Atomic Analog Special OP-6

Can an 80‑year‑old mic preamp design still cut it in the 21st Century studio?

Standalone mic preamps weren’t a big deal in the golden age of recording. If you hired a studio, you tracked through the console that was in it. The business of carving up old desks for their input modules only got going in the ’90s, as the need for mixers declined, whereupon the mic preamp came to the fore as a product in its own right.

But, like most things in audio, the standalone mic preamp has much older roots. One of the most revered designs of all time originated as a self‑contained unit, way before large‑format consoles were a glint in Rupert Neve’s eye. Introduced back in the 1930s as a portable solution for outside broadcast and remote recording, the RCA OP‑6 could be run either from mains power or a large number of batteries, using three RCA 1620 (aka 6J7) valves to deliver almost 90dB of gain. Optionally, it could be preceded by the companion OP‑7 four‑channel mixer.

The OP‑6 has long been obsolete in radio, but is regarded as a ‘secret weapon’ by many recording engineers. Its mystique is compounded by the fact that extant schematics are incomplete: the OP‑6 used custom transformers and chokes of unknown specifications, and the values of the resistors in its stepped gain control were never published. This is not a trivial omission, as the gain control performs a complex dual function, simultaneously adjusting the amount of negative feedback around the first gain stage and the signal level feeding the second. Nevertheless, recent years have seen a few attempts to recreate the OP‑6, the most prominent example being Retro Instruments’ now‑discontinued OP6. The latest is Special OP‑6 from Atomic Analog.

Refreshing The OP

Atomic Analog is the trading name of Vinny Wood, an Englishman who lives in Iceland and seems to have a slightly unhealthy obsession with RCA’s classic design. Vinny spent two and a half years researching the OP‑6, going so far as to X‑ray the transformers so that they could be faithfully recreated, as well as reverse‑engineering the gain circuit. The fruits of his labour are available in two forms: as a mono channel built into a portable flightcase, in homage to the original, or as a stereo 3U rack unit. The latter was supplied for review.

Whereas Retro Instruments’ OP‑6 was mostly faithful to the RCA design, Vinny has been bolder in adding his own slant. The front and rear panels are made of wood, and can be custom engraved to order; the review unit proudly proclaims “Fucking handmade in East Iceland” on the front. Personally, I love the aesthetic, which sees retro fitments such as the knobs and meters set off by more modern, functional elements. The build quality of the review unit was...