Recording - October 2009 - 28

B Y PA U L J . S TA M L E R Benchmark MPA1 Clarity is the byword in this remarkable little preamp Mic Preamp The MPA1, from longtime pro equipment maker Benchmark, looks pretty basic. It’s a 2-channel 1RU half-rack device, with gain controls, phantom onoff, a highpass filter and polarity inversion on each channel, plus a pair of LED level meters. Unusually for a box this small, the power supply is built in, rather than an external wall-wart. The only frill is a headphone jack with its own control. A small row of mysterious LEDs by each level control, though, gives hints that this very basic preamp may harbor some subtleties. The level controls are actually switches that control a network of precision resistors. Gain settings are 0–24 dB in increments of 2 dB, and when you advance past 24 dB, things get interesting. As you keep turning the switch clockwise, it doesn’t run against a stop. Instead, a relay clicks inside and a “+24 dB” LED lights. Now the gain settings are 26–48 dB. This continues through two more revolutions, until you reach the highest gain, a whopping 74 dB. That’d be good for classic ribbon mics. It’s clear from looking at the spec sheet (in the cleanly-written manual) that Benchmark was taking ribbon mics seriously when they designed this preamp; the input impedance is 8100 ohms rather than the usual 1000–2000 ohms. As Scott Dorsey has discussed in these pages, classic ribbon microphones were designed for and work best into high 28 load impedances. (I’ll have more to say about input impedance later on.) The equivalent input noise spec of –128 dBu unweighted (with a 150 ohm microphone) places the MPA1 in the quietest bracket of the preamp world. The highpass filter is 12 dB/octave and cuts off at an unusually low frequency, 40 Hz. Clearly Benchmark intends this to be used for filtering low-frequency room noise, breath blasts, and the like, rather than compensating for microphone proximity effect. The MPA1 is built like a little tank, with tough-feeling step switches and positive function switches. Usually variable resistors (gain controls and the like) are the least reliable parts in a preamp; in this case, the only one is the headphone’s level control, and that’s a heavy-duty pot. Everything else is done with switches. I’d expect this box to last for decades of heavy use. Under the hood I cooked the preamp on the counter for 24 hours; it got quite warm, suggesting generously-biased audio circuits. When I opened the box, I found a whopper of a toroidal power transformer in the middle. This is good; more transformer typically means more available current. It was hard to tell the quality of the resistors and capacitors by inspection, since they’re almost all surface-mount devices, but the switches and relays looked unusually high-grade. [Benchmark informs us that the resistors are metal film, with many 0.1% resistors in the gain control circuits and 0.5% and 1% resistors elsewhere in the audio path, and that the oversized AC-coupling capacitors on the mic inputs are a composite of low-ESR electrolytics in parallel with film caps for low distortion over the entire audio band.—Ed.] One thing about the interior surprised me. For decades, Benchmark has relied on the classic NE5534 and NE5532 integrated circuits for their designs, and I expected the same in the MPA1. Instead, along with some custom matched input transistors I found several National Semiconductor LM4562 dual opamp chips doing the heavy lifting. These are from a new generation of designs that achieve distortion performance at least an order of magnitude better than older chips; indeed, the spec sheet suggested very clean numbers even at high output levels (the MPA1 is spec’d up to +29 dBu output). And so it was. Without giving you a long list of figures, I ran a series of harmonic and intermodulation distortion tests at +24 dBu (the highest most people will use in a pro recording environment). The highest distortion levels in any of the tests were about 0.0016%, which is very clean—and I suspect much of what I was measuring was the residual distortion of my test setup. At +18 dBu, or 14 dB RECORDING OCTOBER 2009

Recording - October 2009

Table of Contents for the Digital Edition of Recording - October 2009

Recording - October 2009
Fade In
Contents
Talkback
Fast Forward
Joe Chiccarelli And The Perfect Mix
Zoom R16
The Mixing Workshop
Lauten ST-221 Torch Microphones
Mastering Engineer Bob Katz—The Voice of Experience
Benchmark MPA1 Preamplifier
What The Heck IS Mastering?
Reviewed and Revisited: M-Audio ProFire 610
Universal Audio 710 Twin-Finity
Recording’s Drum Column
Plug-In Outlet
Readers' Tapes
Chameleon Labs 7802 Tube Compressor
Recording’s Showcase of Sounds
Advertiser Index
Fade Out
Recording - October 2009 - Recording - October 2009
Recording - October 2009 - Cover2
Recording - October 2009 - 1
Recording - October 2009 - Fade In
Recording - October 2009 - 3
Recording - October 2009 - Contents
Recording - October 2009 - 5
Recording - October 2009 - Talkback
Recording - October 2009 - 7
Recording - October 2009 - Fast Forward
Recording - October 2009 - 9
Recording - October 2009 - 10
Recording - October 2009 - 11
Recording - October 2009 - Joe Chiccarelli And The Perfect Mix
Recording - October 2009 - 13
Recording - October 2009 - Zoom R16
Recording - October 2009 - 15
Recording - October 2009 - 16
Recording - October 2009 - 17
Recording - October 2009 - The Mixing Workshop
Recording - October 2009 - 19
Recording - October 2009 - 20
Recording - October 2009 - 21
Recording - October 2009 - Lauten ST-221 Torch Microphones
Recording - October 2009 - 23
Recording - October 2009 - Mastering Engineer Bob Katz—The Voice of Experience
Recording - October 2009 - 25
Recording - October 2009 - 26
Recording - October 2009 - 27
Recording - October 2009 - Benchmark MPA1 Preamplifier
Recording - October 2009 - 29
Recording - October 2009 - 30
Recording - October 2009 - 31
Recording - October 2009 - What The Heck IS Mastering?
Recording - October 2009 - 33
Recording - October 2009 - 34
Recording - October 2009 - 35
Recording - October 2009 - Reviewed and Revisited: M-Audio ProFire 610
Recording - October 2009 - 37
Recording - October 2009 - Universal Audio 710 Twin-Finity
Recording - October 2009 - 39
Recording - October 2009 - 40
Recording - October 2009 - 41
Recording - October 2009 - 42
Recording - October 2009 - 43
Recording - October 2009 - 44
Recording - October 2009 - 45
Recording - October 2009 - Recording’s Drum Column
Recording - October 2009 - 47
Recording - October 2009 - Plug-In Outlet
Recording - October 2009 - 49
Recording - October 2009 - 50
Recording - October 2009 - 51
Recording - October 2009 - Readers' Tapes
Recording - October 2009 - 53
Recording - October 2009 - Chameleon Labs 7802 Tube Compressor
Recording - October 2009 - 55
Recording - October 2009 - Recording’s Showcase of Sounds
Recording - October 2009 - Advertiser Index
Recording - October 2009 - 58
Recording - October 2009 - 59
Recording - October 2009 - 60
Recording - October 2009 - 61
Recording - October 2009 - 62
Recording - October 2009 - 63
Recording - October 2009 - Fade Out
Recording - October 2009 - Cover3
Recording - October 2009 - Cover4
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