Teardown of Sound Devices MixPre 6 (gen 1)

It’s good to know your gear, and I’ve been a real big fan of Sound Devices’ MixPres because they are generally free of the analog corner-cutting you find in most prosumer stuff. This should be celebrated, because that’s the way the market should be. Companies generally gut analog stages then add all kinds of bells and whistles that no one actually needs or cares about longer than 15 minutes after removing shrink-wrap. SD cannot comment on component-level decisions because, understandably, this is proprietary knowledge they’d rather keep to themselves so to not undercut their profit margin. So allow me…

Internal photos from a mixpre-6 (1st gen), showing two preamp channels, headphone amp and aux input, ADC, and DAC. They indeed feature a discrete front end with dual transistor pairs. On my 10T these are great low-noise Hitachi 2SA1084, which I was surprised to see because they’ve long been discontinued, but in this particular unit they’re using ZTX789.

What is really nice, however, are the super low-noise/low-distortion OPA1662 op amps (marked OUQI) acting as buffers after the transistor pairs. There are two per preamp, doubling the balanced differential signal from the mic all the way to the ADC, AK5578, an 8-channel chip. This means that for the MixPre-6, with 4 preamps*, there are 2 ADC channels per mic input. These are summed together in the ADC itself, which further reduces noise and increases dynamic range. They could easily have used one dual op amp (something bland like NJM4850 or NJM2068) to sum the incoming differential signal to mono, then used a cheaper ADC with four mono channels (or two stereo ADCs, like the Zoom H5). But they didn’t, the signal is double balanced (e.g., Millennia) through low-distortion parts all the way home, which is why these things eat batteries for breakfast.

*The mixpre 10 has two of these 8 channel ADCs

DAC is fantastic, and uses OPA1662 for aux output, and the headphone amp is a super snappy XR8052, with loads of output current, which is why it can drive anything.

The only place they seem to have cut corners is on the aux input, which uses the MC33178 (a kinda blah low-power opamp) and AK5359 ADC, same as Zoom H5/6 with rather disappointing dynamic range and noise specs. This can be substituted with AK5381, which has *slightly* better dynamic range (I did this in my wife’s H5, which I completely overhauled), but this is no longer manufactured and can only be found, at some risk, on eBay. The MC33178 could easily be substituted with OPA1662, or (if battery life is the concern here) OPA1692, a new audio-optimized opamp from Texas Instruments (successor to the 1662) designed specifically for battery-powered applications. Perhaps they’ve improved this in the gen 2 series, I don’t know, I haven’t popped the hood on one. They probably figure it doesn’t matter much because folks will mostly use plugin lav mics or other lesser-than-hifi sources. Just know that if you’re running signal from a nice external preamp or stereo feed from another mixpre as a backup, this is the one audio “bottleneck” (which is to say, it’s *fine*, but Zoom H5 quality).

All the ADC/DAC chips have ample power supply filtering (big capacitors), skimping on this is another common space-saving measure in most cheap gear. All the analog stuff appears to be running on +/-5V rails (10V rail-to-rail). +/-15V rails (30V) would have been even better, allowing even snappier response in analog stages (higher slew rate, for one), but they’ve done so much else WAY better than average, it doesn’t really matter. Most chips in the Zoom H5, for example, are running on a single-ended +5V rail which, plus the low-end ADC, makes for noise, slew rate limiting, and higher distortion.

Anyway, here’s to good sound! Cheers, sound devices!

One thought on “Teardown of Sound Devices MixPre 6 (gen 1)

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