Nakamichi Model CM-700 Electret Condenser Microphone Systems (Equip. Profile, Sept. 1978)

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MANUFACTURER'S SPECIFICATIONS:

Generating Element Type: Electret condenser

Directivity: Cardioid or omnidirectional.

Frequency Response: 20-20,000 Hz 13 dB.

Output Impedance: 600 ohms, ±15 percent, balanced.

Sensitivity, Cardioid: -65 dB (1 kHz, 0 dB=1 V/ pbar).

Sensitivity, Omnidirectional: -63 dB (1 kHz. 0 dB=1 V/ pbar).

Maximum Input SPL: Cardioid, 130 dB (145 dB with 15-dB attenuator); Omnidirectional, 128 dB (143 dB with 15-dB attenuator); both re: 20 microPascals for 3 percent distortion.

D.C. Power Requirement: 6 V, 1 mA using silver-oxide battery--Eveready type no 544.

Battery Life: 200 hours, continuous use.

Accessories Supplied: Battery, windscreen, two-conductor cable with XLR and phone plugs, foam-lined vinyl case, stand adaptor, 15-dB attenuator section, and CP-702 omnidirectional capsule.

Accessory Available: CP-703 shotgun ultra-directional capsule with foam windscreen.

Price: CM-700, $185.00 and CP-703, $85.00.

Note: The CM-700 microphone includes CP-701 cardioid capsule, standard center section (FET preamp), plus the microphone body which includes the Off-Flat-Lo Cut switch and 3-pin output connector. It also includes the accessories listed above, but not the shotgun capsule.

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Fig. 1 Impedance of the Nakamichi CM-700 microphone system with the CP-701, 702, 703 capsules, and 0 or-15 dB preamps (center section).


FIG. 2 Frequency response vs. distance with the CP-701 cardioid capsule.


Fig. 3 Effects of loading on "Lo-Cut" frequency response with CP-701 cardioid capsule.


Fig. 4 Frequency response vs. angle with the CP-701 cardioid capsule.

This is a high-quality electret system, having a very small diaphragm (16 mm), and is suited to audiophile music recording applications as well as in sound reinforcement for the performing arts. The cardioid and omnidirectional capsules are most useful for the former application and the shotgun capsule for the latter.

The system is suited to professional use from the viewpoint of audio quality, but applications in studios and permanent sound systems may be limited because the CM-700 cannot be remote powered. (Remote-powered mikes usually cost more than the CM-700.) The shotgun attachment is good for special purposes Involving long distance sound pickup such as wildlife sound recording and surveillance applications.

The CM-700 microphone system is perhaps unique among electret or air-condenser microphone systems because the FET preamps, as well as the condenser transducer capsules, can be interchanged. All of these pieces are included in the CM-700 system, and there is nothing extra to buy except perhaps the CP-703 shotgun capsule. This is logical because not all users will need this accessory. Our first impression was that you get quite a lot of parts for the money, and if worked only half as well as advertised, it would still be a bargain. The CM-700 system plus the CP-703 shotgun posed a formidable challenge for our laboratory because if we tested all possible combinations of pieces in all possible ways, it would require ten-to-the-nth hours! The principal parts of the CM-700, starting at the connector end, are the main body with the Off-Flat-Lo-cut Switch, the battery, the center piece containing the FET preamp, and the condenser-transducer capsule. The battery fits into the barrel of the main body and the center piece screws on and holds the battery in place. You have a choice of two preamps plus two capsules (three including the shotgun). One preamp (unmarked) yields flat response and normal sensitivity. The preamp marked "-1 5 dB" yields flat response, 15 dB lower sensitivity, and correspondingly higher sound pressure capability. The cardioid or omni capsule may be used with either preamp. The rigid plastic windscreen can be slipped over the capsule to a positive stop at the base of the preamp, and this screen is more durable than a foam sock which eventually deteriorates.

The CP-703 shotgun capsule is not simply a capsule ... it consists of a condenser capsule attached to a 16-in. long tube, of 1/2-inch diameter, with a series of small holes along the length of the tube. This assembly is screwed onto a FET preamp, which is unmarked, but appears identical to the preamp/center section that is used with the other capsules. We mixed up these two preamps and had to make an acoustic response test to sort them out. They are electrically different the shotgun preamp frequency response is specially tailored for maximum speech intelligibility. A foam windscreen covers the entire length of the shotgun pipe, capsule, and preamp.

Our entire kit included three capsules, three preamps, two windscreens, a mounting swivel, and a cable assembly. The cable is terminated in a '/a-inch phone plug for use with unbalanced inputs such as on Nakamichi equipment, but the mike may be used with balanced inputs, center-tap grounded or floating, by using a standard mike cable with an A3F connector on the mike end. The system is available in any color as long as you choose black, in keeping with Nakamichi's "black look" Laboratory Tests

Figure 1 shows the impedance curves. The impedance on "flat" setting conforms to the 600-ohm nominal spec value. EIA Standard SE-105 states that a low impedance microphone shall be 600 ohms or less, but states a maximum value of 180 ohms as standard. This is being ignored by many makers, and the Audio October, 1977, Directory issue shows that today's hi-fi microphones range from 1 50 to 600 ohms. We experienced no loading effects when testing the mike into the 150-ohm "unloaded" input of our broadcast mike preamp with the mike switch on "Flat" response, but on "Lo Cut" experienced loading effects because the mike impedance exceeds 2000 ohms at 150 Hz. More about this later, but we recommend that the user make certain that his equipment input is "unloaded" before connecting the mike to a 1 50-250 ohm input. Most inputs are "unloaded," that is, the actual impedance is 5 or 10 times nominal value.

Figure 2 shows the on-axis frequency response of the CM 700 with cardioid capsule and normal gain FET preamp. At 12 inches, the response is ruler-flat and equal or better than any air or electret-condenser mike we've tested. The rolloff above 16 kHz may be ignored. If we interpolate to a distance of 50 cm (20 inches), our curve matches the factory curve supplied (this may not be supplied with your mike), within about ±1 dB. Results on sensitivity agree precisely with the factory value. The factory curve was not corrected to plane-wave conditions so the low end rolloff appeared slight. Our "Testing Update" article discusses the plane-wave testing problem. For distant miking, the cardioid capsule response is lacking in bass. Some cardioid condenser mikes have a bass rolloff which is good for many field uses, where very low frequency ambient noise is present and for pop music which has lots of bass. For classical music, bass boost may be desired, and we will describe an inexpensive way to equalize the mike.

Figure 3 shows the impedance loading effects. For "Flat" setting, response was the same into our preamp or into an open circuit. With "Lo Cut" setting, the preamp produced more bass rolloff than an open circuit. In the voice frequency range above 200 Hz, the preamp loading helps to compensate for proximity effect, but your preamp may not exhibit the same loading as ours.

The frequency response at various angles (Fig. 4) gives a hint of the superlative quality of the CM-700/CP-701. The 90-degree curve is nearly identical to the 0-degree curve. Presumably, the frequency response is unchanged throughout the entire front hemisphere. The small diaphragm size is responsible for this uniformity. All ½ or 5/8-inch diameter laboratory microphones generally have rather uniform directivity to 15,000 Hz, but 7/8- or 1-inch diameter microphones do not. Precision sound level meters are now equipped with 1/2-inch diameter microphones to permit accurate measurements at high frequencies in reverberant rooms where sound wave direction is random.

The CP-702 omnidirectional capsule (Fig. 5) has ruler-flat bass response and a nice compromise adjustment of the high frequency response. There is a slight excess of highs in the 0 degree curve for close miking and a slight deficiency in the 90 degree or random incidence responses for distant miking, such as when the mike is suspended over an orchestra. The small variation in response from 0 to 90 degrees is scarcely audible and, again, is a result of the small diaphragm.

The plastic windscreen produced little change in response (Fig. 5), less than most foam screens. The screen was found to be effective outdoors in winds up to 10-15 knots. There is a problem with screen I.D. tolerances, because one screen fit too tightly and marred the mike finish, while another fit too loosely.

The CP-703 shotgun capsule was tested at six feet outdoors due to its great length. (See the "Testing Update" article.) Figure 6 shows the frequency response, and it may be assumed that a plane wave exists at this distance. Our data does not correlate with the factory curve, undoubtedly because of the big difference in distances to the source 50 cm vs. 6 feet. We presume our test is more accurate. The "Flat" response is generally undesirable for shotgun applications. The rising response on "Lo-Cut" appears ideal for long distance outdoor speech pickup, as it will reduce low-frequency noise and emphasize the higher frequencies so critical to intelligibility. Figure 7 shows a non-standard condition where we used the preamp section from the cardioid/omni capsules. The response is considerably flatter, and we thought might be more ideal for quiet locations, as in a studio or auditorium. Note the increased sensitivity. The foam screen for the shotgun was not terribly effective on a breezy day, and tests had to be postponed until the wind was less than five mph.

Figure 8 reveals the excellent directional characteristics of the CP-703. Essentially null response is obtained at 90 and 180 degrees. A microphone of this type should degrade to a cardioid pattern below about 500 Hz, but Nakamichi has apparently defied the laws of physics, maintaining uniform directivity at low as well as mid and high frequencies. (Some years ago we stated in a Patent Application that a shotgun mike must be 22-feet long to obtain this performance!) So far, all of these data show that the CM-700 is equal (and dare we say superior?) to any air or electret condenser we have tested. The small diaphragm size is responsible. But it is well known that 1/2or %-inch laboratory condenser mikes have poor signal-to-noise ratio; that is the noise level expressed in equivalent SPL is high, reducing usable dynamic range. The noise test (see "Testing Update" article) is critical to proving or disproving the superiority of this microphone.

Our test results shown in Fig. 9 reveal a fairly uniform noise spectrum, and the unweighted and "A" weighted levels are 29 and 22 dB, respectively. The one-third octave levels are a few dB higher than a good air condenser at mid frequencies (see our January, 1978, review), but the low frequency noise levels are significantly lower. The 22 dB "A" weighted scale level means that microphone noise will be less than ambient noise in a very quiet studio, church, or auditorium. Our BK-5B reference mike measured only 3 dB "A" weighted scale less noise, and it has no internal amplifier to make noise! Again, Nakamichi appears to have repealed the laws of physics. We see little reason to purchase a larger, more costly air-condenser microphone, unless you need special features such as remote powering.

The dynamic range, frequency response, and directional properties of the CM-700 with the omni capsule are ideal for sound measurement as well as for speech and music pickup.

Specifically, the omni capsule is suitable for Type 1 precision sound level meters meeting ANSI Standard S1.4-1971. The cardioid is particularly suited to loudspeaker tests in rooms because of its uniform frequency response and ability to reject reflected sound. A cardioid mike may be ideal to use for system/room equalization because, not unlike your ears, it discriminates against reverberant sound. [The editor reminds us that TDS equipment, a la Dick Heyser, discriminates against reflections, permitting an omni mike to be used for speaker testing in rooms.] The phasing test showed pin-2 positive in accordance with the prepared revision to the EIA standard. The clipping level with cardioid capsule and close-up speech was 129 dB re: 20 micro Pascals. The measured output of the "-15 dB" preamp was approximately 15 dB less than the standard preamp, and we can only assume that the clipping level increased to at least 129 + 15.0= 144 dB re: 20 microPascals.

Listening and Use Tests

The Nakamichi CM-700 sounds identical to our BK-5B reference microphone except for a slight decrease of "presence" when reproducing speech at 6 to 12-inches distance. The BK 5B has a smoothly rising response from 50 to 6000 Hz and a ruler-flat mike sounds a bit dull by comparison. Similar results were obtained with CP-701 capsule and "Lo-Cut," and CP-702 capsule and "Flat" response.

Then we tried the shake test and heard a loud rattle in the earphones. It required considerable patience and skill to find that there were two similar rattles caused by loose-threaded rings in the bottom of the cardioid capsule and in its preamp. We removed the ring from the capsule because of curiosity, and the entire back plate and insulator assembly fell out! Looking inside the capsule housing, we saw the foil electret which was translucent to light. This was the first electret mike we've taken apart where the foil electret wasn't cemented to the rim of the back plate. We held out little hope of making the capsule work again.

We recalled that electret foils more or less sit on the back plate and, hoping that no critical airspace was involved, we reassembled the capsule.

Well, the cardioid capsule talked again, so we repeated the frequency response test. Lo and behold the response and sensitivity were within about ± 1/4 dB of that shown in Fig. 2! Has Nakamichi repealed the laws of physics again? [Editor's note: Nakamichi promises they'll either tighten the rings or supply a spanner wrench with every mike!] With a sigh of relief, we resumed our listening tests. Now, nothing rattled. Vibration sensitivity with the cardioid capsule was higher than the BK-5B, and rubbing the mike generated a substantial amount of high frequency sound. This is no worse than a good air condenser, and, similarly, the CM-700 should be resiliently mounted where vibration is a problem. The CM 700 with omni capsule has equal vibration sensitivity, and is not highly recommended as a hand-held vocalist's mike. Another reason not to let a vocalist hold it (besides not wanting it dropped) is that the switch may be accidentally actuated.

The windscreen affords excellent "pop" protection for either capsule, as well as the previously mentioned wind-noise reduction. "Pop" sensitivity was only a little higher than the BK-5B with its large 4-inch diameter accessory screen. We recommend using the screen in all but the most pristine environments and on all occasions when picking up speech or close-miking instruments. Without the screen, the CM-700 has very high wind and pop sensitivity.

The CM-700 has less hum pickup than the BK-5B, which puts it in the "excellent" category. This, no doubt, contributes to the low noise output and lack of 60 Hz and harmonics in the spectrum.


Fig. 5 Frequency response with the CP-702 omnidirectional capsule.


Fig. 6 Frequency response with the CP-703 shotgun capsule.


Fig. 7 Frequency response with the CP-703 shotgun capsule using preamp (center section) for cardioid and omni capsules.


Fig. 8 Frequency response vs. angle with the CP-703 shotgun capsule on "Lo-Cut," 150 ohms impedance, and "Unloaded" input.


Fig. 9 One-third octave band noise spectrum with the CP-701 capsule on the "Flat" response setting.

Continuing our talk test versus the BK-5B, we compared sound at 90° off-axis and discovered that the CM-700/CP-701 sounds just the same as on-axis, but the BK-5B has a distinct nasal quality off axis. This aroused our interest, and we fired-up our RCA MI-10006A condenser mike (See Audio Cyclopedia and "Update" article) for comparison. We were amazed by the result: On-axis, the two sounded identical, but off-axis the Ml 10006A sounded completely muffled compared to the natural sound of the CM-700. Then we compared the CM-700/CP-701 /CP-702 in the studio with recorded music and were astonished to hear an entire top-octave not present in the BK-5B! (The BK-5B has fairly uniform axial response to 15,000 Hz, but at 90° it is not uniform.) Our recorded sources did not have much music in the top octave because the tapes were recorded with BK-5s or 77-DXs, so we tried an acoustic guitar. A lot of very high pitched harmonics were heard with both CP-701 and CP-702 capsules that were inaudible with the BK-5B. The Nakamichi microphone plus earphones enabled us to perceive these sounds more clearly than by ear alone.

The strikingly beautiful sound of the CM-700 on strings inspired us to borrow a second system and substitute CM-700/ CP-701 for BK-5Bs to make a recording of the Bach B Minor Mass with chorus, soloists, and orchestra, including pipe organ and harpsichord. The location was a high-ceilinged church, seating about 600, and with a reverberation time of approximately three seconds. We used a ReVox A77 reel-to-reel tape deck that we tuned up with Maxell UD tape for ±1 dB, 3020,000 Hz frequency response and 10-dB headroom above OVU. Fortunately, we were able to compare our tape to a previous B Minor Mass recorded in the same place with the same machine, but using 77-DX microphones.

Playback in our studio revealed that although the 77s sounded a bit better on the chorus, the top-octave-plus was missing.

Of course, the (on-axis) string section sounded better with the CM-700/CP-701, but the sound of off-axis instruments was most startling. The harpsichord on the right could be heard at all times, but at my location in the church I could not always hear it.

The brass on the far left were almost too loud, and rich with harmonics. By comparison, the 77-DX sounded as if it were "focused" on the centrally located sources which were chorus, strings, flute, and woodwinds, but off-axis instruments were muffled. In this church, the orchestra stretches from left to right and subtends a wide angle from the microphones which are positioned in the third row. Bass viol was slightly stronger with the 77-DX, which exhibits uniform response to 40 Hz for plane waves.

The CM-700/CP-703 shotgun was subjected to an entirely different set of listening tests. First we positioned a portable radio, set on a news station, outdoors about 100 feet from the building. It was adjusted for normal speech, 65 dB re: 20 micro Pascals at one meter. We set up our listening equipment on the roof and compared the Nakamichi to the MI-10006A condenser with its set of pipes, which are the same length as the Nakamichi. We would expect these mikes to be similar. With our unaided ears, we could understand a very few words above the (low frequency) ambient noise, primarily auto and truck traffic. Aided by the Nakamichi, we understood about 80 to 90 percent of the words. It had to be precisely aimed at the source, but actually performed a little better when aimed at a sloping section of roof which apparently acted as a reflector. The RCA mike picked up only low-frequency noise no speech was heard at all! The preamp that is attached to the CP-703 worked best the other flat response preamp yielded a poorer signal-to-noise ratio. "Lo Cut" had to be used at all times to reject noise.

Then, our son tried the CM-700/CP-703 for stage reinforcement of vocalists in a high school musical show. The outstanding discrimination enabled weak vocalists to be heard above the orchestra and with realistic audio quality, although the rising response preamp was used, plus "Lo Cut." The director, who has a lot of professional experience, indicated the CP-703 was the best stage mike he'd heard. To achieve these results, a fulltime operator was needed in the orchestra pit to aim the mike accurately. In our large auditorium, we need three mikes and three operators to cover the entire stage. The operators should have had monitor phones to assist aiming and handling of the mike.


Fig. 10 -- Diagram of a corrective equalizer and 12-dB pad for the CM-700 microphone and CP-701 cardioid capsule.


Fig. 11 -- Measured response of equalizer for the CM-700 microphone and the CP-701 cardioid capsule, and the computed response of the system.

The mechanical quality of the CM-700 system is excellent, save for the easily-fixed problem of loose rings. The system components have fine threads which may be damaged by careless cross-threading. On our mike, the external threads were not masked during finishing, and we experienced some binding during the first few engagements. The swivel mount incorporates what appears to be a tapered 1/2-inch pipe thread. It fits an iron pipe having a tapered thread, but not the Atlas AD-1 RCA Adaptor which has a straight pipe thread. Microphones should have a straight thread, specified as "'/z-inch NPSM." An adaptor to fit 5/8-27 mike stands is included but it doesn't have a knurled shoulder as the AD-1 does, and pliers were needed to remove it from the stand each time we used the microphones.

The acoustical performance of the Nakamichi CM-700 system is superior to comparable microphones tested to date. The CP-701 cardioid capsule is perfect, save for the bass rolloff on distant sources. It occurred to us that a simple passive equalizer could provide linear bass response.

Figure 10 shows a suitable equalizer that may be inserted into the mike line. Figure 11 shows the measured frequency response of the equalizer, plus our BA-31 preamp, and the resulting ruler-flat acoustical response for plane waves. Of course, electrolytic capacitors are a "no-no" for this application; we use small 50-V mylar capacitors.

This equalizer does two additional useful things: 1) The 12 dB loss reduces the acoustic sensitivity to equal ribbons or dynamics and restores a "normal" gain setting on our ReVox, which in close miking, could prevent recorder overload and would eliminate the need for the-15 dB preamp in cases where SPL is less than 130 dB; and 2) the output impedance (at mid and high frequencies) is reduced to 225 ohms which presents a more suitable source impedance to 150/250-ohm inputs, which are still required to be "unloaded." In the future we will try this equalizer at a live recording session. We hope it will provide the desired bass without picking up too much VLF ambient noise, and we will also be checking for noise from the recorder preamp. This equalizer is not needed when the source is closer than three feet from the microphone, so it's mostly for classical recording applications. Others, who want an extra heavy bass sound, may find it useful.

The CM-700/CP-703 omni capsule is an excellent system, requiring no bass equalization, but we can't use them as permanently suspended recording mikes until we figure out how to power them from a remote source. We would appreciate learning of schemes for remote powering, experiences with equalization, and other comments from users. We tried the omni capsule for recording a concert where the mikes were aimed straight up and the instruments and vocalists surrounded the mikes. Sound quality was as good as with the cardioid capsules, and we were spared the complexities of setting up many cardioid mikes.

We highly recommend the Nakamichi CM-700 system for the most exacting applications in music recording and sound reinforcement for the performing arts. To those who doubt that a moderate cost mike system can do justice to a multi-thousand dollar recording or reinforcement system, we say "Try it we think you'll like it."

--Jon Sank

(Source: Audio magazine, Sept. 1978; )

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