#61 MOTU 896 mk3 Hybrid teardown (post-recap)

Since relatively recently acquiring the MOTU 8M, once i got around to swapping it out for the 896 mk3 in my recording rack, i figured it might be worth doing a quick teardown of the latter, before i sell it off. I've done a really brief one over on the BadCaps.net forums when i first bought it, but it deserves some higher-res photos and a more detailed documentation of its guts. Do keep in mind that it got re-capped as soon as i got my hands on (or into?) it, so the electrolytics are not stock. But
without further ado...

Quite "fully-featured", one might say - but then again, with a 2RU unit, one has a lot more front & back panel space to play with. I'll let you peruse the specs in the manual at your own leisure, and won't repeat it all here.

Popping off the top cover, we're greeted with a two-"layer" design, and plenty of empty space in there. But as one might expect, the physical connections to the world outside, as so often is the case, take up the most room.

Starting off at the power supply, it's the exact same design as the one "still" used in the much more recent units. For a more detailed analysis, i refer you (once again) to the MOTU 8M teardown posted earlier on this blog.

Down in a corner of the mainboard is the boost-converter for producing the voltage for the phantom-power required for condenser mics (and some active ribbon mics, even). I can't say i recall ever needing to repair this part of the circuit so... That's either a factory bodge, or it was repaired before i had bought this (used, "obviously"). Regardless, couldn't manage to make out the chip markings, but "feels" quite similar to the 8M - likely a Texas Instruments part, with a voltage-multiplier tacked onto the output. The TO252-packaged part is a Texas Instruments TLV1117; the "TV1117" marking, according to the datasheet, means this is a fixed 1.25V regulator, likely for supplying the core voltage for the DSP. Also, a pair of Analog Devices ADM709 power supply monitors (aka. reset controllers) make sure the hefty silicon parts only get "woken up" once all the necessary power rails have stabilized.

Staying on the power side of things, at the other end of the mainboard we find the regulators for the preamps and ADC's, comprised of a Texas Instruments TPS54140 step-down converter for the +5V, and an OnSemi MC7905 for the -5V.

At the heart of things is a trifecta of silicon. Well, quad-fecta, if you wish to count the external RAM there. But regardless, the main controller is an Atmel AT91SAM7S256 ARM7 microcontroller. Next to it is an Atmel AT45DB161D 16Mbit (2Mbyte) flash chip holding the firmware. The next larger chip is a Xilinx XC3S700A Spartan 3A FPGA. My best guess would be that it's in charge of marshalling the various audio signals between the converters, the DSP and the digital I/O, perhaps also handling some format encoding and decoding (thinking of ADAT here, for instance). The small chip to the left of the FPGA is an AKM AK4125 asynchronous sample rate converter.

And speaking of the DSP, a Texas Instruments TMS320C6722 is the brawn of the operation, crunching the numbers for all the audio processing this unit is capable of. At its side is an Alliance AS4C4M16S 64Mbit (8Mbyte) SDRAM chip.

And just above the two are a pair of Cirrus Logic CS2000-CP clock synthesizers, while the little SOT23-6 part is a Texas Instruments TP73001 linear regulator, likely for the FPGA core supply.

In the vicinity, a now-classic Texas Instruments TSB41AB2 handles the comms through the two Firewire ports, and an SMSC USB3300 interfaces the USB connection with the internals of the unit.

Temporarily obscured by the ribbon cable going to the front-panel LCD is the headphone output section, as well as some further power circuitry. The latter is comprised of a Texas Instruments TPS79650 5V linear regulator (i would reckon, powering the analog-to-digital converters, and potentially the mic preamps). The former is made up of a Rohm BA4580R driving the "Main Out" (mirroring the signal present at the main pair of outputs on the back), and a further two BA4580R's as well as an AKM AK4382 digital-to-analog converter for the independent "Phones" output.

The preamps are, as you may or may not have expected, a set of Texas Instruments PGA2500's - can you start to see a pattern emerging, perhaps? Regardless, each of the eight has an associated Taiwan Semiconductor DBL103G rectifier bridge used as part of the input protection.

In addition to a pair of Omron G6J-2FS-Y relays for each input (one for switching the attenuator pad, and one for switching the phantom-power to the mic inputs), hidden underneith the input board lurks an ST Microelectronics LM833 per channel. No doubt, for handling the line / instrument side of the inputs.

Slightly further "downstream", signal-path-wise, are 17 - yes, seventeen - ST Microelectronics TS922 rail-to-rail opamps, forming the input buffers and filters for the analog-to-digital converters, as well as one to buffer the half-supply voltage references from the two converter chips. Speaking of which, they're a pair of Cirrus Logic CS5368 8-channel units.

Moving on to the top (output) board, we find not four, but five AKM AK4396 stereo digital-to-analog converters, due to the 896mk3 having an extra "main stereo" output pair, in addition to the quasi-standard eight. The DAC output filtering is taken care of by a group of Rohm BA4580R's, while the converters are powered by another TPS79650 regulator.

Yet more BA4580R's drive the line outputs, and a nice touch that one does not see very often, diode-clamping protection on the outputs, with a few more DBL103G bridge rectifiers.

The rear of the front panel doesn't show much, but on the other hand, there's not all that much to show in the first place - a whole load of LED's, a couple of encoders, and that's about it.