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The U87 LDC multi-pattern condenser Microphone On November 02, 2019


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.


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