You are here

Unlikely Stories: The AKG DX11 Reverb Mic

AKG's 1963 microphone with built-in spring reverb By Sam Inglis
Published January 2024

The 1960s: a time of sexual revolution, radical politics and microphones with built‑in spring reverbs.

Unlikely StoriesToday, live sound is more high‑tech than studio recording. Whilst we cling to our vintage consoles and valve preamps, front‑of‑house engineers operate in a world of networked audio and digital effects. When big‑name bands take their latest album on the road, the hire budget for the tour often dwarfs the cost of recording the album in the first place. But it wasn’t always like that.

In the early ’60s, even the best PA systems of the day were barely adequate. Sound reinforcement might consist of a single vocal mic, with amplifier and speakers of dubious provenance. Stage monitors were unheard of, and any control over the vocal sound beyond turning it up or down was a luxury. Then an Austrian mic manufacturer decided to bring some colour to this grey sonic world.

Spring Theory

Introduced in 1963, the AKG DX11 may have been the first active solid‑state microphone, pre‑dating the Fairchild F‑22 / Syncron AU‑7A by a couple of years. But it was not a capacitor mic, and it was not intended for studio use. Early transistor circuits couldn’t compete with valves from the point of view of fidelity, noise performance or dynamic range. Their advantages were more to do with small size and low power consumption, and it was these qualities that AKG’s engineers exploited to create the DX11.

By the early ’60s, the use of effects in the studio was well established. Pop vocals on record dripped with slapback delay, plate, spring or chamber reverb — but there was no way to reproduce most of these effects live. Until, that is, someone at AKG had the bright idea of building them into the mic itself.

No advance in solid‑state electronics would allow a plate, chamber or tape delay to be squeezed into a mic, and besides, AKG had no experience of building these devices. They had, however, spent several years investigating the acoustic properties of springs. That research would eventually lead to the unparalleled BX series of studio reverb boxes, but in its first public outing, it was paired with a moving‑coil capsule and a battery‑powered preamp to create the DX11.

A mic with a spring reverb built in worked about as well handheld as you’d expect it to.

Goes To 11

Either hideous or possessed of rugged industrial chic, depending on your point of view, the DX11 wasn’t exactly a slimline mic. Even though the casing was made entirely of rigid, off‑white plastic, the packed internals made it a pretty weighty affair. Not that this mattered much, because a mic with a spring reverb built in worked about as well handheld as you’d expect it to.

Some AKG dynamic mics from this period, like the D12 and D19, remain sought‑after classics. However, the company also made a lot of cheaper models intended for consumer markets, and the DX11 supposedly repurposed the capsule from the D14, a plucky underdog that looks a little like the D12’s younger brother.

Getting useful signal from a reverb spring requires a much hotter input than any microphone capsule can put out directly. Enter the DX11’s newfangled solid‑state electronics, which used germanium transistors to amplify the signal and drive the mic’s single spring. These were powered by a single 9V battery. An on/off switch helped to preserve battery life when the mic was not in use, and the only other control was a wheel‑like rotary which varied the level of the wet signal. This was thoughtfully positioned in such a way that the user could operate it with their thumb.

Like many AKG mics of the time, the DX11 was sold as an OEM product under several other brand names. It was even rebadged by Fender and sold into the US market as the FR570. But it never exactly set the world on fire, and was quietly discontinued in the early ’70s. Was it too expensive to make? Was it ahead of its time? Or was it simply not very good? The answer, it seems, is a little from column A, a little from column B, but also quite a lot from column C.

The DX Factor

I’ve been curious about the DX11 since I first saw a photo of it, but not curious enough to pay the hundreds of pounds that some sellers ask. And when I finally found one, some work was required to get it up and running. As supplied, the DX11 had a captive cable to which the user attached their own output connector; depending on how it was wired up, this gave the mic either a 15kΩ or a 200Ω output impedance, but it was unbalanced either way. The cable on mine had frayed and needed replacement, so I deposited it with the long‑suffering Stewart Tavener at Xaudia, who found space to fit a small balancing transformer, as well as a mini‑jack socket that would allow the reverb to be used independently of the mic.

AKG’s product literature for the DX11 claimed a frequency response of 50Hz to 18kHz, which seems a little optimistic. In practice, it has an obvious roll‑off below 150Hz or so, which probably suits its intended role as a vocal mic, and a midrange presence peak that is fairly typical for a dynamic mic. The active circuitry is, as you’d expect, somewhat noisy and limited in headroom. The reverb, however, is surprisingly epic, especially when you consider that it’s generated by a single, short spring. The decay time must be at least five or six seconds — perfect for those torch songs and ballads, less so for rock & roll. In mine, at least, it also has a characteristically ‘ringy’ tone, with a prominent midrange resonance that needs notching out with EQ. On the plus side, the thumbwheel that controls the wet level works perfectly, allowing you to fade the reverb in and out in real time.

Does it have a place in the modern studio? I’ve never heard of it being used on a well‑known track, and doubt I’ll ever record any source with the DX11 alone, but if you have space, there are certainly interesting results to be had by putting it up as a second mic on snare drum, percussion or vocals. It’s more of a historical curiosity than a mic you’d use on every session, but it’s different enough and distinctive enough that it might inspire something special once in a while. And, if nothing else, it’s undoubtedly a talking point.