every recording and broadcast application.  Perhaps the most surprising aspect of this microphone is the simplicity of its electronic design.  It is amazing how good it sounds with such a simple and uncluttered design.  Its only drawback was that it sold for around $400 in the early 50’s. This is about $3,600 in today’s currency!  The same price as a U87 sells for today.  At that time, the U47 was priced about three times higher than the price of the best ribbon microphone available on the market.  But the price didn’t stop the U47 from being purchased by all the best recording studios during the 1950’s and early 60’s.  Neumann stopped production of the CM47 & CM48 in the early 1960’s but with a modest supply of VF14 tubes. To this day Neumann have never made a reissue or replica of the U47. Over the years Neumann have occasionally created special versions of the U47 for people in the industry.  For example, Bryan Adams received a pair of custom built U47’s in the early 90s.

Early U47 capsule suspension with M7

Early U47 capsule suspension with M7

For recording engineers, the U47 is the most recognizable microphone:

Even the average person will recognize the U47 due to the fact that so many famous recording artists have been pictured with the microphone throughout the years. The U47 can be found in virtually every book on recording applications, biographies of singers, documentaries as well as videos and even immortalized in song lyrics.  It simply is one of the vintage microphones that everyone wants to have.  If imitation is truly the sincerest form of flattery, then the U47 is admired by folks around the globe.  There are more clones and derivatives based on the U47 design than any other microphone in recording history!
So, is it truly special or is it a status symbol?  The answer is both.  To a skilled engineer who understands microphone placement and optimum gain structure, there definitely is some magic that happens with the U47.  The combination of capsule, tube, amplifier circuit and transformer creates harmonic overtones characteristics that are pleasing to the human ear.  This effect provides warmth, richness and thickens the sound of the source being recorded.  For example, during the height of his greatest 1950's and 1960's recordings, Frank Sinatra recorded almost exclusively on Telefunken branded microphones.
But the U47 was limited to production of an estimated 5000-6000 units.  While that sounds like a lot, when you consider the number of recording studios around the world and the number of home studios that are in use, there is simply not enough left to go around.  As a result, a lot of people see the U47 as the ultimate status symbol.  If you have one, you’re viewed as a serious, professional (and successful) studio that can produce quality recordings.  Additionally, as engineers, producers and artists specifically request the Neumann U47, it has become one of the “must have” microphones to secure named artists to work at one’s studio. The U47 is a 'go to' mic for vocals, acoustic guitar, kick drum, bass amp and many other recording applications.

The U47/U48 circuit is elegant in its simplicity:

It involves a pentode tube wired as a triode.  This lowers the plate impedance of the tube circuit allowing the tube to drive the BV8 6.5:1 output transformer without any significant tube loading effects.  The VF14 tube was designed to run from a 60v heater (filament) supply.  Neumann chose to power the filament at about 34v fed by R4 which slowly bring the voltage up to 34v and this is why a U47 must be powered up for at least 45 minutes before the circuit stabilizes and the cathode is brought up to the correct temperature which creates and efficient electron “cloud”.  There is some belief that this under powering of the filament voltage contributes to the U47 sound.  However, it is this writer’s opinion from his 45 year of experience with tube circuits that once the cathode is brought up to the correct temperature it will react as the same tube with 60v on the filament.  It just takes it much longer to achieve this optimum temperature and this is what contributes to the longevity of the VF14 tube that has been operating in some U47 microphones for 60 years.
There are only two capacitors in the audio chain of the U47.  C1 bypasses the audio to ground so the back-plate can remain polarized by R1.  C2 couples the output of the tube to the output transformer blocking the plate voltage from magnetizing the output transformer.

 u47 shematic

U47 schematic

The first consideration is to separate myth from fact regarding the circuit and capsule construction to understand the reasons for some of the unique U47 design features:


There is no reason to under power the filament with today’s solid state regulation which will hold the filament at a constant voltage + or – 2%.  
This was necessary in the late 40’s and early 50’s in order to lengthen the life of the tubes filament.  Today, solid state regulators hold the filament voltage in check no matter how much the A/C line voltage might change.

2: Does the VF14 provide a sound signature of its own?  
There is no proof of this other than the original tube had an internal capacitance that was higher than a more modern sub-miniature pentode like the GE/Jan 5654w.  The internal capacitor of the tube according to the Miller Effect states that the internal capacitance of the tube is multiplied by the gain of the tube.  So, the VF14 will roll out the very high frequencies above 15khz earlier than a circuit using a more modern 5654W(6AQ5) low plate voltage tube having a similar gain structure.  This would not be noticeable on most program material unless it had very hf content.  The output impedance of the 5654w is very close to the venerable VF14 as the 5654W was designed to work on a lower plate voltage more like the VF14 compared to other modern tube types.

3: Can self-bias (cathode bias) be used and get the same sonic result?

Cathode bias requires larger bypass capacitors in the order of 220ufd to produce the same low frequency response as the original U47.  In 1947 a 220ufd capacitor would be larger than U47 body.  However, today as a result of the space race smaller, larger value high quality capacitors are readily available.  For, example the .5 ufd transformer coupling capacitor can be increased to a 2.2ufd Wima metal film and is ¼ the size of the original capacitor in the U47/U48 microphones.

4: can a modern capsule be manufactured to reproduce the frequency response and sound signature of the original k47 type capsule?

Today modern CNC milling machine can produce exacting capsule metalwork identical to the original K47 form.  These can be skilfully skinned with 6 micron mylar and will produce a response curve and sound signature within the tolerances set for original K47 capsules.

Can, a high quality vintage sounding tube microphone alternative be built that can be afforded by a wide section of amateur recorders?
Yes, this can be accomplished by using microphone bodies already manufactured for our CM87 and CM48FET microphones along with their 3 polar pattern switching circuit boards.  Now we only have to fit a new tube circuit board for the economical GE/JAN 5654w tube, a 7 pin connector and our BV18 transformer used in the CM251 and CM67se microphones.
This allows us to produce a world class microphone with 3-patterns that sells for $595 USD including case, shockmount and power supply.



aa cm48tnos tubeBV18 transformer

CM48T in shockmount   GE/JAN5654W NOS Tube in socket  BV 18 transformer

U47fet Microphone versus Affordable Alternative Posted on April 22, 2016




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.


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.



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.


“Vacuum-tube amplifiers differ from transistor and operational amplifiers because they can be operated in the overload region without adding objectionable distortion. The combination of the slow rising edge and the open harmonic structure of the overload characteristics form an almost ideal sound- recording compressor. Within the 15-20 dB "safe" overload range, the electrical output of the tube amplifier increases by only 2-4 dB, acting like a limiter. However, since the edge is increasing within this range, the subjective loudness remains uncompressed to the ear. This effect causes tube-amplified signals to have a high apparent level, which is not indicated on a volume indicator (VU meter). Tubes sound louder and have a better signal-to-noise ratio because of this extra subjective headroom that transistor amplifiers do not have. Tubes get punch from their naturally brassy overload characteristics. Since the loud signals can be recorded at higher levels, the softer signals are also louder, so they are not lost in tape hiss and they effectively give the tube sound greater clarity. The feeling of more bass response is directly related to the strong second and third harmonic components which reinforce the "natural" bass with "synthetic" bass [5]. In the context of a limited dynamic range system like the phonograph, recordings made with vacuum-tube preamplifiers will have more apparent level and a greater signal to system noise ratio than recordings made with transistors or operational amplifiers.”


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

U47fet Schematic


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)


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.














The AKG C414 LDC Condenser microphones Posted on February 09, 2016


The AKG 414 is dual sided LDC solid state multi-pattern microphone. Unlike the U87 multi-pattern microphone which has two electronic variations since it was released in 1968, the 414 has had nearly a dozen incarnations since its first inception as the C414 comb in 1971.

These variations have significantly changed the sound and circuit designs over the last 4 decades garnishing many opposing opinions on its sound.

All versions of the C414 from the “EB” to present day were fitted with a 4   pattern variation switch on the front of the microphone which allowed the polar or directionality of the microphone’s pickup pattern to be switched between OMNI, CARDIOD, SUPER CARDIOD and FIGURE 8.

All version since the C414eb featured a -10db and -20db attenuation switch plus a 75hz and 150z low cut. Also known as a rumble or HP filter. This HP or low cut filter will reduce the proximity effect which is the phenomenon of the low frequencies building up as the source is addressed closer than 30cm from the front of the microphone.

The first changes from the 414 comb to the 414eb were more subtle. Within 5 year of its inception the size was reduced and the more durable XLR 3 pin connector became standard. But for all practical purposes the capsule and circuit remained the same except for an improvement in the bass response by redesigning the output transformer in the 414eb compared to the 414 comb. The C414eb was released in 1976 and I remember buying a matched pair for our studio, Ocean Sound located in Vancouver circa1977 at a price of $400 each CDN.


The 414eb proved invaluable on acoustic instruments and it could be placed very close to the hammers of a grand piano or used as drum overheads because of the extra headroom and dynamics that is provided by the -10/-20db pad switches and the 2 stage emitter/follower circuit.

The emitter follower circuit used in the early 414 microphones increased the headroom of the electronics compared to the venerable U87. This improved transient response of the 414 on percussive sources allowing the 414 to be used over the hammers of a grand piano. This was very useful when the piano and drums were recorded in the same time in the same room. The emitter follower circuit had a much lower output impedance compared to the single FET circuit used in the U87 allowing a transformer with 14db less loss to be be used. This means the first stage of the 414 required 14db less gain to provide the same final output level as the U87.

The 414eb microphones were often favoured for percussion recording and drum overheads. Several examples can be found on-line including Paul Horn and Ralph Dyck’s Jupiter 8 album that this writer recorded in 1980. Track #5 the title track called Jupiter 8 features an original 414eb on all the percussion overdub. This is a good example of an early 414eb on different percussion as the conga, shaker, scraper and wood block was all recorded in one take with the 414 in the centre of the room.


Both the U87 and 414 had an extended top end above 10khz but rather than de-emphasize the high frequency rise with active EQ like the 87 circuit the 414eb had an amplifier circuit that could more easily handle this extended HF response. The U87 was a bit more mid forward having more rise at 3khz while the 414e did not start to rise till and octave later but having a greater rise above 10khz creating an “airy” sound signature.

The c414 offers and alternative for acoustic guitar recording compared to the U87. For example, the difference between a U87 and 414eb on acoustic guitar is a bit like the difference between a Martin D28 and a Gibson J200. The Martin sounds wonderful on a simple folky/blues arrangement but in a pop tune the jangle of the J200 helps the guitar sound cut through a busy mix.

Likewise, the 414 tends to capture more of the pick striking the strings sound while the U87 captures more of the body. The 414 would often be used on BG vocals as it tended to let the vocals cut through the mix even though they were mixed further back in the track. This wasn’t always the case but it gives the reader a closer idea of how the 414 could be used in a recording situation.


In the late 70’s the capsule’s construction was dramatically changed to drop the price and time taken to manufacture the very complex CK12 capsule.

Recording professionals including this writer believe that the sound of the original 414e and its class “A” discrete solid state circuit provides a sound signature that is often preferred for music recording and drum miking compared to later variations of the 414.

The sound of the original CK12 seems a bit more open and the use of the teflon pressed capsule seems slightly darker sounding than the original 414eb microphone. Engineer/producer Hugh Padgham preferred the 414eb to record the vocals for Sting’s “Every Breath You Take”.

By 1980, the 414eb-P48 was introduced which optimized the 414 power circuit for 48 volt operation. Earlier versions could be powered from 9-48v by changing a jumper wire inside the microphone. The 414-P48 power circuit re-design increased the signal to noise ratio by 6db to take advantage of modern digital recording techniques with its better noise ratio.

The 414eb-P48 remained in production until 1986 when the C414B-ULS was released. The 414b-ULS circuit reduced the noise floor even further but the this contributed to the microphone becoming even darker sounding compared to the early 414eb with its CK12 capsule. The 414b-ULS had a more “compressed” an less clear sound compared to earlier versions.

However, the 414 remained popular as it was still more affordable than the more expensive U87 which costs nearly 3 times the price of a 414.

In 1993 a capsule with a brighter response was added to the 414 and a transformerless circuit was introduced. This model became the C414 TL-II. It was a brighter, sounding microphone with a better transient response but again fell slightly short of the sound produced by the earlier 414eb versions. It was characterized by some engineers as sounding too bright.

The latest C414 XLS/XLII versions have the brighter teflon capsule. AKG has improved the capsule mount in the new version but the circuit still remains transformerless with surface soldered components making the microphone nearly impossible to repair but much cheaper to manufacture.

The current list price of the C414XLII is $1549 USD but the microphones can can be purchased at a street price around $1000 with shock-mount.

The 414 still remains a multi-pattern solid state LDC microphone which has become a workhorse of the recording business. However, recording engineers still look for alternate versions that more closely represent the more favoured sound of the original 414eb manufactured in the early 70’s.

On the left is the early CK12 capsule which was very time consuming to manufacture having over 50 individual parts. On the right is the Teflon 2072z which has a slightly “darker” response curve.


Here is a picture of the darker ULS which still incorporated an output transformer and components that are soldered through the circuit board.



Designing the Advanced Audio CM414 multi-pattern professional LDC microphone.

The Advanced Audio CM414 features a dual diaphragm edge fed brass capsule with a response within 2db of the original CK12 capsule used in the original 414eb. The Advanced Audio AK12 capsule has the same “airy” lift found in the original 414comb and early 414eb microphones loved by recording engineers.




Modern CNC “computer controlled milling machines” allow nearly twice as many very accurate holes to be drilled in the capsule’s back-plate negating the need for two resonator disks that have to be accurately hand fitted and aligned under each of the mylar diaphragms of the original CK12 capsule. This gives the Advanced Audio AK12 capsule a response which delivers a very similar sound as the original capsule while allowing its to be manufactured for much less cost than the vintage CK12 capsule.





The body of the CM414 is also manufactured via computer control milling machines reducing production costs significantly plus the construction is much more ergonomic and efficient in its construction. It much faster to fit the capsule and circuit board components into the CM414 body. The CM414 also uses more reasonable priced rugged metal toggle switches compared to the custom made switches used in the original 414.


While reviewing the use of the 414eb on sessions over a 20 year period we discovered the -20db pad was never used. So, in the design of the CM414, only a -10db pad was incorporated for use with high SPL sources. It is possible to use the CM414 just a few centimetres above a snare drum with just the -10db pad. This is because of the CM414’s emitter/follower circuit having 14db more headroom than a U87 before its 10db pad is engaged. Also, the more complex 3 position HP filter switch was replaced with a much more economical and sturdy SPST metal toggle switch engaging a roll-off frequency of 10ohz, 1/2 way between the 75hz and 150hz roll-off frequency of the original 414. Also, we discovered that hyper-cardiod pattern was under utilized and the CM414 was designed with just 3-patterns like the U87. This also means a lower cost and more rugged 3 position metal toggle switch could be used to choose the polar pattern.

These design considerations allow for a microphone to be built that accurately captures the sound signature of the vintage 414eb microphone for a price of $379 complete with a HD shock-mount and flight case.

The CM414 has high quality components that are solder through on the circuit board and can be much more easily and economically replaced than modern day surface soldered components. The CM414 also uses high quality and more expensive tantalum capacitors in the audio chain just like the original 414eb and the U87 vintage microphones.

The CM414 has become a popular choice for percussion, grand piano, vocals from professional to project studios alike.


The U87 LDC multi-pattern condenser Microphone Posted on February 08, 2016


If you have reached this page then you are most likely looking for information on the U87 recording and broadcast Microphone:

The U87 is one the most recognizable and widely used condenser Microphones in modern broadcast and recording studio history.

Why was the U87 Microphone a favourite for voice recording and radio/tv personalities as well as a recording studio spot mic?:

 The U87 features a large dual sided (double diaphragm) condenser capsule. This double sided capsule in conjunction with a 3-pattern switch on the front of the microphone allowed for the microphone’s variable polar pickup patterns to be easily switched from omni-directional, to cardioid and through to figure 8.  

The Cardioid pattern is preferred for voice work because it rejects sound from both the rear and each side of the microphone. The U87 provided a present but warm sound for voice work.

In the recording studio OMNI and Figure 8 polar patterns are very useful features as well being able to roll out the low frequencies plus reduce the sensitivity of the mic when its used in close proximity to capture loud sound sources.

Rear of U87 showing the Low Cut/HP filter and the -10db attenuation switches.

headgrill U87

On the rear of the U87 are two switches; one switch provides the -10 dB of attenuation enabling the microphone to handle sound pressure levels up to 1o dB louder before the on-set of mic distortion. The 2nd switch will reduce the low-frequency response below 150hz to compensate for an increase in low frequencies know as proximity effect. This effect becomes more prominent as the source get closer than 30cm from the microphone in Cardiod or Fig 8 patterns. This high pass filter also known as a rumble filter or low cut will reduce “P’s” popping or low frequency rumble picked up by the very sensitive condenser capsule of the U87.

An advantage of the OMNI directional polar pattern in modern recording?:

Note, when a microphone is placed in OMNI there will be no proximity effect present. In a quiet and well treated studio room OMNI can be quite effective in reducing acoustic guitar boom.

Where can the U87 be used?:

The U87 is often chosen for general purpose recording     applications in professional recording studios, radio stations, television stations and film sound stages. The U87 can be used as a main microphone for orchestral recordings or as a spot microphone for single instruments. Today it is primarily used as a vocal microphone for many different types of music vocal and speech recording applications as well as for radio announcers.

The U87 is known as a front address microphone. In cardiod the microphone will only pickup from the front where the polar pattern switch is located. In Figure 8 it will pickup from the front and rear but sound reaching the left and right sides of the microphone will be rejected. In OMNI the U87 will pickup 360 degrees around the front and rear. By engaging the HP (low cut) filter, low frequency audio interference or “masking” through subsonic and low frequencies sources are effectively reduced.

How many versions of the U87 are there and what are the differences between these two?:

U87i vs U87Ai

There are two basic versions of the U87. The early version was designated the U87i microphone which was built from 1967 to 1986. This version was designed to work from internal batteries as well as “phantom power”. Phantom Power is remotely sent to the microphone through the microphone cable from 48 volts dc emanating at a mixer, microphone preamplifier or recording console. Phantom power came into common use in recording and broadcast studios during the early 70’s and by the 80’s most location mixers had also become “phantom power” capable.

The later U87Ai was made from 1986 to the present day. In the U87Ai the battery compartment was removed and the newer U87Ai can only be powered from a phantom power supply which connects 48v dc through the mic cable to the electronics without negatively effecting the audio quality of the microphone.

However, in the U87Ai the capsule’s working voltage was increased by a factor of 1/3 which increased the output coming from the mic’s capsule compared to the earlier U87i. This higher polarization voltage also increases the tension of the capsule’s diaphragm which creates a slight rise in the HF response of 2-3db.

This increase in the U87Ai’s capsule polarization voltage circuitry also reduces the operational headroom of the U87Ai compared to the U87i. This is due to the higher capsule sensitivity or output level from the higher polarization voltage placed on the capsule of the U87Ai. This yields a much higher sensitivity of 10 dB for identical sound pressure levels but there is a benefit as the signal-to-noise ratio is improved by 3 dB. The audio schematic or circuit diagram of the U87i vs the U87Ai are nearly identical except for how the K87 capsule is polarized.

You can review the information in the chart below and compare both the electrical specs of the U87i verses the U87Ai. This chart shows that the U87Ai has 5db less headroom. That is, the original U87i can handle a source level 5db louder before the on-set of distortion compared to the new U87Ai. Even, though the U87 is more often than not used for close miking audio sources, the U87Ai designers choose not to increase the headroom of the audio portion of the U87Ai circuit keeping the single FET class “A”circuit identical to the U87i.

The U87Ai has a nearly 3db better (unweighted) signal to noise but in this writers opinion the signal to noise is not as important as headroom when microphones are used in close proximity to a loud acoustical sources such as “rock” vocals, percussion or even dialogue associated with dramatic and animation voice recording.

Review the U87i specs compared to the U87Ai here?:

Parameter   U87    U87A(i)    rel. difference U87A/U87
Sensitivity (cardioid) -42 dBv      31.1 dBv 10.9 dB
weighted per CCIR 468-2)                          18 dBA 12 dBA    -6 dB
unweighted     25 dB    23 dB -2 dB
S/N (CCIR) 76 dB  82 dB  6 dB
max. SPL (0.5% THD) 122 dBspl 117 dBspl -5 dB
(with 10 dB pad)     13 RE2 dBspl 127 dBspl -5dB
dyn. range of amplifier  104 dB 105 dB   1 dB
max. output (<0.5% THD)    200 mV  390 mV 
-14 dBv  -8.2 dBv  5.8 dB
max. input (test port) 320 mV 390 mV 1.7 dB
Transducer capsule K87 K870 (K67.87A)   


More on the audio electronics circuitry of the U87?:

The U87i and the U87ai both use the same K87 dual diaphragm capsule. Due to the laws of physics a condenser microphone’s capsule has a rising high frequency response. This means high frequency content which is roughly those frequencies above 3khz are emphasized more than lower frequencies and the result is that the mic’s amplifier has to work harder above 3khz. The U87i and U87Ai both incorporate a HF de-emphasis circuit to reduce this phenomenon which helps to retain the delicate headroom of the U87 audio circuitry.   However, this de-emphasis circuit like any active equalizer circuit will cause slight phase anomalies to the audio signal.

Can an alternative microphone to the U87 be built for a more reasonable price and will it work for professional users?:


Does the Advanced Audio CM87 compare to the U87?:

The Advanced Audio CM87 is a microphone designed to provide the same features and sound signature as the venerable U87 but at a much lower price. The CM87 mic has a frequency response within 2db of a U87i or U87Ai across its audio bandwidth.

The CM87 vs the U87?:


The audio circuit of the CM87 increases the headroom of the mic’s electronics significantly.

This allows the CM87 microphone to handle louder sound sources than even the U87i while retaining the increased signal to noise ratio of the U87Ai. The CM87 can be purchased for nearly 1/10th the price of U87Ai. It offers an alternative to any U87 version at a much better price point.

How is this possible?:

This is possible because in the 21st century high quality electronic components are much cheaper and more accurate that those components made in the 1960’s. Also, the metalwork can be now manufactured with computer controlled milling machines dropping the manufacturing time significantly. The structure of the CM87 is also much more efficient and easy to assemble than the venerable U87. The price of the CM87 is also based on direct internet sales and there are no distributors or retail outlets taking a percentage of the profit. Plus any servicing goes directly back to the Advanced Audio shop and not back through a retailer or distributor which can significantly slow down service.

The CM87 like the U87 features a similar dual diaphragm, dual back-plate design which is polarized with the same voltage as the K87 capsule in the U87Ai. However, the AK67 capsule has a slightly larger surface allowing more damping holes to be computer drilled into the metal back-plate slightly reducing the high frequency increase of the capsules response compared to the K87 and providing a somewhat flatter response from the capsule.

How has the circuit of the CM87 been improved?:

 The audio circuitry has also been improved by using a 2-stage circuit while still remaining class “A”. The CM87 has an emitter/follower class “A” discrete/fet circuit that utilizes the full 48v to energize the FET and the following silicon transistor increasing the headroom the audio circuit by 6db.

The other advantage of the emitter/follower circuit is that that output transformer can have a much lower turns ratio decreasing the loss in the transformer circuitry by 14db compared to the U87 circuit. This has the added advantage of reducing the signal to noise of the circuitry and increasing the headroom significantly.

The lower ratio transformer of the CM87 with its much lower loss allows a medium gain FET (field effect transistor) to be used and the overall headroom of the CM87 compared to the U87 is increased respectively. This allows the CM87 to even be used in front of a bass drum or inches above the hammers of a grand piano without any significant increase in harmonic distortion.

The CM87 also features a 3-pattern selector switch on the front of the microphone plus a -10db pad and HP filter on the rear of the microphone. These switches are high quality metal toggle switches that are more easily sourced than the custom made switches in the U87. The switches in the U87 are incredibly expensive and time consuming to fit and replace when they eventually become defective. Also, the head grill of the CM87 can be more easily removed than in the U87 without any chance of damaging the more delicate plastic switch levers used in the U87i and U87Ai.

Does the CM87 use de-emphasis in its circuit?:

The slightly larger AK67 with its smoother response and the extended headroom of the CM87 means that no de-emphasis is required in the circuit and the phase response is no longer compromised. The CM87 in field testing has been found to be on average slightly brighter sounding than an original U87i but not noticeably brighter than the later CM87ai. Like the U87 the CM87 has a very nice present an warm sound for voice work without being overtly sibilant.

Who uses Advanced Audio CM87 microphones?:

Sirius Satellite XM recently purchased 28pc CM87 microphones for all their on-air studios in Nashville. Plus the CM87 has quickly become one of the most used microphones in the ever expanding project studio market.

More specs and user information can be found on the Advanced Audio Web-site.