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PAR Studio Review: Retro Instruments OP6 Portable Amplifier by Lynn Fuston

When I asked Phil Moore why he decided to build a recreation of the classic 1940s-vintage RCA OP6 preamp, his answer was brief and straight to the point.

Lynn Fuston
PAR Technical Editor
When I asked Phil Moore why he decided to build a recreation of the classic 1940s-vintage RCA OP6 preamp, his answer was brief and straight to the point. “Because I heard one.” There you have it. “I have a friend who collects old gear and he loaned me one. It adds such a nice polish to the sound.”

For those who may not recognize Phil Moore’s name, his company, Retro Instruments, has been manufacturing his reinterpretations of the likes of the Pultec EQP-1A, UA 176 and Sta-Level limiters and his own creations, the Powerstrip and Doublewide. This is his latest project, the Retro OP6, based on RCA’s “portable amplifier” which came in a small breadbox-size case with huge VU meter and Bakelite “volume” knob.

So is it a replica of the original or a reinterpretation? “It’s a re-engineering of the original.” He kept all of the circuit design, the VU meter (optional on the original), a big plastic knob and the hammer tone gray all-steel case. To these he added things that the original didn’t offer, things that weren’t needed (or even invented) in the 1940s: XLR in and out, IEC power connector, phantom power, an input pad, selectable mic impedance and multitap transformer. Phil used the original tubes (“There’s still a big stash of these tubes around, 6J7 and 1620”) and added a fewer other tricks along the way. ��I doubled the tubes on the input and output stages. They run paralleled.” Why is that? “It lowers the noise floor by 6 dB and it actually sounds better. It’s like a Fairchild thing. And it allows you to have a spare tube, in case you lose one. Just pull one of the tubes and it will work just like the vintage original. And you can listen to it either way and decide which you like best. But it’s better with the dual tubes” he adds coyly.

Retro’s OP6 sits left of its inspiration, the ’40s-era RCA OP6 preamp. Other nice touches include stepped main “Volume” knob, switchable polarity reverse, cool colored jewel light indicators, along with a Hi-Z instrument input and a continuously-variable Output level control. There’s a meter switch to indicate output levels (+4, +20, OFF) or to check the status of the tubes. Nice. Input impedance settings are 37, 150 and 600 ohms. I noted those seemed very low. “That’s for selecting appropriate termination for certain mics, not the actual input impedance. The output of the transformer is terminated and reflected back to the mic.” The Retro OP6 even mimics the original’s dual-log pot and it offers a spec’d 82 dB of gain. “Part of the sound of the original is the metal tubes and part of it is the transformers. I used a custom input transformer and the steel output transformer is also custom made for us.”

Curious potential purchasers so far have asked if there will be rack-mount version of this unit. Phil, who is still gearing up for the first “non-prototype” production unit, is not inclined to agree to that. Also I joked with Phil that a 500-series version could probably fill an entire 10-slot 500 rack, three spaces for the meter, two for the knob, etc. That got a laugh.

So how does it sound?

I assembled a small but elite group of engineers to audition the Retro OP6 against two very worthy preamps. First was an original RCA OP6 that came from the vintage RCA gear collection of Blackbird Audio Rentals ( Thanks, Rolff. This was a unit that has been refurbished with caps replaced, XLR connectors added, etc. The other contestant was a Neve 1073 preamp in the racks at Addiction Sound Studio. (

We listened to acoustic guitar through the Retro OP6 and the 1073, using a U87. I preferred the OP6 as it seemed livelier to me, with more definition and clarity, less thick. Gains were precisely matched and mic and instrument position were identical. I have included 24-bit/48 kHz .wav samples so you can hear what I heard. ( Next we compared the Retro to the original RCA OP6 on drums with an R84 placed in front of the kit, then with a Peluso 67 placed over the kit, and then with the Peluso 67 on acoustic guitar.

Overall, the RCA felt softer on transients to my ear, like I could hear softness in the power supply (listen to the first crash cymbal of the Peluso 67 drum sample for comparison). The RCA also sounded woolier on the low end and possibly a bit hyped on the extreme top (compare the Retro/RCA guitar samples). That huskiness is part of the RCA’s appeal. According to Phil, the sound will get “mushier” when removing one of the doubled tubes but I didn’t get to hear that. On the drum samples, I heard a difference in the ambience (amount of room) picked up the two, which surprised me, almost as if the mic was moved. The way the snare sat in the overall drum picture also changed between the two. The Retro felt overall more desirable to me but mostly in terms of fidelity. If someone wants a vintage sound, that seems a bit dated, then the RCA would get the nod. The RCA definitely has a distinctive sound to it, but for overall usage, I’d pick the Retro recreation.

From a purely practical standpoint, I found that working with the Retro was preferable. The Retro’s ability to pad the input allows use with a wider range of mic (when using the RCA, a tube LDC’s output was delivering too hot an output [+21 dBu] even with the RCA’s input turned all the way down), having phantom power available at the flip of a switch allows using any mic (the original RCA never offered phantom), plus the stepped input control and the ability to trim the output, all of these are not only nice but features we take for granted these days.

The Retro OP6 is really an amazing recreation of a classic piece, an update both sonically and practically, and I think it sounds great. Nice job. The final score? For retro vibe (pun intended) and cool factor, there are very few things comparable, you’ll spend time scouring Ebay trying to score an original, which may be in questionable condition. The sound? Unique and worth the price. And that price? It’s high, at $3500, but the cost of finding and refurb’ing a very elusive original may be comparable. The upside to such a unique piece is that it will definitely be a focal piece, both visually and sonically, in the studio. The downside? Only one. It won’t fit in your rack.

Second Opinion: Russ Long

The Retro OP-6 performed beautifully on acoustic guitar. The rich, pristine top end coupled with a smooth, lush body was gorgeous. Capturing drums with a single mic was also pretty amazing. The impact of each individual element of the drum kit combined with the tone of Addiction Sound Studio’s tracking space was enough to capture the ear of any discerning listener.

RCA’s Portable Amplifer OP-6

Designed at a time when a six-channel broadcast “console” required two men to lift, the nearly 21 lb lunchbox-sized (9.5-inch x 12.5-inch x 7.25-inch) “Remote Pickup Amplifier” was designed for field work and was designed for use with any kind of microphone, including the then-current crop of low output RCA “velocity” mics (ribbon mics, like the 44-BX and 77D) and offered 88 dB of gain. Frequency response was 40 Hz to 10,000 Hz, +/- 1 dB and it offered a built-in power supply. Accompanying kit included the OP-7 microphone mixer, which offered four mic inputs and a mic-level output that would then plug into the OP-6, and also a similar-sized Battery Box for use where there was no electricity. The complete kit for the recorder-on- the-go offered a leather shoulder strap so one could sling the preamp over one shoulder and the 44 lb battery box over the other. Just add a mic and boom and something to record to and you’re ready to go. What was unique about its circuit design? I’ll just quote from the 1950 RCA manual: “The OP-6 is a three-stage resistance coupled amplifier using RCA 1620 low noise, non-microphonic tubes. The amplifier circuit is unique in that it utilizes two feedback loops. One loop is around the first stage and is varied with the main gain control thus maintaining a maximum feedback consistent with required gain. This arrangement prevents overloading the first tube by high output microphones. The gain control is located between the first and second stage and is a high quality step-by-step device equipped with a large knob. The second feedback loop is fixed and is connected around the second and third stages.”