[sdiy] MIDI output current and MIDI-driven gadgets

[Neil Johnson]

Hi Mattias,

The MIDI spec was never about voltage - it is specified as a 5mA current
loop into a receiver comprising an LED with a series 220 ohm resistor,
giving enough current to turn on the majority of opto-couplers. It just
happens that the reference schematic shown in the MIDI Spec (originally
drafted in 1983) used 5V. The newer specs now also support 3V3 with the
accompanying lower transmitter resistors. I don't believe there is anything
in the spec that would prevent you from selling a device with a MIDI OUT
that worked on -48V and two 4k5 resistors.

Neil

On Thu, 4 Jun 2026 at 12:08, Mattias Rickardsson <mr at analogue.org> wrote:

> Hi folks,
>
> Do any manufacturers intentionally design their MIDI Outs to improve
> reliability with MIDI-driven devices?
>
> Even though the MIDI standard only specifies communication, some gadgets
> use it for driving electronics by the voltage and current typically
> appearing on a MIDI Out. Like CME WIDI Master etc, adding Bluetooth MIDI in
> a plug directly on the MIDI port, and in many other hobby projects and
> commercial products throughout the years.
> https://www.cme-pro.com/widi-master/
>
> The MIDI standard allows quite wide tolerances on everything: Supply
> voltage +5V +/-10%, resistors +/- 5%. Hence, the possible voltages and
> currents available for a MIDI-driven device can differ quite a lot, at
> least in theory and in older equipment. Moreover, the MIDI Out can also run
> off a +3.3V +/-5% supply with low-valued resistors, potentially allowing
> more current if more than MIDI's 5 mA is sucked out, but also never giving
> anywhere near 5 V (or even 4 V) in low-current situations. There seem to be
> observations of certain external devices not working on certain MIDI gear,
> which is not very unexpected given the out-of-spec application.
>
> For example I'm thinking that it would be possible and fully allowed to
> choose 1% resistors near the more current-generous end of the 5% spec
> range, and then give slightly more current and/or higher voltage to an
> external device, while still being compliant with the MIDI spec (and
> hopefully well-working with all MIDI In ports). There are also other
> potential tweaks to the MIDI Outs, optimizing its use for certain "illegal"
> operations. The driving voltage can be tweaked to give more current or more
> voltage at certain non-MIDI-typical usecases, and I guess the MIDI Out
> could also be designed more as a true current source than the logic buffer
> with output resistors shown in the spec.
>
> Now that the MIDI spec has allowed both 3.3V and TRS since some years
> back, it should have triggered more innovation and I figured some of you
> might have encountered some interesting concepts and design variations. :-)
>
> Curious regards,
> /mr
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