The TTL MIDI signals of your sound cards game port should be buffered to ensure that you get enough current to power the LED in the optoisolator on your MIDI device. Here's an example of a proper adapter using a buffer chip.

Many thanks for the answer! 😀

But would such a circuit solve my problem of the voltage offset? I can relatively easily adjust the resistors so that the optoisolator gets enough current. This should be at the usual 5 mA.
I thought that my problem is the voltage difference and as a result the signals high and low levels become wrong. Instead of being between 5V and 0V, my signal is between 1.7V and 5V. Or am I on the wrong track??

One other problem is that the sound card probably has it own resistor between the sound IC and DB15 connector, perhaps as much as 10kΩ, so even on a short circuit you will only manage 0,3mA output current. You can see from this datasheet that the minimum high voltage is 2V for a typical part so it will solve your issue.

theseim wrote on 2020-05-13, 13:59:

Hello everybody! Not sure if the question has been discussed here yet, but I can't find anything. It's about the Gameport/MIDI i […]
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Hello everybody!
Not sure if the question has been discussed here yet, but I can't find anything. It's about the Gameport/MIDI interface of the Soundblaster PCI cards. I know there is no working Dos driver. But should the MIDI-Out work under Windows?
I can't get an external MIDI synth to accept the signals from my card (Audigy 2).

The whole thing is connected to the gameport, via a gameport to MIDI adapter cable. What I find strange is that the MIDI voltage on the Audigy gameport is 5V, but the MIDI signal is 3.3V, which will not work due to the design of the MIDI interface. Is this a known problem? I only know the topic that the voltage as well as the signal is 3.3V. But this can be solved by changing resistors in the adapter cable.

The MIDI bus is a current supply circuit. The supply voltage doesn't matter as long as it delivers the proper signalling current. The standard includes a reference circuit based around a 5V rail, but that's only an example implementation.

Okay, so build it yourself! 😀

Kamerat wrote on 2020-05-13, 14:42:

The TTL MIDI signals of your sound cards game port should be buffered to ensure that you get enough current to power the LED in the optoisolator on your MIDI device. Here's an example of a proper adapter using a buffer chip.

I finally found the time to build an adapter according to this schematic.
Works perfectly, thank you very much!

theseim wrote on 2020-08-09, 20:04:

I finally found the time to build an adapter according to this schematic.
Works perfectly, thank you very much!

Glad you got it working. Nice little adapter you made BTW.