Question about electrostatic pickups in old electro-acoustic pianos

[Greg]

Hi,
Some old electro-acoustic pianos have metal reeds which vibrate. One end
only of
each reed is secured to a metal bar, with the other end of the reed able to
vibrate
freely. At the free end of each reed, there is a plate of metal (a pickup),
which is very close
to the end of the reed. All these pickups are connected to together. The
whole assembly
of the bar, reeds, and pickups, forms a giant capacitor - it is charged to
hundreds of volts (DC),
and a preamplifier amplifies the voltage which is developed across the
capacitor
when the reeds are set into vibration with a plucking mechanism.

Now, some say that the reeds need to be *magnetised*. This is something I
don't
understand. Do you think that magnetising the reeds (along the length of the
reed),
would cause any appreciable change in signal amplitude? Assume that the
magnetic
attraction between the end of the reed and the pickup is neglible, for the
sakes of this
discussion.

Thanks,
Greg.

 

[Greg]

Note that the source impedance of the DC supply across the electrodes is
high.

Greg.

 

[Don Kelly]

Hi,
Some old electro-acoustic pianos have metal reeds which vibrate. One end
only of
each reed is secured to a metal bar, with the other end of the reed able to
vibrate
freely. At the free end of each reed, there is a plate of metal (a pickup),
which is very close
to the end of the reed. All these pickups are connected to together. The
whole assembly
of the bar, reeds, and pickups, forms a giant capacitor - it is charged to
hundreds of volts (DC),
and a preamplifier amplifies the voltage which is developed across the
capacitor
when the reeds are set into vibration with a plucking mechanism.

Now, some say that the reeds need to be *magnetised*. This is something I
don't
understand. Do you think that magnetising the reeds (along the length of the
reed),
would cause any appreciable change in signal amplitude? Assume that the
magnetic
attraction between the end of the reed and the pickup is neglible, for the
sakes of this
discussion.

Thanks,
Greg.

No- this is an electrostatic device- no magnetisation needed. There are also
(or have been) electrostatic speakers. Generally electrostatic devices for
electromechanical conversion suffer from low energy density compeared to
electromagnetic devices. - that is- it must be big to produce appreciable
power. In this case only a few microwatts (if that) would be involved.