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 flawless.

The modifications to the design are more than just cosmetic, though. By default, the Special OP‑6 is supplied not with 1620 valves but with the 5879 pentodes that superseded them; the mono version is available with 1620s as a special order, but Vinny says the only sonic difference is that the older valves are noisier and more microphonic. The all‑important gain control has been reworked to operate in 2dB steps and to cover a 44dB range, 8dB more than on the original, and augmented with a 20dB pad. Finally, the Special OP‑6 adds something Vinny calls a “saturating output attenuator”. Some sort of output level control is essential or at least highly desirable with a preamp that offers this much gain, and over the top half of its travel, this operates conventionally, bringing down the level in a controllable way to a point where an A‑D converter can safely accommodate it. Go past the 12 o’clock position, though, and the output stage not only attenuates even further, but begins to distort itself. If you prefer something more subtle, this can optionally be substituted with a standard attenuator.

The RCA OP‑6 offered a choice of 250Ω or 50Ω input impedance settings. Both are very low by today’s standards, and so the Special OP‑6 offers 2kΩ or 600Ω as switched options; the input pad, if used, changes this to 1.5kΩ regardless of the switch setting. In this circuit, impedance also increases with input gain. Vinny has added a high‑impedance input for connecting electric guitars and basses, which bypasses the pad and can be used as an unbalanced line input at a pinch. However, if you anticipate getting a lot of use from the Special OP‑6 as a line‑level processor, you can take advantage of another build option. Like the original, the Special OP‑6 has two inputs per channel, and on ordering, you can choose to have the second input configured either for line‑level signals, with a fixed 40dB pad, or as an alternative mic input, which can have phantom power disabled if you like. This is a nice idea for anyone anxious about their ribbon mics, though as the phantom power is designed to ramp slowly, they should be safe enough in any case. The outputs are on male XLRs, with recessed ground‑lift switches.

The RCA OP‑6’s surprisingly sophisticated circuit retains lots of headroom at all gain levels, and for a design that uses three pentode valves, has good distortion and noise performance throughout the gain range. RCA’s own publicity materials claimed a frequency response that was flat within ±2dB from 30Hz to 15kHz. The published specs for the Special OP‑6 at least match those of the original, and typically improve on them. A variance of +3/‑2 dB is quoted for the full 20Hz to 20kHz frequency spectrum, and Equivalent Input Noise is given as ‑125dB unweighted, which is pretty impressive. Incidentally, all of these specs are measured into a 600Ω load; Vinny says that with a typical modern high‑impedance load such as an audio interface’s line input, you’ll see a couple of dB more gain and a very slight boost above 15kHz.

You might expect a design of this vintage to be of interest only as a character piece in the modern studio, but the Special OP‑6 blows that notion out of the water.

Special Forces

You might expect a design of this vintage to be of interest only as a character piece in the modern studio, but the Special OP‑6 blows that notion out of the water. If anything, it is actually more versatile than many modern solid‑state designs, because it doesn’t really have a ‘sweet spot’. Or, to put it another way, it doesn’t really have a spot that isn’t sweet. Run the output fully open and back off the input gain, and it sounds ‘clean’, but with a subtly forward and present quality that you’ll miss when you switch back to your interface preamps. Push the gain or drop the input impedance to 600Ω and it begins to feel almost like you have an EQ on tap, with a surprising degree of control over the tone of the signal. Even when things get obviously hairy, there’s no sense of the sound being constricted or hitting a brick wall; you never run into the situation where signal peaks are obviously clipped while the rest of the sound remains resolutely neutral.

What I particularly loved about the Special OP‑6 was its ability to get the best from fairly humble dynamic and ribbon mics. For my sins I have a fairly large and ragtag collection of oddball models from the ’60s and ’70s, and I’ve got into the habit of thinking of some of them more as historical curiosities than as useful studio tools. The Special OP‑6 seemed to breathe new life into them; my Bang & Olufsen ribbons have never sounded better, and an ancient Sennheiser MD405 delivered a vocal sound I had no idea it was capable of. Anyone lucky enough to own an original RCA 44 or 77 will be in heaven.

The saturating attenuator is a really nice idea, too. The upper half of its range offers more than enough clean attenuation where needed, and its nether regions open up new realms of thick, chewy saturation. With the gain fully cranked and the attenuator at its lowest setting, you can get pretty close to the infamous Beatles ‘Revolution’ DI guitar sound, whilst slightly more conservative settings will deliver a wide range of bass tones or distorted rock vocals. Finally, although the review Special OP‑6 wasn’t configured with the line input option, I experimented with running complete mixes through the front‑panel DI inputs, and found it highly addictive. At low gain settings, it doesn’t sound obviously distorted, but somehow pulls the mix right to the front of the speakers and tucks in the lower midrange in effortless fashion. I couldn’t bear to part with the review unit at the end of the testing period, so I’ve bought it for my own use — but I’ll have the second inputs modded for line‑level use sooner rather than later.

I’m not someone who often gets excited about mic preamps, but the Special OP‑6 is, well, a bit special. And although a £3k+ piece of kit will never be an impulse buy, I actually think it’s also good value for money. When you consider the labour involved in building it, the cost of valves, meters and custom transformers, and the fact that you’re getting two channels in a single unit, the price seems to me more than reasonable. And if you disagree, try to find a pair of RCA or Retro OP‑6s for less than twice as much. The Special OP‑6 is simply a fantastic preamp at any price.