[sdiy] MIDI output current and MIDI-driven gadgets

[Mike Bryant]

Agree on the spec, but 48V would destroy many of these Chinese MIDI devices that omit the opto-isolator 😄
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From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Neil Johnson via Synth-diy <synth-diy at synth-diy.org>
Sent: 04 June 2026 12:46
To: Mattias Rickardsson <mr at analogue.org>
Cc: Synth DIY <synth-diy at synth-diy.org>
Subject: Re: [sdiy] MIDI output current and MIDI-driven gadgets

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<mailto: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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