July 1932 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Potassium sodium tartrate (Rochelle
salt) was used in commercial speakers for a while in place
of electromagnetic driver coils. The relatively large mechanical
deformation produced when subject to an electric field made
them attractive as coil alternatives because a separate energization
circuit was not required. The drawback, at least early on, was
unavailability of crystals large enough to drive anything other
than a headphone size speaker cone. This article tells of the
time when a process was created to grow large crystals from
a seed, similar to how silicon, gallium-arsenide, and other
modern semiconductors are grown from seeds. According to author
C.B. Scott, it revolutionized the speaker industry - at least
for a while. I don't know if a 1930s' lawyer advised companies
to place "Do not put in mouth" warning labels on the speakers
since, according to Wikipedia, Rochelle salts were also used
The Rochelle-Salt Crystal Reproducer
By C. B. Scott*
A revolutionary idea in commercial reproducer design. A Rochelle-Salt
Crystal, of improved construction, mechanically coupled to a
cone gives remarkable clarity and volume. Consuming no power,
it has high efficiency.
Fig. B - Picture of the new Crystal speaker
which is bound to be used in all receivers that require a maximum
Piezo electricity, a phenomenon well known to most radio
engineers, demonstrable in quartz and other crystals, is present
in much greater quantity in Rochelle-salt crystals. Until quite
recently, however, it had been impossible to obtain these crystals
in size and quantity to make their commercial application practicable.
At last, a process has been developed for growing large, homogeneous
crystals, and methods have been devised for machining and shaping
them into usable sizes and shapes.
Fig. 1 - The speaker's crystals.
Direction of Motion
The Type R-95 "Crystal Reproducer," illustrated in Figs.
A and B uses a double or "bimorph" crystal element consisting
of two slabs 2 1/2 x 2 1/2 x 1/8-in. thick, metal-foiled on
each surface, and cemented together in opposition to each other
so that a torsional motion of the combined slabs, illustrated
in Fig. 1 results; when three corners of the element are held
semi-rigid, the vibration of the fourth corner is in a direction
vertical to the flat surfaces. (This action is similar to that
in thermostats, where the expansion of one metal and contraction
of the other in one [parallel] plane, produces a "wiggling"
motion in another. Technical Editor.)
Fig. 2 - Vacuum tube speaker driver circuits
This corner is fitted with a metal cap provided with a connecting
link which in turn is soldered to a tone arm, providing a mechanical
amplification of motion of 2 1/2 to 1.
The end of the tone-arm is then fastened to the center of
a conventional paper cone and results in a very light, compact
reproducer, suitable for general use, with an outside diameter
of 9 1/2 in., a depth overall of 3 3/4, in.; and a weight of
two pounds (as compared with the typical "dynamic" type of reproducer,
weighing approximately 5 1/2 pounds).
Fig. A - This speaker was tested by Radio
Craft and found to have excellent tone quality and volume.
The "crystal" reproducer has several advantages from the
electrical point of view.
It is voltage-operated and the power consumption is very
low, as it requires neither field current nor polarizing voltage.
For this reason it has been recognized as being especially valuable
in multiple reproducer work, such as installations in schools,
hotels, and hospitals. Due to its very high "sensitivity," several
may be operated on the same power required to operate one of
the present type of reproducers. Likewise in this type of installation,
one of the important factors is that there be a minimum of service
and repair work. "Crystal" reproducers have been operated for
a continuous period of four years under a wide variation in
temperature and humidity change, with no deterioration in output.
Fig. 3 - Impedance variations.
The "crystal" reproducer covers a much wider range of frequencies
than is now covered by the present types of reproducer units.
The Rochelle salt crystal itself is responsive from 0 to 500,000
cycles and when built into a reproducer the only limiting factor
is the method by which the crystal is made to reproduce through
an associated tone-arm, cone, etc., and the frequencies which
are brought to it as a result of broadcast station equipment
and radio receiving sets.
The reproducer is considered to have a "negative impedance"
(condenser effect) of about 25,000 ohms at 1,000 cycles, and
characteristics similar to those of a 0.03-mf. condenser.
It operates extremely satisfactorily when connected directly
across an output choke in the plate circuit of a type '47 pentode
tube. It also has an astonishing volume when connected across
the output inductance in the plate circuit of a pair of type
'30 tubes connected in push pull, and is especially good for
battery-operated sets when operated by a pair of these tubes
in "push-push" or class B amplifier connection. The diagrams
shown below indicate the possible hookups with pentode and '45
tubes either singly or in push-pull.
* Brush Development Co.
Posted July 1, 2015