[ARC5] Crystal Headphones

[Richard Knoppow]

     Harry Olson has a page on the RCA design for sound powered 
phones in his book "Elements of Acoustical Engineering" and the 
later edition titled "Acoustical Engineering". The earlier one is 
on the web at Tubebooks and I think also at Archive.org.
    The drawing shows both elements to be of the balanced 
armature type. I have one and it does appear the receiver and 
transmitter are the same. I also have an Automatic Electric set 
which uses different elements for receiving and transmitting. I 
have a third type but its not working and I am not sure why, both 
elements are identical and test OK. The RCA and AE sets have the 
microphone and earphone in parallel with a DC resistance of about 
35 ohms. I have not yet checked the impedance but its likely low. 
Both units are very efficient but also make use of resonance to 
achieve the efficiency. Olson says that the intensity of the 
reproduced voice at the ear is higher than the level at the 
microphone. This sounds like something for nothing but is the 
result of the concentration because of the short horn on the 
microphone and the resonances along with a carefully limited 
frequency response.
    There is another form of sound powered phone, this one with 
dual headphones and a microphone on a chest plate. I think these 
were used for Naval gun emplacements. Probably both used the same 
elements.
     Because crystal sets operate best into a high impedance I 
rather wonder at these having a good reputation for use with them.
     There may be another technical description of sound powered 
phones somewhere but the one in Olson's book is the only one I am 
familiar with.
     The third set I have appear to be moving coil elements but 
they are sealed and I don't want to dissect them. The DC 
resistance is substantially higher than either the RCA or AE 
sets. I will try to make an impedance measurement in a day or so. 
I don't know why I never did it in the past.
     Crystal speakers were available as tweeters around the 
1950s. They have a problem of limited power handling capability. 
There were also speakers of the type used under pillows for 
personal listening using crystal elements. Crystal elements are 
quite simple so they can have good frequency response and are not 
too expensive to make. Crystal microphones and phonograph pickups 
were very popular from about the mid 1930s, when they first 
appeared, until the late 1950s when they were essentially 
supplanted by ceramic units.
    Crystal mics and pickups have the advantages of good 
frequency response, simple construction, meaning good performance 
at low cost, light in weight and are very sensitive often 
eliminating a stage of amplification. This was important at the 
time since a vacuum tube amplifier stage was expensive. However 
Rochelle salt crystals also have some problems: they are 
sensitive to moisture and must be well sealed, they are sensitive 
to heat and are limited to an ambient of around 125F, they are 
sensitive to shock so can't withstand being dropped and the 
phonograph pickups don't like being dropped on the record or 
scraped across the grooves. They were also used as cutting heads 
for phonograph records, mostly on home recorders. Here they had 
the advantage of good low cost for good performance, low power 
consumption and simple construction. However, all the 
aforementioned faults existed plus the crystal could be fractured 
by over driving it. Brush and Fairchild made some quite high 
fidelity cutters around the late 1930s to early 1950s.

On 9/18/2018 6:46 PM, Brooke Clarke wrote:
> Hi:
> 
> There are some LS-685/U Crystal Loudspeakers on the market.  But 
> they have a non standard connector that would need to be replaced.
> https://www.prc68.com/I/LS685.html
> 
> Sound powered phones are 10 to 20 dB more sensitive than 
> conventional designs.  See the comparison chart at:
> https://www.prc68.com/I/SoundPoweredTelephone.html#Background
> Early loudspeakers made use of the sound powered phone type drivers.
> Note sound powered receivers have cone movement at right angles 
> to the magnetic center line, while conventional receivers have 
> parallel or coaxial center lines.
> See drawing for the TP-3-T1 at 
> https://www.prc68.com/I/SoundPoweredTelephone.html#TP-3

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL