Louigi Verona wrote:

So I see. But I am still interested in how he did what he reported since I still cannot get
his scheme to work.

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Sorry for the late replies,

The Vocoder track needs to be a 2 channel track, one input will be assigned to the carrier input of the vocoder LADSPA plugin and the other to the formant input. Input 1 of the Vocoder track/bus is the carrier, input 2 the formant.
It also doesn't matter if the sources for the carrier and formant are internal or external, in my case I connected Yoshimi to the Formant Synth in port, but you could also connect the output of a specific track. The same goes for the carrier, in my case it's the built-in mic (system capture), but it could also be the output of a specific track.
I think what is important is that you create separate buses for all your tracks and use these buses for both input and output on all your tracks except on the mix tracks. All tracks have to be monitored, not their inputs or outputs but the tracks themselves, so the middle part of the Qtractor Mixer panel.
Hope that helps a bit. I have access to only my netbook (I'm on vacation, long live Android and mobile internet) but maybe I can make some screenshots of how to set up the Qtractor Mixer panel.

Best,

Jeremy