In 2015, Dieter Binkert asked me to make a Chardon / Casadesus 7 + 7, with a special setup for the sympathetics, as described by Henry Casadesus.
The Symps. are tuned F#,A#,G#,Gnat, Fnat, Enat, Eb. Where Henry Cas. recommends to use 7 steel violin E strings for the symps, I've used the 3 thinnest symps made by pirastro for better tension.
The playiing strings are tuned to a D major chord. ADADF#AD.
Casadesus even describes what to use for the playing strings "1st steel vioin string, 2nd thin gut viola A string, 3rd thick gut viola A string, 4th Aluminium wound viola D string, 5th silver wound viola G string, 6th silver wound viola C string, 7th doubled gut silver or platinum wound."
From this description by Henri Csadesus and my Chardon original built for Henri himself I made Dieter an instrument for studying the Casadesus Preludes.
The tuning makes for an inetresting tone. The A and D modes still have the strongest resonance but the general tone colour and character change subtly.
Here are some of my favourite recordings of Dieters. He has recorded all 24 preludes available on his youtube channel. Link to Dieters Youtube