synthv1 is an old-school all-digital 4-oscillator subtractive polyphonic synthesizer with stereo fx made by rncbc aka. Rui Nuno Capela.
synthv1 is free, open-source software, distributed under the terms of the GNU General Public License (GPL) version 2 or later.
The software requirements for build and runtime are listed as follows:
Qt framework (core, gui, xml), C++ class library and tools for cross-platform application and UI development http://qt.io/
Optional (opted-in at build time):
JACK Audio Connection Kit http://jackaudio.org/ ALSA, Advanced Linux Sound Architecture http://www.alsa-project.org/ LV2, Audio Plugin Standard, the extensible successor of LADSPA http://lv2plug.in/ NSM, Non Session Management http://non.tuxfamily.org/nsm/ liblo, Lightweight OSC implementation (needed for NSM support) http://liblo.sourceforge.net/
This synthesizer is made for alternative DAW producers.
HOW I DID
To use synthv1, you must download QJackCTL which will allow you to connect a virtual keyboard (that you also have to download) to the synthv1 app.
QjackCtl is a simple Qt application to control the JACK sound server daemon, specific for the Linux Audio Desktop infrastructure.
Written in C++ around the Qt framework for X11, most exclusively using Qt Designer.
Provides a simple GUI dialog for setting several JACK daemon parameters, which are properly saved between sessions, and a way control of the status of the audio server daemon. With time, this primordial interface has become richer by including a enhanced patchbay and connection control features.
QjackCtl is free, open-source software, distributed under the terms of the GNU General Public License (GPL) version 2 or later.
Project page: http://sourceforge.net/projects/qjackctl
download link : 
virtual keyboard example :
Jack Keyboard link : 
In AUDIO part :
Connect synthv1 with System
In MIDI part :
Connect your Virtual Keyboard (out) to SynthV1 (in)
Now you can hear sound when pressing keys on the computer keyboard.
Modify your sound by using the tools of the interface.