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Keyboard Monitor In port and MIDI in/out

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Keyboard Monitor In port and MIDI in/out

Postby Steven11 » Mon Jun 29, 2020 3:11 pm

Hello,

I play keyboards as a hobby and sound engineering is not really my area of expertise, so I've found this forum and figured I'd ask here. I hope this is the right section.

My question is about connections between devices in general, be it audio or MIDI.

I have been connecting all kind of PCs, smartphones and tables into the "monitor in" port of my keyboards for years to play along and stuff, and the same goes for MIDI to connect keyboards between themselves.

Now, I have heard that if the device you are plugging in the "monitor in" has the headset out, as opposed to simply the headphone out, you can potentially damage your keyboard because the headset jack will send 5 V DC to it along with the audio signal, and this could somewhat damage the monitor in circuitry.

I am talking about regular 3 pin stereo 3,5 mm plugs here. Using these, the "mic" source of the jack, which is the one sending the 5 V, is even shorted out by the plug. How can it possibly send any voltage to the keyboard? Maybe connecting order plays a role?

Similar argument goes for wrongly connected MIDI cables, for example OUT to OUT as opposed to the rightly connected OUT to IN and the like. Is there a risk to fry anything?

I ask this because I have been doing the above for a long time, totally ignoring it (including mistakenly connecting devices to the wrong midi port, for ex. IN to IN).

All of this seems very strange to me, I'd think we'd see keyboards failing all the time. But who knows, maybe I am doing indeed something potentially harmful to the keyboard.

Am I missing something?
Thanks guys
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Re: Keyboard Monitor In port and MIDI in/out

Postby Hugh Robjohns » Mon Jun 29, 2020 5:37 pm

Steven11 wrote:I have heard that if the device you are plugging in the "monitor in" has the headset out, as opposed to simply the headphone out, you can potentially damage your keyboard because the headset jack will send 5 V DC to it along with the audio signal, and this could somewhat damage the monitor in circuitry.

Extremely unlikely and it wouldn't permanently damage it anyway, it would just lead to a distorted sound.

Audio inputs into devices like your keyboard monitor in are (almost) always 'DC-blocked' meaning they have a capacitor in series with the input signal specifically to let the audio pass and stop any DC that might be on the connection. And these capacitors are typically specified to cope with at least 16-20V and often 50V, so a puny 5V isn't going to worry anyone! Some specialist gear -- some interfaces and some hi-fi gear is 'DC-coupled' and doesn't have those blocking capacitors... the usual reason being that some people with aurically-enhanced ears believe they can hear distortion artefacts from series capacitors. But most equipment is designed to be more robust and practical!

Similar argument goes for wrongly connected MIDI cables, for example OUT to OUT as opposed to the rightly connected OUT to IN and the like. Is there a risk to fry anything?

Nope. the 5-pin DIN MIDI out socket has a 5V supply on pin 4, buffered through a 220 Ohm resistor. If you connect two MIDI Out sockets together the 5V supplies from both devices will meet in the middle of two 220 resistors... and absolutely nothing at all will happen! At the same time, the active MIDI data is on pin 5, again via a 220 Ohms resistor, so the two devices will be trying to shout over each other, but the 220 Ohm resistors are there specifically to prevent any damage being done to either side. equally, you can't hurt anything by plugging MIDI In connectors together. If that was possible, an awful lot of MIDI gear would be in landfill right now, because we've all accidentally mis-plugged MIDI equipment at one time or another!
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Re: Keyboard Monitor In port and MIDI in/out

Postby James Perrett » Mon Jun 29, 2020 5:59 pm

It may also be worth saying that, provided you are using standard 3.5mm TRS connectors and the connector is pushed fully into the socket on the phone/tablet, there will be no DC voltage as the sleeve on the TRS connector will short the microphone voltage to ground.
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Re: Keyboard Monitor In port and MIDI in/out

Postby ef37a » Mon Jun 29, 2020 6:24 pm

Hello Steven and welome.
As the two top chaps said, there is really no danger here.
Two things to be careful of..

The 48 volts present at the microphone XLR sockets of almost all interfaces and mixers. There is almost no risk of this going to anything but microphones but very few other electronic devices will be happy to receive it. Again an almost never happens event but as well to be aware.

Speaker outputs on amplifiers, especially powerful amplifiers. Always follow the instructions and don't make ad hoc connections. Shorts can destroy amps and big amps can destroy other things that should not be connected. No real shock risk but could be one of fire.

In general audio gear seems to be phenomenally reliable. I am on forums every day and cannot recall the last "It blowed up" post!

Finally, keep mains kit in good order, do not EVER modify anything and buy good quality cables and diss' strips. If you have any sign of a problem, call in a professional.

Dave.
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Re: Keyboard Monitor In port and MIDI in/out

Postby Steven11 » Sat Jul 04, 2020 9:56 am

Thanks so much guys!

I was sure it couldn't be THAT easy to damage a keyboard just like that...

So if I get it correctly, the worst possible scenario is that those 5 V DC are actually there (connector not fully pushed in, or any other reason), and that the device isn't protected against those unexpected current flowing into it. But even in this case, you get a distorted sound as the worst result.

So pretty reassuring. You know, In music circles (or any other art circle for the matter) you'll find guys who explain to you how easy it is to improvise over a diminished chord with a super-mixo-phrygian-whatnot scale, but then when it comes to gear technicality, tribal knowledge reigns supreme.

Anyway, thank you again for the good info. :)
Steven11
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