Mojave MA-37

Mojave’s new valve mic pays tribute to a neglected classic.

The classic valve mics we all know, love and wish we’d bought before prices went through the roof are mostly European. Mics like the Neumann U47 have become Platonic ideals, eternally out of reach and forcing us mortals to make do with shadowy imitations and inferior copies. Yet even before Neumann discontinued the U47, it was already facing serious competition from a surprising source.

Out Of The East

Like many national broadcasters, the Japanese NHK were heavily involved in developing audio technology after the Second World War. Ribbon mics from this period sometimes surface in the West, and are usually close copies of RCA models, but the first Japanese capacitor mic to enjoy major export sales was a more original design.

Created by NHK engineer Heitaro Nakajima along with Kanane Nakatsuru from Sony, the Sony C‑37A resembled the U47 insofar as it was a large‑diaphragm capacitor mic with a valve‑based impedance converter. However, Sony took a very different approach from European manufacturers. The classic Neumann and AKG large‑diaphragm capsules are all Braunmühl‑Weber designs, in which two tensioned diaphragms and backplates are positioned back to back. These backplates are drilled with complex patterns of holes to create an acoustic labyrinth. Each side of the capsule can provide cardioid pickup, and when both diaphragms are energised, their respective contributions can be combined in the mic’s electronics to synthesize other polar patterns such as omni or figure‑8.

Sony’s design, by contrast, used only a single diaphragm, with a much simpler backplate. Despite this, it still offered a choice of cardioid or omnidirectional polar patterns, thanks to a mechanical shutter which was operated by inserting a screwdriver into the back of the capsule. In omni mode, this blocked off a vent in the rear of the capsule and turned the C‑37A into a pure pressure mic.

There have been small‑diaphragm mics that used mechanical pattern switching, including several Schoeps models and the Shure KSM141. The same principle was also employed in the Philips EL6033/10, an obscure moving‑coil dynamic mic, while Sony’s use of it was possibly inspired by the RCA 77 ribbon mic. As far as I know, however, the C‑37A and its derivatives are the only large‑diaphragm capacitor mics that operate this way.

Sony’s distinctive design approach also extended to the mic’s impedance converter, a 6AU6 pentode valve wired as a triode in a cathode follower circuit. In this configuration, the valve can supply current gain but not voltage gain. This means the output level of a cathode‑follower mic is typically very low, but on the plus side, it allows the output transformer and other electronics to be located at the other end of the mic cable, within the PSU. For this reason, European mic manufacturers tended to employ cathode‑follower circuits mainly where there was a need for the mic to be as small as possible, such as in the AKG C60 and C12A.

This choice on Sony’s part seems, by accident or design, to have helped the C‑37A catch on in the US studio market. At its launch in the late ’50s, most American studios were still using consoles and preamps created to work with passive ribbon mics. With close‑miking coming into vogue, the C‑37A’s low sensitivity and high headroom made it easier to handle than the hotter European mics. Its relative simplicity also made it competitive on price.

Rediscovery

Though it never seems to have made inroads in Europe, the C‑37A achieved widespread popularity in the USA, where it was used on countless classic recordings. Other Sony models also enjoyed some vogue through the ’60s and ’70s, but Neumann and AKG eventually reasserted their market dominance. Perhaps as a result, the C‑37A remained something of a ‘sleeper’ while other valve mics such as the U47 and C12 were being rediscovered in the ’90s. This particular sleeper, however, has long since been roused from slumber, and if you can find an original C‑37A for sale, it will now set you back many thousands.

One of the reasons why C‑37A prices are so high is that, until recently, no‑one was making a modern recreation. The Chinese suppliers who churn out K47 and K67 copies have yet to produce a version of Sony’s distinctive single‑diaphragm design, and of the Western high‑end specialists, only David Josephson seems to have tackled this particular capsule.

This situation may be about to change, however, thanks to another David. Most of us associate David Royer with ribbon microphones, thanks to the company that bears his name, but he has also designed many capacitor microphones, both valve and solid‑state. Marketed under the brand Mojave Audio, these often draw inspiration from vintage models, but for the most part, they are not clones or remakes. The new MA‑37 is, to some extent, an exception to this rule. David has admired the C‑37A for many years, and in the MA‑37, he’s set out to produce a “modern take” on this unassuming classic.

Out Of The Box

The MA‑37 ships in a large and robust Peli‑style case. The mic itself is relatively compact, at about six inches long by two across, but the power supply is a hefty beast. A good‑quality 5m cable connects the two, with the help of a short captive cable emerging from the mic. Like the Sony original, the MA‑37 has an integrated yoke mount which attaches directly to a mic stand, in this case using a 3/8‑inch thread. This doesn’t provide much shockmounting, but generally makes positioning the mic quick and easy.

The US‑made capsule at the heart of the MA‑37 faithfully recreates the original, including the mechanical pattern switching. The mic’s headbasket and body shape are also closely modelled on the C‑37, but with the distinctive addition of small pegboard‑style perforations all over the body. These provide some ventilation for the valve as well as making a distinctive visual statement. The overall build quality seems very good indeed, and the entire system oozes class in a nicely understated way.

The 6AU6 was never a particularly high‑grade valve, and supplying low‑noise examples today would be difficult, so the Mojave MA‑37 uses an EF86 instead. The basic circuit topology is similar, however, with the Lundahl output transformer mounted in the PSU. Sony’s own power supply went through a number of design iterations offering various different low‑ and high‑cut options. For the MA‑37, Mojave have settled on a three‑way rotary switch labelled M, V1 and V2. In the M (music) position the low end is allowed through unmolested, while the V (voice) positions introduce bass‑cut filters with 6dB attenuation at 40 and 100 Hz respectively.

The large and weighty PSU incorporates both the output transformer and the high‑pass filter.

In the M setting, the MA‑37’s frequency response is specified as 30Hz to 18kHz within a ±3dB tolerance. It’s not a mic you’d choose for recording bats, but those figures suggest a slightly greater high‑frequency extension compared with the C‑37A. Interestingly, David also says that the Sony capsule has a much greater bass extension than the classic Neumann or AKG large‑diaphragm designs. The frequency response chart bears this out, with the mic in cardioid mode being only 5dB or so down at 20Hz. In omni mode, the plot shows a low shelving boost that begins around 700Hz and is still 2dB up even at 20Hz. Elsewhere, the published response in both modes is very similar, being flat through the mid‑range and rising to a gentle, broad peak around 12kHz.

The MA‑37 can accommodate SPLs of up to 135dB before 0.5 percent distortion is reached, and self‑noise is specified at 18dB or better. At 5mV/Pa, its sensitivity is more akin to moving‑coil mics than capacitor models; by way of comparison, the current Neumann U87Ai offers 28mV/Pa in cardioid mode, and the Shure SM57 clocks in at 1.7mV/Pa.

The Sony Sound

The Sony C‑37A is a rare mic in the United Kingdom. I’ve never seen one in any British studio, even Abbey Road, and wasn’t able to get hold of one for comparison. So I can’t tell you whether the MA‑37 sounds exactly like the C‑37A, but that is perhaps beside the point. It’s not intended as an obsessive copy of that mic, more as an attempt to create a new mic that shares its good qualities. So it’s encouraging that the clichés that sprang to mind when I tried it exactly matched the clichés that are widely shared regarding the C‑37A.

If I had to pick one adjective above all to describe the sound of this microphone, it would be ‘smooth’, and that’s much more than just a function of its relatively neutral frequency response. There are plenty of flat mics out there, but the MA‑37 isn’t merely flat. On the one hand, it captures a nicely balanced sound, and at sensible working distances, doesn’t come across as muffled or veiled; yet, on the other, it’s mysteriously deaf to unwanted resonances and harshness. It’s almost as though it incorporated some sort of sophisticated dynamic equaliser that can seek out and minimise edginess in the source. (Given its extended bass response, it’s also surprisingly resistant to plosive pops.)

It’s not a universally great vocal mic — the sort of voice that is flattered by a U87 or similar mic can sound a little pillowy with the MA‑37 — but it is a really useful option for those ‘difficult’ voices that don’t work well on other capacitor mics, especially female vocals with a nasal quality or a tendency towards screechiness. In cardioid mode, it also has quite a bit of proximity effect, which you can use to your advantage on thin‑sounding vocalists. I didn’t get the chance to try it on brass instruments, but I imagine it would make a fantastic close mic for trumpets and their ilk.

I’m not one of those people who’s obsessive about mic choices for electric guitar, but I was really surprised by how different the MA‑37 sounds from other mics I’m familiar with.

Where I really loved the MA‑37, however, was on sources where you perhaps wouldn’t usually consider using a large‑diaphragm valve capacitor mic at all. It feels all wrong to position a mic like this right up against a snare drum, for example — but overcome your squeamishness and the results can be spectacular. There’s none of the usual emphasis on ringing and grittiness; instead, you get a soft, inviting bloom in the low mids, with a gorgeous ‘puffy’ quality at the top end. And my favourite application of all was on guitar amps. I’m not one of those people who’s obsessive about mic choices for electric guitar, but I was really surprised by how different the MA‑37 sounds from other mics I’m familiar with. It de‑emphasises the scratchy stuff in the upper midrange, delivering a richness and polished quality to the sound without making it boomy or congested. The only way I can fully describe the effect is to say that it makes the amp sound a lot more expensive than it really is!

Open The Shutters

Switching patterns on the MA37 requires a screwdriver and a deep breath...Talking of squeamishness, I imagine most of us will take a deep breath before plunging a screwdriver into the back of a capsule to change the polar pattern. Assuming you have a suitable implement lying around, though, it’s actually no harder than flicking a switch, and the only real downside is that there’s no visual indication to tell you whether the mic is in cardioid or omni mode. As you’d expect, the omni pattern is pretty approximate; there’s more acoustic shadowing going on than in a typical pencil mic or a conventional multipattern capacitor model, and sound arriving from the rear loses a fair bit of top end. There are useful applications for omni mics that narrow to a more focused pattern at high frequencies — that’s what is usually prescribed for a Decca Tree, for example — but in this case I suspect the omni setting might be more valuable for the on‑axis tonal changes it brings, especially at distances where the proximity effect would otherwise be apparent. On guitar amps, for example, the omni pattern sometimes seemed to deliver a thicker low midrange. I can also imagine using it off‑axis in omni mode to tame a particularly bright or harsh source.

Inasmuch as the MA‑37 has any negatives, they’re ones it shares with the Sony original. The biggest is probably the mic’s low sensitivity. This can be helpful in high‑SPL situations, but it can raise issues too. Naturally, the mic uses a good-quality screened cable, but even so, a small amount of hum pickup was audible in my studio space when I wanted to use it on quiet sources. This isn’t a criticism of the mic, just a caution that running a very small signal voltage through a relatively long cable can make you newly aware of a less‑than‑perfect electromagnetic environment! Some physical hum was also audible from the power transformer, though it wouldn’t be enough to bother me in practice. In terms of self‑noise, the review mic was more than quiet enough for general studio use, and I suspect quieter than most original C‑37As.

All things considered, then, this is perhaps not the ideal first mic, or only mic. But, again, that’s not intended as a criticism. What makes it great is that however many mics you already have, the MA‑37 will bring something genuinely different to the party (unless you’re lucky enough to own a C‑37A, of course). And unlike many ‘interesting’ or ‘different’ mics, it’s truly useful on a wide variety of sources. I’d even go so far as to say that on some of them, it does things you won’t get from any other type of mic.