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Milli Keyboard Interface

Exploring the Milli Keyboard interface.
Published June 1994

Modern keyboards have many good points, but a properly conceived output stage isn't always one of them. Even upmarket models have unbalanced outputs, with at best nominal operating levels, while further down the scale are products designed to interface with home organs and hi‑fi systems, which operate at levels as low as ‑20dBv. To compound the problem, none that I know have any form of ground lifting facility, while the output impedance on some devices may be too high even to work comfortably into a typical desk's line input, let alone the mic input. The result of this dubious compatibility is noise, hum (due to ground loops), and sometimes a deterioration in sound quality due to loading of the output stage.

Milli solves these problems, but because it's built to professional standards, it isn't cheap; channel for channel, it works out about the same as buying a bank of DI boxes, though it is more convenient. Milli provides 12 identical channels, each with an unbalanced jack input and output and a balanced XLR output. The jacks are on the front panel, while the XLRs and mains inlet are on the rear. Each channel has a three‑position gain switch (0dB, 10dB or 20dB), a 20dB pad switch and a ground lift switch.

Internally, the circuit is simple but thoughtfully designed and involves an input op‑amp with switchable gain followed by two output buffer circuits, one balanced for the XLR output and one unbalanced for the jack output. Both benefit from the ground lift switch, which interposes a resistance of around 1K between the input ground and the output ground when the switch is set to ground lift mode. The unbalanced output is a straightforward 5532 buffer stage, though internal links may be set to feed the output jack directly from the input jack if that is preferable. By contrast, the balanced output utilises a fully floating, transformerless circuit based on the two halves of a 5532 op‑amp, the output being isolated by properly biased electrolytic capacitors with diode protection. This circuit topography ensures that no output level is sacrificed if either the hot or cold pin of the XLR is grounded to provide an unbalanced output; a further op‑amp provides a clean signal ground reference for each channel of the audio circuitry.

I have no real criticisms of Milli, though I do have a couple of comments which will be particularly relevant in today's budget‑conscious climate. Because so many keyboards are stereo, the inclusion of at least some dedicated stereo channels would simplify setting up, reduce the cost and do away with several switches. This aside, Milli is a functional device that works as transparently as any good DI box. Ground loops can be a studio nightmare, but Milli takes these in its stride, along with level matching.