UnderTone Audio (UTA) is a relative newcomer to the world of boutique gear, the brainchild of producer/engineer Eric Valentine and electronic designer Larry Jasper, both mad geniuses in their own right. Eric has made a variety of amazing-sounding records (for Queens of the Stone Age, Slash, Persephone's Bees, Nickel Creek, and many others), and Larry (who has worked for Quad Eight and GML) has helped maintain and modify Eric's vast collection of vintage gear over the years. Full disclosure: Eric has been a friend for years, having recorded a couple of albums for my band Deathray. His passionately detailed approach to music, engineering, and production really carries over into the design philosophy behind this four-channel mic preamp. The pair started out when they decided to build a new, "no compromises" large-format analog console from scratch. After four years of design, fabrication, and obsessive testing, they landed on an amazing custom console that had some really innovative technology, but was priced well beyond the reach of most home or project studio recordists. Now they are working to integrate the Class A circuitry used in the UTA consoles in more affordable applications.
The MPDI-4 is a 1RU-height, four-channel, do-it-all-and- do-it-right beast. Each mic preamp/DI circuit has bypassable input and output transformers, adjustable output headroom, and variable harmonic distortion characteristics. The four inputs are accessible from the front and rear, with XLR I/O on the back and Neutrik Combo jack inputs on the front panel. Each channel has an input pad (-20 dB), a rotary switch for gain (-60 to -10 dB in 5 dB increments), and three fantastically flexible options to help shape your signal.
First is a 600 Ω output load switch; when enabled, it drives the mic preamp into a harmonic distortion range similar to vintage preamps. I found this particularly nice with drum overhead mics; it gave the highs a smoothness which was emphasized even further when pushing the preamp into distortion.
Speaking of distortion, the second factor that sets this unit apart from my other outboard preamps is a convenient (some would say necessary) output pad (-10 dB). Without a fader or line trim on the preamp, this is the only way to really drive the bejeezus out of it and not clip the input on your ADC of choice. In most instances, my penchant for (or addiction to) distortion (subtle or not) demanded that I leave this output pad switch engaged.
The third tone-shaping element that I haven't seen elsewhere is the ability to bypass both the input and output transformers on the MPDI-4. Apparently, on the other side of the THD spectrum, is a place I rarely go — a place where sounds are detailed and clean, with hardly any audible distortion. The MPDI-4 will take you there if you want to go; the manual claims that, with both transformers bypassed and no load applied, the mic preamp will pass +27 dBu with only 0.003% THD. That is a crazy low spec for distortion. In my experience, using the preamp in this mode yielded clean, accurate, insanely precise captures of sources like grand piano, vocals, and acoustic guitar. Zero coloration. If anything, it exposed some of the shortcomings of my mic cabinet and even helped me uncover a subtle difference between a pair of AKG C 414 mics that I had always assumed were functionally the same, but in practice were found to deliver slightly different frequency responses. (I suspect that may be the result of a wild drummer nicking one of them a couple of times when close mic'ing the toms.)
I did test the DI circuit with bass and analog synth, as well. The unit switches over automatically when you plug into the 1/4'' jack of the front-panel Combo input, and (as I learned from the joyfully meticulous manual) an active Class A JFET stage is automatically inserted to effectively lower the output impedance, which is then fed directly to the input of the mic preamp stage. I dunno, whatever — it works, man! I tended to again give in to my fuzz habit, and pile on the transformers and 600 Ω load to these sources, which sounded freaking great, but the DI would also be convenient for capturing bass or guitar for later re-amping, in which case, I would likely clean up my act a bit and go transformerless. Just the idea that I can do all of this with a single-rackspace preamp has me in my happy place.
Now, all of these tone-shaping options come at a small ergonomic price: There are a ton of switches on the front face of the MPDI-4 (seven per channel, plus a gain pot). In fact, there's room for only one output meter (which you switch between channels). However, everything is clearly labeled, and I had no problem checking settings at a glance. I'm not hugely fond of the somewhat stiff feel of the rotary switch, particularly when paired with UTA's knob of choice, which isn't terribly "grippy." Nonetheless, the switch is clearly well-built, like everything on this unit, and it functions just fine — I just have big bass player fingers, I guess. Also, as expected, this unit gets super hot, so take care to leave a space above and below it in your rack.
Overall, I absolutely loved this thing, and it's exciting to see smaller gear manufacturers take on the challenges of a "no compromises" attitude towards gear design. An audio-nerd tip of the hat to UTA.