Coil Audio: CA-70 PS2 Mic/Line Preamp

Coil Audio is a company in Lawrence, Kansas, that manufacturers retro, vintage-styled, industrial-looking mic preamps, audio transformers, and accompanying power supplies and chassis. Coil’s visual aesthetics and circuit topologies are firmly rooted in vintage 1930s and 1940s American tube recording equipment designs from the likes of RCA, Altec, Western Electric, Langevin, and Gates. Coil Audio CA-70 and CA-286 [Tape Op #107] preamps can be purchased in a squarish horizontal configuration for use in the Coil Audio PS2, a two-channel, 3RU-height rackmount chassis and power supply; or in a tall vertical layout for the PS6, a six-channel, 9RU-height modular rack and supply. (The latter configuration looks like a small vintage console.) CA-70 and CA-286 modules can be mixed and matched in the same chassis, as desired, because they share the same power requirements. Also of note, input and output transformers mount onto exterior octal sockets (like vintage vacuum tubes), and Coil Audio produces four different transformer options, which can be swapped by the user to color the sound in various ways. (A new, single-channel CA-70s model, with a discrete, built-in power supply and fixed transformers, will be available soon.)

Recently, I had a few weeks to get to know the CA-70 mic preamp, which was inspired by a 1913 Western Electric preamp that was licensed and manufactured by various companies through the 1950s. The original circuit is extremely simple, utilizing two octal tubes and two transformers, along with a few simple gain and tone controls. The CA-70/CA-286 PS2 configuration I demoed sports a mid-century look with matte grey and red faces on matte stainless-steel chassis. The indicator lamps, knobs, and switches also scream vintage industrial design, while imparting a sense of durability and quality.

Anyone who has had the good fortune to use a vintage Langevin 5117 or Gates SA-70 mic preamp knows how special these preamps sound, and Coil has faithfully reproduced the circuit and sonics of those boxes while adding some unique additional features. The front panel of the CA-70 sports two power switches, one for 300 V and one for 6.3 V, which are switched on by the user in sequence to provide a soft-start and to extend tube life. Each channel includes a stepped, passive, input attenuator, which accommodates various mic levels as well as line level, which allows for processing DAW signals and mix buses, or even cascading mic preamps for a dual-channel, high-gain signal path. An output control sets the gain of the output tube stage and provides up to 42–54 dB of gain, depending on the negative feedback setting. The magic of the preamp comes from the three knobs to the right of the aforementioned gain controls. These knobs adjust the negative feedback, high-frequency tilt, and low-frequency boost for the feedback circuit. Normally, the design engineer would set and lock down these controls as part of the circuit design, but Coil has chosen to allow the user to manipulate these circuit parameters for a wide range of control over the preamp’s tone. Without getting to techy, negative feedback in an amplifier circuit creates a more controlled, lower distortion output signal by feeding some of the amplifier’s output, polarity inverted, back into the input of the circuit. Reducing the amount of negative feedback creates a brighter, more energetic sounding tone with higher gain; while increasing the negative feedback creates a smoother, more damped, or mellow tone. The CA-70’s LF and HF controls work inside the negative feedback circuit to provide subtle, musical tilting of the balance between low and high frequencies, in a manner that sounds slightly more natural than a standard equalizer. On drum overheads, for example, tweaking negative feedback and tone controls can produce a mellow and creamy cymbal sound, or a bright and transient-rich presentation, from the same mic position and musical performance. The CA-70 does not provide phantom power for condenser mics, but Coil also produces a well-designed, handsome, two-channel phantom power supply.

In the studio, on male vocals through a Cathedral Pipes St. Jean Baptiste (U 47 fet–style) mic, the CA-70 produced a larger-than-life vocal with a huge but controlled bottom end, and a clear but not overly bright top end. The tonal balance felt very natural, and starting with the gain set at 42 dB on this medium-loud singer, I had to bring the input pad down a click and trim the output down to produce the appropriate output level. As I turned the negative feedback down, the vocal became brighter and slightly midrange focused. Conversely, increasing the negative feedback provided a creamier and slightly darker vocal tone. Once I found an appropriate negative feedback setting, I tweaked the high and low tilt controls until the tone was dialed in. Even though the preamp provides a wide swing of tonal options, finding a suitable setting only took a few moments and never felt cumbersome.

Looking at my recall notes, I noticed that on female vocals for folk and jazz recordings with a Bock Audio 251 tube mic, I tended to decrease negative feedback and also turn the low frequency down a click or two. For tenor male vocals, I tended to keep the low and high frequencies at the stock position and increase the negative feedback slightly. During mixing, I found the CA-70 terrific at “re-amping” vocals and acoustic and electric bass, to tune in the tonal balance without EQ or compression. If you’ve ever wanted to recreate the sweet, rich tone of Patsy Cline’s voice, this preamp is a great place to start! On lead electric guitar, increasing the negative feedback rounded off the transients and saturated the highs in a way that set the guitar nicely into a crowded mix. As a mix processor, I found the transparency, image size, and tonal options interesting enough that I can’t wait to try a pair of these mic preamps on stereo bus duty or even as a 2-mix processor.

My demo unit had one channel of CA-70 and one channel of CA-286, so I had to compare the two units. I found the CA-286 to be similar to my Telefunken V72 preamps, which have the edgy character of rock music — detailed and forward. The CA-70 could easily be coaxed into a creamier, more laid-back place, or pushed for more clarity and a bit of edge. If they were artists, the CA-286 would be Lenny Kravitz, and the CA-70 would be Beck — two different but equally strong attitudes.

The folks at Coil Audio spent a lot of time choosing circuit components, designing power supplies, and creating flexible, musical controls — and the results are rewarding. If you’re looking for a boutique and sonically special preamp, be sure to check out the CA-70 from Coil Audio.