Sound Devices MixPre review (for linguists)
Sound Devices, a a company based in Reedsburg, WI specializing in professional field recording equipment, calls the MixPre "The Ultimate Portable Mixer for Production, ENG, and Film." A rather is a bold statement, isn't it? Typically, such marketing statements should be taken with a grain of salt, but in this particular case I must say I agree. The MixPre is an extremely versatile, robust, and reliable field microphone pre-amplifier and two-channel mixer. The only thing that I wish were better is battery life, but more on this later.
The MixPre is built around an all-metal chassis. All connectors, dials, ports, etc., are robust and have never failed me. The mixer is truly a pleasure to use. It is so difficult to find well-designed electronic devices; devices that perform exactly the functions that are useful, without fluff. Sound Devices is the Apple of field recording. Their designs are as beautiful as they are functional.
Figure 1. MixPre front panel, photo courtesy Sound Devices
The MixPre is jam-packed with critically useful features. The spec sheet can be found here. It features two high-quality, high-gain, low-noise pre-amplifiers with phantom power available on-board. You should be able to interface just about any professional microphone with the MixPre. It works perfectly with all the microphones I typically recommend for speech analysis: Beyerdynamic Opus 55 Mk II, Shure Beta 53, Sennheiser HMD25, Audio-Technica AT831b, and even the lowly RadioShack 33003.
Balanced line-level XLR outputs
The MixPre is designed to route a microphone signal into a line-level device, such as a mixing console, a digital recorder, or a live audio feed. I typically route the signal into a digital recorder, such as the M-Audio MicroTrack II or my M-Audio Audiophile 2496 sound card. The advantage of using the MixPre in the field is that it is likely to have much better microphone-preamplifiers than most portable digital recorders. The two-channel mixing functionality comes as a nice bonus. It can be very useful for live mixing of one-on-one interviews.
I would recommend getting dedicated connecting cables from Hosa. For example, if your recorder has a 1/4-inch line input, you will need a female XLR to male 1/4-inch cable.
In addition, the MixPre has a stereo tape-level output, which works very nicely with my iRiver ifp-899, and should work equally well with other portable MP3 players with a line interface.
Figure 2. MixPre outputs and battery chamber, photo courtesy Sound Devices
Seven-segment, sunlight-readable LED meters
The LED metering is available on the dBu scale. One cannot overestimate the importance of responsive, reliable metering right on the front of the unit. In practice, in a properly calibrated recording circuit, the LED metering is all you need to ascertain proper recording levels either on a digital recorder or in recording software.
The MixPre has yet another extremely useful feature - the 1 kHz tone oscillator. It is a tone of known frequency and known volume that the unit generates in order to, among other things, help calibrate consistent recording levels throughout the recording circuit, especially if each devices uses a different volume scale (e.g., VU, dBu, dB FS, etc.). Once calibration has been performed, the only level adjustment the recordist needs to worry about is on the MixPre itself. In other words, a 5 dB gain on the MixPre will be equivalent to a 5 dB overall gain in the captured recording. Figure 3 shows a typical calibration scheme that the MixPre enables.
Figure 3. Typical tone calibration across a recording circuit and different level scales (VU, dBu and dB FS)
Phantom power, limiter, and high-pass filter
The MixPre offers a switchable phantom power (48 V or 15 V), a limiter, and a high-pass filter. These are rare on portable field equipment, but turn out to be extremely useful.
Figure 4. XLR inputs, along with phantom power, high-pass filter, and limiter, photo courtesy Sound Devices
While the need for phantom power is fairly obvsious (you need it to power condenser microphones), the use of a high-pass filter is not only less clear but also rather controversial as well. The controversy lies primarily in the fact that the filter changes the spectral envelope of the signal. Figure 5 below illustrates the point. I used the MixPre's filter in at its 80 Hz and 160 Hz settings to demonstrate how the low-frequency spectrum is affected by it. Note the progressive attenuation of low frequencies that the filter introduces. If you are interested in low frequencies (e.g., nasalization analysis), you will probably not want to filter your signal.
Figure 5. The effect of high-pass filtering on low frequency spectra
At the same time, the high-pass filter may be very useful in attenuating unwanted low-frequency environmental noise, such as the sound of traffic, air conditioners, computer fans, and the so-called 60 Hz hum (or 50 Hz outside of the US). While you don't always need to use the filter, it is certainly very nice to have it available on the MixPre.
The unit operates on two AA batteries. The specification quotes 6 hours without phantom power. My experience has been closer to 3 hours using phantom power, which is what I do most of the time. This is the only disappointing feature of the MixPre. However, it is only disappointing if one has unrealistic expectations! Instead, let's just always carry a bunch of charged spare AA batteries.
The MixPre offers real-time stereo headphone monitoring with a high-gain, clean sound. You will really enjoying listening to your recording session through the MixPre.
Overall, the device performs really well. The sound is clean and virtually noise and distortion-free. Please, refer to my iRiver review to look at an example of how the MixPre enables really respectable quality with a consumer MP3 player. In the tests below, I interfaced the MixPre directly with my Audiophile 2496 PCI sound card via the line-in interface at 24-bit/48,000 Hz and captured the recording with Adobe Soundbooth CS4. Overall, the results closely match those obtained with the USBPre, with the same microphones and in similar acoustic conditions. In this particular case, I used the MixPre as a desktop mixer, but you could route the audio signal out of the MixPre into a digital recorder, such as the M-Audio Autotrack II, or similar devices with a line-in interface.
The Sennheiser HMD25 is a professional, dynamic headset microphone that is ideal for recording speech in noisy environments. Being a dynamic microphone, the HMD25 requires a pre-amplifier with low noise and high gain. It is problematic for many field recorders, but the MixPre handles the microphone really well. Figure 6 shows a spectrogram of a Polish phrase "Czarna krowa" recorded at approximately -12 dBFS (a reasonably high level). You can see very good spectral detail with very little noise, and virtually no distortion. DOWNLOAD ORIGINAL FILE.
Figure 6. Spectrogram of a phrase "Czarna krowa" with the Sennheiser HMD25 microphone
Similarly, a narrow-band FFT spectrum of the vowel /a/ in "czarna" (Figure 7) shows a very natural spectral envelope and sharply defined formant peaks.
Figure 7. FFT of the vowel /a/ with the Sennheiser HMD25 microphone
If you want the ultimate in portable microphone pre-amplifiers, look no further. The MixPre has all the features you will ever need in a small, sturdy, and reliable package. I strongly recommend using the MixPre with portable digital recorders such as the Samson Zoom-series recorders, M-Audio Microtrack II, Edirol R-09HR, etc.
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