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Interesting features and amazing audio hide behind the MPA1's simple faceplate.
Interesting features and amazing audio hide behind the MPA1's simple faceplate.

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Benchmark MPA1 Mic Preamp
By Paul J. Stamler
Date: June 2009

The MPA1, from longtime pro equipment maker Benchmark, looks pretty basic. It's a 2-channel 1RU half-rack device, with gain controls, phantom on-off, 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 takin g 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 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 above the standard professional operating level, I couldn't measure any distortion at all. None.

I checked for hum susceptibility, and found that the Wall Wart from Hell couldn't induce any detectable noise, even when I pressed it directly against the empty input jack. [Benchmark tells us the interior is heavily magnetically shielded to protect the signal from the internal power transformer, and its resistance to external fields is a beneficial side effect.—Ed.] Based on that, I'd say there are no constraints about mounting the MPA1 in a rack with other gear.

I did find one anomaly: when the LEDs on the front panel read +24 dBu output, my own meters measured 6 dB lower, or +18 dBu. I exchanged e-mails with Benchmark; as it turned out, one batch of MPA1 preamps was built with incorrect resistors in the metering circuit, and they read exactly 6 dB high. Most of the erroneous batch was still in the warehouse, but a few had been shipped, and of course one of them came to me. Benchmark's people are thoroughly professional; by the time you read this, the ones which got sold will have been recalled and fixed at the factory, with Benchmark paying the shipping both ways.

So what did it sound like?

Before I did formal comparisons against a reference preamp, I got a clue what the Benchmark was about. I needed to do several takes of a guitar piece to give my students practice material for tape editing, so I set up a Microtech Gefell M930 in front of the old Martin and recorded a few quickies through the Benchmark into the DAW, to be dubbed onto analog later. And I noticed something: the recording sounded exceptionally good. Now, I've made a lot of nice remarks about the M930 over the years; it's my go-to microphone for a lot of things. But I'd never heard it sound quite as good as this; in particular, the clarity, from the bottom of the audio spectrum to the top, was exceptional.

Time for some more testing. I brought the MPA1 to class; one of my colleagues, Bill Schulenburg, was nice enough to lend us an old RCA ribbon microphone, as I wanted to see how the Benchmark dealt with ribbons.

All the preamps we tried in comparison were transformerless, so that was a constant; we matched levels to within 0.1 dB, and we tested the microphone on a very nice Baldwin concert grand piano (thanks to our uki, Matt Nichols, for the good playing). Well, the result was unequivocal: the MPA1 smoked a whole roomful of well-regarded (and expensive) preamps. The main difference was clarity: the Benchmark preamp was cleaner, more open, more transparent—all the words you use when you want to talk about a preamp that stays out of the music's way. There was simply less electronic stuff between us and the music. The Benchmark also removed a "tubby" sound in the upper bass / lower midrange, probably because of the higher-impedance loading.

We used the Benchmark for more tests, this time using my Gefells and a pair of Neumann KM 84s for some stereo miking experiments (the class session, done at the students' request, was really aimed at learning more about recording pianos). In every case the Benchmark was clear and clean, revealing much more detail than we heard through other preamps. Now, one of the big clichés of equipment reviewing is to say, "It was like a veil was lifted between us and the instruments." I've tried hard to avoid such a hackneyed phrase, but, dammit, that's what happened.

I went home and did some more formal tests, comparing the MPA1 to a version of the project-r preamp built using 5534 chips and Jensen JT-110k transformers. I pulled out the Martin again, recording it using a Gefell M930 placed about 10" from the 14th fret. I set the Benchmark for 32 dB of gain, and matched the project-r to within 0.1dB. The two preamps were tonally similar in the midrange and treble, but the Benchmark was a good deal clearer, with better pitch definition, in the low frequencies. Funny; I've always thought the project-r was a low-coloration preamp, and it is, but the Benchmark is lower. I liked both sounds, though.

I tried switching in the Benchmark's highpass filter, and confirmed that it had little or no audible effect on the sound. As suggested above, it's really for getting rid of subsonic crud, not compensating for proximity effect.

Next I tried vocals (mine) through an Electro-Voice RE15, which required 48 dB of gain. The two preamps sounded quite similar, but there was a slight nasal resonance audible through the project-r which was absent when I used the Benchmark. Since I prefer my voice without 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 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 is a recording engineer and musician in St. Louis, whose students at Webster University happily help him test boxes like this in their recording classes. Write to Paul via our authors' mailbox at talkback@recordingmag.com.

 

Sidebar

Making a Gizmo

The longer tale of the Gizmo is found in "The Taming of the Shure" (Recording, May 2006). In brief, you can make one by taking a short (6-12") mic cable, opening up the male XLR connector, and soldering a resistor between pins 2 and 3. Be careful that the cable's own wires also stay soldered, and make sure neither pin shorts to the connector's shell when you close it back up. I recommend 1/4 watt 1% metal-film resistors, which you can get from Mouser Electronics or a local surplus store. You'll need to snip the resistor's leads off real short.—PJS

 

 




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