MiniDSP chassis done, at last

Not strictly related to the speakers themselves, but as the miniDSP is a core component in driving the speakers I’ve noted this in the “beyond-econowave” posts:
For a long time the miniDSP was sitting bolted to a lump of wood, the “temporary” test chassis, with a cardboard box sitting on top of it to cover it. But this test chassis was quite noisy.

I’ve made a simple pine enclosure that makes up in solidity what it lacks in sophistication. It still containes the miniDSP and 2 simple OPA2134 based pre-amps to give me a bit of gain over the quiet miniDSP output, but I’ve got a 4 channel stepped attenuator (actually two stereo stepped attenuators ganged together) as a master volume, plus the software volume control as a fine volume pot.

There’s also a switch to switch over to the analogue input for “guest devices” .e.g. mates dropping over with DJ mixers or synths – so the sockets are on the front for easy access.
Made with bits of pine from the local hardware warehouse, and painted with leftover stain/varnish from my big speakers so it matches.

The tricky bit was mounting the two preamp PCBs (which didnt come with mounting holes) in a way that they were braced down firmly to cope with the stepped attenuators being turned.. solution some clamps fashioned from “polymorph plastic” and brass bolts to hold the PCBs firmly in place.

As the USB input on the miniDSP I find is super sensitive to USB cables quality, so a USB extender would be unreliable, and I couldnt think of an easy way to provide an SP/DIF optical socket on the outside, the left side has a simple “porthole” in the side to access the optical SPDIF (primary input) and USB (for making adjustments to the crossover).

Jaycar linear PSU is mounted inside the steel box from a PC power supply at the rear of the box, keeping all the mains/power stuff isolated from the rest of the circuitry.

Just to recap what is in the box:

  • A miniDSP 2×4 with a miniDIGI board mounted on top. The 4 outputs are fed to:
  • Two simple ebay opamp preamplifier boards. The input coupling capacitors have been bypassed, as the miniDSP has no DC offset, and the opamps replaced with OPA2134 chips. The volume pots have been replaced with two stereo stepped attenuators that are ganged together to make a single 4 way volume control. This allows boosting the output level of the miniDSP, as I find it runs too quiet.
  • Jaycar linear power supply to drive both the miniDSP and the two pre-amp boards.
  • Software volume control attached to the miniDSP (the is labelled “fine” volume), with the stepped attenuator as master volume.

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