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MMD > Archives > April 2001 > 2001.04.12 > 08Prev  Next

Duo-Art Expression Box Adjustment
By Spencer Chase

I just have to say that the letter from Craig Brougher says it all.
I have fiddled with my Duo-Art regulator for about 5 years and finally
got it right, after rebuilding it perfectly with respect to free play
and friction.

I chose to substitute Teflon and ball bearings just to make sure, but
this is not necessary with low-wear components.  What is necessary is
to get all slop out and minimize the friction.  After that it is a
matter of getting the chart linear.  This is essential for the
interleaving of expression levels as intended by the editors.

The exact slope required is dependant on the particular piano, the room
it is in, the voicing and the taste of the listener.  If the initial
rate of the springs is too low or they are too weak, the curve will top
out prematurely.  Modern springs are not rated for initial rate and can
be much lower than the original springs.  This can force the springs to
reach their elastic limit before the top is finished.

It is a simple matter to wind a spring with a desired rate but it is
much more difficult to set the initial rate.  Even the professionals
today do not promise accuracy in this.  I do not consider this to be
an easy task as others have suggested.  Choosing a spring merely by
wire size and coil diameter is not enough.  Every spring I have wound
has had a different initial rate.  I do not know what the professionals
did with the original springs but I find that forcing the coil closed
while winding and going slowly to prevent overlapping the turns can
produce a spring with a higher initial rate.

If you have enough room, the initial rate is not that important.  You
can increase the effective length of the spring and jack it up to the
correct initial tension and still have enough spring left for the job.
You can lower the bottom fixing of the spring to gain this extra room.
If you have to stretch it out to get the initial tension you will not
have enough linear spring left to cover the whole span of the

I also had problems with a piano that was too loud.  I have a large
Steinway upright in a small room.  After replacing the hammers and
easing the action I was able to get it to play at 3.5" vacuum at the
zero level.  Of course this requires very careful regulation of the
action and evenness of all center pins, etc., etc.

When you achieve this soft playing it is amazing what realism appears
Perfect condition of the expression box is essential as the slightest
free play or binding will result in non playing notes on occasion.

Spencer Chase

(Message sent Thu 12 Apr 2001, 07:28:24 GMT, from time zone GMT-0700.)

Key Words in Subject:  Adjustment, Box, Duo-Art, Expression

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