Seventh Circle Audio: T15 mic preamp

Long before the 500-series format craze, Seventh Circle Audio (SCA) was helping ambitious engineers build phenomenal preamps (Tape Op #54). Continuing my DIY quest, I was very excited to learn of the recently introduced SCA T15 preamp, a transformerless, IC-based design. It is a commonly held belief that ICs are not as good as discrete op-amps, particularly in areas of noise, headroom, and output drive. While that may be true of older designs, the gap is getting smaller. For the T15, SCA has chosen state-of-the-art chips manufactured by THAT Corporation. These ICs (models 1512 and 1646) were designed specifically for audio preamplifiers. Consequently, the performance of the T15 approaches sophisticated discrete and hybrid designs (such as the SCA C84) while using far fewer components. And since the ICs replace more expensive components, the kit is less costly to build. Note only does the T15 come close to the C84 spec-wise, it excels in the same applications. Unlike the C84, the T15 has a small number of components, requires no adjustment or trimming, and is easier to assemble.

Speaking of assembly, build time was just a few hours, with the most time spent double-checking resistors. But unlike self-sourced DIY, the resistors come from SCA taped in the Bill of Materials order, making it hard to get them wrong. You’ll also want to triple-check that you’re putting the correct IC in the correct space, as the instructions have you solder them direct to the PC board (of course, high-quality machined pins could be added to allow IC-swapping, but most people won’t need this). I say triple-check because twice is obviously not enough for me, having swapped, and thus burned up, one set of ICs. Heavy sigh. Those who succeed in building their T15s can expect up to 70 dB of gain, which is more than enough for today’s condensers and dynamics. The high-quality 12-position Grayhill switch provides repeatable settings. There is also polarity reverse, and slow-rise 48 V phantom power to reduce pops and thumps.

We tried the T15 on drum overheads, electric guitars, and male vocals. On drums, the T15 provided a clear, accurate picture of the kit. It was similar to the stock preamp in our Sony MXP-3036 console. It does the job. It does not harm the record or burn you for choosing it. Compared to the much more expensive Millennia Media TD-1 in FET mode, the T15 lacked some of the air, depth, and overall dimension found in the Millennia. But at 1/5th the price, one would expect that would be the case. On vocals, we thought the T15 was neutral for male singers and a tad underwhelming on female leads. On women, the T15 had some high-end sizzle without that airy sparkle commonly heard on contemporary recordings. Quiet vocalists were also the only time we had to reach into the gain of the T15, as most of our other sources were plenty loud. Even cranked, the T15 did not add noise until we went to the maximum settings, something you might never do in a real-world session.

We loved the T15 on electric guitar, irrespective of mic choice. It was tone, tone, tone. From the twang of a Telecaster through a Twin mic’ed with a Royer R-121 (Tape Op #19), to the intestine-shaking heft of a Les Paul through a Marshall mic’ed with a Sennheiser MD 421 or Shure SM57, the T15’s accurate portrayal of midrange frequencies made it a hands-down winner in our tests. The only exception was against the API 212, where it was a matter of preference. As of this writing, a pair of T15s have a permanent slot in our patchbay, waiting for the next guitar session. We occasionally do location and live work, and in a dream world, I would have a rack of 16 or 24 T15s for those jobs alone. It would be a big jump over the standard live console. You would have to spend five or six times the price (not counting labor) to do better.

Because the T15 is a less colored preamp, it can be difficult to describe what kind of sound it has. In terms of fidelity, I would put it above the preamps found in the current revision of Mackie mixers, but lower than a solid-state Millennia Media, TRUE Systems, or Grace Designs. (For that kind of performance you would want to look into the SCA C84.) One reason the T15 is an improvement over current console preamps is the SCA power supply. In comparison to VPR Alliance standards for 500-series, the SCA power supply provides almost twice the voltage and more than twice the total power per slot. Any good designer will tell you that good sound starts at the power supply, and the SCA chassis supply is overbuilt with top-shelf, audio-friendly components. (All SCA preamp kits fit in the company’s 8-space chassis, which was covered in Tape Op #54.) Specifically, the SCA supply allows the ICs to operate at their full-rated supply voltage of +/-18 V, which is something that other formats can only achieve if the individual module has an onboard voltage converter. Consequently, the THAT Corporation ICs do a tremendous job, especially with midrange, proving what kind of accuracy can be achieved when the IC is purpose-designed for audio.

The T15 is the most affordable preamp in the SCA catalog, and for those with patience and decent soldering skills, it could be one of the easiest to assemble. No, it’s not the finest preamp on the planet, but it is a competent, economical, all-around workhorse. Whether an eight-channel rack of them is right for your set-up is a personal decision, but we’re very pleased to have a pair in our rack, plus it leaves us room to try some of the other tasty preamps in the SCA line.

(T15 kit $99 direct, assembled $199; power supply and chassis required; www.seventhcircleaudio.com)