Recording - October 2009 - 31

extra resonances in that region (it has enough of them built in, thank you), the Benchmark led by a nose. What would happen if I tried a transformer-coupled condenser microphone? I had two handy, a Neumann KM 84 and an obscure Groove Tubes 5sc, the latter an early example of Chinese condenser mics. I was looking for high-frequency resonance effects (I’ll explain that in a moment), so I played the mandolin. With the 5sc and a gain of 28 dB, the two preamps sounded pretty similar, both good, with the Benchmark perhaps sounding a hair cleaner. The Neumann was a somewhat different story. The reason I tried the experiment is that a higher-impedance preamp input could theoretically cause the output transformer in a microphone to show a resonant peak due to the light loading; the MPA1’s 8100 ohm impedance seemed like a good candidate to show the effect. Well, it did; the mandolin, miked with a KM 84, sounded distinctly brighter through the Benchmark than through the project-r. Both sounded good, but they were definitely different. Was this because of a resonating transformer in the Neumann? I removed the mic’s capsule and fed a signal generator through a capacitor into the KM 84’s amplifier module, running it through both preamps in turn. The project-r showed a simple rolloff with no obvious resonant peaks, primarily from its own transformer’s bandlimited response. When I measured the response through the Benchmark, however, and subtracted the preamp’s own frequency response, I found a 3.7 dB peak out at 120 kHz. As Flanders and Swann said, “The ear can’t hear as high as that / Still it ought to please any passing bat.” Well, the frequency response in the band where we can hear is a few fractions of a dB hotter on the Benchmark than on the project-r, and that’s enough to account for the difference I heard. Still, it’s nice to minimize peaks of this sort, and luckily it’s easy to do—adding a Gizmo of 1620 ohms (1.62k; see sidebar) reduces the Benchmark’s effective input impedance to 1350 ohms, just what the KM 84 likes to see. (A Gizmo of 536 ohms would make mics from the SM57 family happy.) As I said earlier, the mandolin via the KM 84 sounded good through both preamps. I’m not criticizing the Benchmark at all, just noting that theory predicts the possibility of certain effects when using a preamp with its input impedance on transformer-coupled microphones, and the data suggest that the theory is correct. Summing up So what’s my verdict on the MPA1? Simply that it’s a superb example of a low-coloration preamplifier, and it looks RECORDING OCTOBER 2009 like it’s built to keep on ticking through a lot of tough use. Do I have any criticisms? Well, sometimes it might be nice to have gain steps smaller than 2 dB for recording stereo pairs that aren’t exactly matched in sensitivity—but you can always fix that in post-production. And, given my druthers, I’d have liked to see a switch on each channel to add loading resistors to the input, bringing the impedance down for mics like KM 84s and SM57s, which care about such things. (On the other hand, Gizmos are easy and cheap—and there’s not much spare real estate on the MPA1’s front panel.) These are quibbles. The fact is that, if you’re looking for a low-coloration transformerless preamp that’s built like a tank, the MPA1 may be the one to beat. In fact, you could call it a new benchmark. So sue me. Price: $1795 More from: Benchmark Media Systems, Inc., 203 E. Hampton Place, Syracuse, NY 13206. 315/437-6300, www.benchmarkmedia.com. Paul J. Stamler (stamler@recordingmag .com) is a recording engineer, musician, and educator in St. Louis, whose students at Webster University happily help him test equipment like this in their recording classes. 31

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Recording - October 2009

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