Warm Audio WA-CX24

Unlike the original C24, the WA‑CX24’s rotation system has markings to help you set the correct angle.

Warm Audio escalate their mission to make vintage gear affordable with a recreation of a classic stereo mic.

Iconic stereo valve mics are like buses. You wait years for a modern‑day reproduction of one — and then three come along at once.

In May this year, I reviewed Vanguard Audio’s V24, which attempts to capture the strengths of AKG’s immortal C24 whilst correcting some of its perceived weaknesses. Announced at January’s NAMM Show but yet to reach SOS Towers, the Peluso P24 is described as a “tribute to the legendary AKG C24”. And now Warm Audio have pitched in with the WA‑CX24, “engineered as a faithful recreation of one of the most sought‑after stereo microphones in recording history”.

Quite how many people have been standing at the bus stop waiting for the number 24 from Vienna is debatable, but it’s clearly having a bit of a moment. And if you’re struggling to decide which vehicle to run for, the cost of the fares definitely favours Warm Audio. Whereas the P24 retails at $3999£3999, and the V24 costs more than twice as much, the WA‑CX24 is extremely keenly priced at £1449$1599.

24 Carat Brass

The AKG C24 is sometimes described as a stereo version of the better‑known C12, and does indeed have much in common with that mic. It uses the same valve, the same output transformers and, most importantly, the same ‘brass ring’ CK12 capsules that are also found in the Telefunken ELA M 251 and in early AKG C414 models. One reason why the prices of vintage AKG mics have remained so high is that these capsules have proven uniquely resistant to mass production. AKG themselves made them by hand, with a scary rejection rate, and eventually ditched it in favour of the cheaper and more amenable ‘nylon ring’ version. Even today, the Far Eastern plants that churn out accurate copies of Neumann K47 and K67 capsules still can’t do the same with the CK12.

The design of the C24 permitted a mutual angle of up to 180 degrees to be obtained by rotating the upper capsule, and rotary switches either on the PSU or a separate controller offered nine polar patterns for each capsule. It was thus possible to use a C24 for Blumlein stereo recording, with both capsules set to figure‑8 and angled at 90 degrees, but that was only the start of it. Many variations on crossed cardioid and hypercardioid could be created by varying the pattern and the mutual angle, and it could also be configured as a Mid‑Sides array; indeed, the stand adaptor offered dedicated settings for X‑Y and M‑S use.

Supersize C

In its ’60s heyday, the C24 wasn’t only employed in recording studios. Radio and TV also needed high‑quality stereo mics, and with the latter in mind, AKG’s designers worked hard to make their offering as compact as possible. The C24’s narrow cylindrical body is filled to bursting with circuitry, and the 6072 valve occupies a cut‑out in the middle of the back‑to‑back printed circuit boards that handle the signal from each capsule. Although they are small, the two Haufe T14/1 transformers barely fit inside.

Makers of modern C24‑alikes have less reason to worry about their mics being visually obtrusive, so tend to opt for more generous dimensions and a less crowded interior. Even so, I was unprepared for the sheer scale of the Warm Audio WA‑CX24. It’s gigantic: by some distance the largest capacitor microphone I’ve ever seen or used, and much larger than either the original C24 or the Vanguard V24. More precisely, it measures 356 x 51mm to the original’s 255 x 43mm, which by my calculations equates to almost double the internal volume. This has allowed, or perhaps been necessitated by, the use of TAB‑Funkenwerk transformers, which are billed as T14 remakes but which are much larger than the originals. It’s also permitted Warm Audio to employ a conventional PCB with the valve base mounted on one face rather than in a cutout. As on the V24, however, the vertical spacing between the capsules is noticeably greater than it is on the C24, making it slightly less of a coincident stereo mic.

Warm Audio make a feature out of their use of high‑quality components such as WIMA capacitors, and build quality seems very good. In this respect, the only obvious way in which the much more expensive V24 perhaps outdoes it is in providing a detented rotation system for the upper capsule. The WA‑CX24 follows the original C24 in making the upper capsule freely adjustable to any angle, though unlike the C24, it has the angles marked.

The WA‑CX24 ships in an appropriately huge tweed case, with the mic itself contained in an orange wooden box that looks as though it ought to house a pool cue. Also included are the power supply, which is home to nine‑position pattern selection switches for each capsule, a 5m cable that terminates in locking connectors at either end, and a shockmount that holds the mic body using two spring‑loaded clips. I preferred this to the V24’s rather uncomfortable mount, which attaches only at the base and isn’t ideal for horizontal use.

The WA‑CX24 ships in a suitably plus‑sized case with its PSU, cables and mount.

12 Tones

A large element of the V24’s high price is down to Vanguard Audio’s choice of capsules. Wth no truly authentic CK12 design available in mass production, they’ve gone for one of the handmade ‘boutique’ options from BeesNeez in Australia. What, then, of Warm Audio’s “edge‑terminated, dual‑backplate, all‑brass vintage CK12‑style capsules”? The company’s founder and President, Bryce Young, told me: “The original AKG CK12 capsules were handcrafted and have internal ‘chambers’ which make them difficult to produce properly at scale. Our all‑brass CK12‑style capsules provide the same frequency response as the vintage original CK12 capsule, but with a different hole pattern that allows for modern manufacturing techniques. Other reproductions use one of the most common types of LDC capsules on the market, the K87 capsule. We want to be clear that our CK12‑style capsule is not a K87: ours is a custom design constructed with different holes to achieve the original CK12 frequency response.”

The specifications of the WA‑CX24 closely follow those of the C24 in many respects. Unlike Vanguard, Warm Audio haven’t attempted to improve the self‑noise of the design, which they list at 22dBA. The C24 reaches 0.5% distortion at a relatively modest 117.5dB SPL, whereas Warm Audio quote a maximum SPL of 149dB. This, however, represents the level at which 1% distortion is reached. Warm Audio also publish frequency response charts for the omni, cardioid and figure‑8 settings. As on the original, these show a flat response through the bass and lower midrange, with a broad presence boost that lifts the high end and varies somewhat with pattern. However, Warm Audio’s plots suggest that this treble boost kicks in nearly an octave lower than that of the C24.

Figure Skating

As I mentioned in my review of the V24, I’m fortunate enough to have an AKG C24 available for comparison, but wary of drawing too many conclusions from such a comparison. This particular mic has been rebuilt with new PCBs and transformers, and has a slightly later iteration of the ‘brass ring’ CK12 than was ever found in original C24s. The upshot is, I suspect, that it’s rather brighter than a typical specimen. It was certainly brighter than the V24, and in comparison with the WA‑CX24, this particular C24 was typically several dB up above 5kHz or so. Conversely, the WA‑CX24 was somewhat more present in the 2‑3 kHz area, bearing out the suggestion that its treble emphasis starts a little lower down the frequency spectrum. This is, of course, relative; objectively speaking it’s not honky or midrange‑forward, and if you were to compare it to something like a U87, the midrange would feel well controlled.

The two capsules in a stereo mic obviously need to be well matched in terms of both sensitivity and frequency response, and as far as I could discern, the review WA‑CX24 performed pretty well in this respect. I couldn’t detect any difference in sensitivity between the two sides, and they were timbrally as close as you could reasonably expect, given the configuration that places one capsule in a significantly more open environment than the other.

Little and large: the WA‑CX24 (right) dwarfs the mic that inspired it.In an ideal world, the two sides of a stereo mic would also maintain a consistent on‑ and off‑axis frequency response across different polar patterns, with the only variability being the sensitivity to sound arriving from different directions. In practice, this is a fairly distant ideal when it comes to large‑diaphragm capsules such as are found in the WA‑CX24. It often seems to be the case that any on‑axis high‑frequency boost gets exaggerated and shifted downwards through the spectrum as the polar pattern approaches figure‑8, and that’s certainly the case with both the C24 and the WA‑CX24. Consequently, I am not generally a fan of figure‑8 settings on large‑diaphragm mics, as I often find them shrill or spitty on‑axis and obviously coloured off‑axis, at least compared with ribbon mics. Subjectively speaking, I’d say the C24 was the more shrill and the WA‑CX24 the more coloured in figure‑8 mode.

Crossing Over

For me, the real advantage of a design like the C24 lies in the number of subtle variations that are possible on the theme of crossed cardioid or supercardioids. Crossed cardioid mics at 90 degrees have a stereo recording angle of more than 180 degrees, which is rarely what you want, but increasing the mutual angle and/or nudging the pattern a step or two towards hypercardioid can achieve a satisfyingly full stereo image with a balanced timbre. The WA‑CX24 didn’t sound exactly like my test C24 when used thus, but it certainly had the same strengths, and it makes an excellent one‑stop shop for drum overheads, grand pianos and the like.

As befits a close relative of the C12, the C24 is also in its element as a close mic. On vocals, for example, you can set both capsules to face forwards and use different gain settings or processing chains to experiment with slightly different sounds. On acoustic guitar, you can align the mic parallel with the fretboard with one capsule angled upwards and the other downwards, for a subtle stereo effect. In my tests, I compared the WA‑CX24 not only with the C24 but also with an older C414 EB that likewise has the ‘brass ring’ capsule. For whatever reason, this C414 is much less bright on‑axis than the C24, but both have a characteristic ‘dryness’ to the midrange. In terms of high‑frequency response, the WA‑CX24 lay somewhere between the two, while its midrange tonality was attractive but somehow subtly different.

Fewer than a thousand C24s were ever made, and when they come up for sale, the asking price is typically north of 10 times what Warm Audio are asking for the WA‑CX24.

For me, the question “Does the WA‑CX24 sound exactly like the AKG C24?” is moot anyway. For one thing, the variability of CK12 capsules and the existence of at least one major design revision through the C24’s history make that sound something of a moving target. More importantly, though, fewer than a thousand C24s were ever made, and when they come up for sale, the asking price is typically north of 10 times what Warm Audio are asking for the WA‑CX24. The few people who are in a position to buy a ‘real’ C24 are unlikely to consider the WA‑CX24 as an alternative, while those who are attracted by the Warm Audio mic simply won’t have the option of going for the vintage original.

The real attraction of the WA‑CX24 isn’t that it mimics a sought‑after vintage model. It’s that it is a high‑quality stereo valve mic at an unprecedented low price. As far as I’m aware, there’s simply no competition below the level of the Peluso P24, which is more than twice as expensive. If you want a multi‑pattern, stereo valve mic for under $£2k, the WA‑CX24 isn’t just the best option. It’s the only game in town. And the fact that it bears more than a passing resemblance to one of the greatest mics of all time is the icing on the cake.