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  • Bock Audio 4-Zero-7

    By Phil O'Keefe |

    Large diaphragm cardioid tube condenser microphone

    By Phil O'Keefe


    When it comes to condenser microphones, possibly the most famous models of all time are the Neumann U47, and its close sibling, the U48. This groundbreaking large diaphragm condenser was first released back in 1947 and has had a huge impact on subsequent condenser microphone designs. Long a favorite of artists from Frank Sinatra to the Beatles, and producers and engineers from George Martin to Bruce Swedien, they still remain incredibly popular. However, they have been out of production since the early 1960s, and with only around 5,000 units produced (plus less than a thousand U48's), they're fairly rare and quite expensive today. Even more of a challenge than finding one is trying to maintain and keep one of those old vintage classics up and running properly, which is no easy task due to the fragile M7 capsules used in some U47s, as well as the scarcity of the VF-14M tubes used in the originals.


    David Bock may not be a household name, but he is very well known and highly respected in professional studio circles. A longtime tech at such prestigious studios as Ocean Way and The Hit Factory, he has extensive experience in keeping their classic vintage microphones up and running, and in the process he learned exactly what makes them tick. He has applied that knowledge to designing and building the highly-regarded high-end Soundelux microphones, and since 2007, his own Bock Audio branded products. His latest microphone is the Bock Audio 4-Zero-7. While it has some similarities and parts in common with their top of the line 507, they are different microphones, with different capsules; the 407 is definitely out to capture the sound and vibe of a good 47. Let's take a closer look and listen…



    What You Need To Know

    • The Bock Audio 4-Zero-7 (407) is manufactured in limited production run quantities of five or ten microphones at a time in Southern California. Designed by legendary microphone expert David Bock, it is hand-built and utilizes point to point wiring and premium "through-hole" style electronic components throughout, and no hard to service surface mount components. While it is highly influenced by, and attempts to recreate the sound of the vintage U47, it is not a direct copy or exact clone of one. 
    • The 407 comes in a cardboard box, and inside you'll find foam cutouts for the microphone as well as the included accessories. In addition to the mic itself, you get a form-fitting foam-lined wooden storage box, a spider style shock mount, the requisite power supply and IEC power cable, and a 20' Gotham GAC-7 cable with Tuchel connectors for connecting the microphone and power supply.
    • The Bock-built true linear N470 power supply is the "brick" type, and externally it's quite simple, with a IEC power connector and power switch on one end, and a power indicator lamp, multi-pin Tuchel connector and XLR output on the other. A label on the supply warns against hot plugging, and you should always connect the power cable and turn it on only after everything else is hooked up. Both 115V and 230V factory-wired versions of the power supply are available.
    • The metalwork of the 407 is all custom made, and extremely well machined. The various components fit together snugly and precisely. The internal design and construction are also first-rate, and the overall build quality is outstanding.


    • In a transformer-balanced microphone, the transformer makes a significant contribution to the microphone's sound. The Bock Audio 4-Zero-7 uses a highly accurate reproduction of the vintage BV8 multi-sectional transformer that was utilized in the original U47. Designed by Oliver Archut, the BV8 in the 407 is manufactured in Gaylord Kansas by AMI/Tab-Funkenwerk.




    • The original U47 used two different capsules over the course of its production run - the PVC based M7, and the K47, which was introduced around 1958. The Bock Audio 4-Zero-7 uses a proprietary German-built modern reproduction of the center-terminated K47 type capsule with a gold sputtered 6 micron mylar diaphragm. The 407's capsule is specified, hand-selected and meticulously tweaked by David Bock. The materials used for a K47 type capsule are much more durable than those used in the M7 capsules (the PVC dries out and breaks down over time), and this choice should result in a much longer lifespan for the capsule than if a M7 type capsule was used. Contrary to popular myth, the two capsule types are both capable of equally excellent sound, so the decision to go with the more durable type isn't any kind of sonic compromise.




    • The original U47 used the metal-bodied VF-14M tube. Nearly every single VF-14 ever made was tested by Neumann, and only around one third met their specifications and received the "M" stamp. Manufacture of these tubes was completely discontinued in 1958 and they are extremely difficult to find today - and if you do find a good one, you can expect to pay a couple thousand dollars for it. Obviously since they're in such short supply, using them in a new microphone is just not practical, and a variety of tube substitutes have been used in U47s and U47-style microphones since the VF-14M was discontinued, with varying degrees of success. Bock Audio has selected a NOS (new old stock) Telefunken EF814k tube as the tube for the 4-Zero-7. According to Bock Audio, it has similar characteristics to the VF-14. The EF814k is a pentode, but is wired as a triode in the 407, with a fixed cathode bias and no negative feedback. The EF814k has also been used in other Soundelux and Bock Audio mic models, including the E47C and 507, and Bock Audio has a large supply of them, so you'll be able to easily re-tube your mic in the future when that eventually becomes necessary.




    • The overall frequency response of the 407 extends a bit further than the typical vintage U47, especially the low end, which goes all the way down to 10Hz, as opposed to the 40Hz of the original U47. This is done without compromising the highs, which are also slightly extended. It is by no means a bright or hyped sounding microphone (it's big, warm, and smoothly articulate), but it does have a bit more air than most vintage U47s that I've heard. The sound of this mic simply has to be heard to be appreciated. Singing into it is a blast, and incredibly confidence-inspiring! The bottom end is as thick and full as you would hope and expect, with tons of proximity when used in close. The midrange is strong and smooth, with the characteristic upper midrange presence you'd expect from a 47, while the top is articulate, yet buttery and silky, rolling off above about 18kHz rather than the usual 15-16kHz of a vintage '47.
    • Like the original U47, the Bock Audio 4-Zero-7 has a tremendous amount of proximity effect boost when placed close to the sound source - upwards of 12dB! Get in close, and you get tons of boost at around 100Hz. If you want HUGE sounding vocals, this mic can definitely deliver them. 
    • Even when used at a distance of 18"-24" or more, the U47 doesn't have a flat frequency response, and neither does the 407. This is due in no small part to the design of the head basket. The grille and head basket size and shape are significant contributors to the "U47 sound" - especially in the upper midrange, and the Bock Audio 407 replicates those characteristics perfectly. It has a heavy external grille, a finer grille behind that, and another heavy grille oriented diagonally behind that, and the same basic shape, dimensions and internal cubic volume as a U47 head basket.





    • The Bock Audio 4-Zero-7 has a 200 Ohm output impedance. Bock Audio recommends using a mic preamp with a 2000 Ohm input impedance for proper matching. Lower / low impedance settings on the mic preamp are not recommended. Having said that, I tried the 407 with a variety of different mic preamps, including models with fixed input impedance such as the API 312 and a 1272 clone, as well as a Radial Engineering Powertube and the board preamps on my Yamaha digital console, and it sounded good with all of them. The ideal mic preamp for use with this mic will, as always, depend on your personal preferences, the sound source you're tracking, as well as the context of the other tracks in the mix.



    • Unlike the original U47, there is no omnidirectional pattern available, nor is there a figure-8 pattern like you'd find on a U48 either. The Bock Audio 407 is cardioid-only. While this may limit your options somewhat, many users would probably never utilize those options and would leave the mic in cardioid anyway, so this may not be a significant concern for you. Besides, by focusing on the cardioid pattern, certain limitations and compromises inherent in the original U47/U48's design (such as lower output in omni or fig-8) can be avoided.
    • There is no low frequency filtering or onboard pad switch - but then again, there isn't one on a U47 either. Most mic preamps and consoles have high pass filters if you feel the need to use one, so this isn't a significant omission either.
    • While there is a decent amount of accessories included with the 407, at this price, it would have been nice to see a hardshell camera style case included too.
    • Let's face it - this is an expensive microphone, and beyond the financial means and/or the "how do I justify the purchase to my spouse" factor of many individuals and project studios. However, if you price vintage U47s (assuming you can find a good one for sale), you'll see that one in good condition can easily cost twice what the 407 runs, making the Bock Audio 4-Zero-7 an attractive alternative from a financial standpoint. Furthermore, since it utilizes an easily obtainable EF814k instead of the nearly unobtainable VF-14M tube of the original U47, it should be relatively easy (and much less expensive) to keep it running far into the future.



    In a word, this microphone is stunning - a world-class work of art. Not only in terms of its cosmetics and the obviously obsessive attention to detail and meticulous care that went into the design and build, but most importantly, the sound of it, which is spectacular. While no microphone is going to be the right choice for every source, the Bock Audio 4-Zero-7 is an excellent choice for multiple applications, including male and female vocals, as well as for instrumental recording duties. It sounds terrific on acoustic guitar and electric guitar amps (hey, if a 47/48 was good enough for the Beatles…), hand percussion, bass, as well as on drum overheads and as a room mic. While I didn't have the opportunity to try it on strings or brass, Bock Audio also recommends it for those applications too. But let's face it - the 4-Zero-7 is going to probably see more use as a vocal mic than anything else, and if it suits the singer and the track, it's quite simply superb - when you put it up you'll be rewarded with a sound that is very reminiscent of a good vintage 47. Between this and my Bock-designed ELUX 251, I feel I could expertly capture just about any vocalist.


    No two vintage U47/U48 microphones sound the same, and because of that it would be difficult to say that the 407 sounds exactly the same as any particular vintage U47. Having said that, the unmistakeable sonic characteristics of a good '47 are all here, and in some ways (such as the slightly extended top end) it actually sounds subjectively better to my ears. It's certainly in much better condition than many / most of the tired 50-60+ year old U47/U48s out there. True, it's far from inexpensive, but a microphone of this caliber is a musical investment that with proper care can last you a lifetime. The Bock Audio 4-Zero-7 is designed and built to provide decades of faithful service. Importantly, and unlike a vintage U47, the parts and tubes are available to readily service this mic when that eventually becomes necessary. For those who seek that legendary sound, you simply must try out a Bock Audio 407. It provides the vibe, tone and fun factor of the coveted originals, but without the maintenance hassles - and for significantly less money. Sounds great to me!




    Bock Audio 4-Zero-7 ($7,175.00 MSRP, $6,795.00 "street") Distributed by TransAudio Group



    Bock Audio 4-Zero-7 product web page





    Patterns:                                      Cardioid

    Frequency Range:                  10Hz to 18kHz, +/-2dB

    Sensitivity:                                  26mv/Pa

    Equivalent Noise:                  18dB (“A” weighted) 32dB (unweighted)


    Distortion vs.SPL @1kHz:

                                                              112dB = 0.5% THD

                                                              118dB = 1% THD

                                                              129dB = 2% THD

                                                              (increasing distortion is non-exponential, nearly linear, and primarily 2nd harmonic)


    Impedance:                               200 Ohms true transformer balanced

    Recommended load:           2k Ohms

    Dynamic range

    (maxSPL-"A"noise):               94dB

    S/N (94dB-noise):                 76dB “A”, 62dB unweighted

    Capsule size:                           1” diameter, single backplate K47 type

    Tube type:                                 New Old Stock EF814k

    Dimensions:                             2” dia x 8.5” oal (52x216mm)

    Weight:                                      1lb 9.3oz (1.58 lbs) =716g

    Shipping weight:                   13 lbs

    Power Supply:                        N470 true linear, factory wired 115v or 230v operation








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