Speaker Development

Re: Where to find Jensen alnico speakers?

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From vanclee–(at)–etcom.com (Henry van Cleef)
Organization Union Graduate School
Date Sun, 28 Apr 1996 17:37:43 GMT
Newsgroups rec.antiques.radio+phono
Message-ID
References 1 2 3

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In article <4lrla5$hh--(at)--anix3.panix.com> ti–(at)–anix.com (Tim Mullen) writes:
>
> So does anybody out there have a good grip on when the first permanent
>magnet speaker appeared? Shortly after I got hold of a 1947 forty-one tube
>RCA 741PCS television I lost the field coil on it’s fifteen inch speaker.
>I replaced it with a PM speaker of the same size and an LR series to bring
>the voltage back down (made a mess out of the input electrolytic when the
>field coil opened up, too) but it never did sound the same. Much less bass.

I don’t have details on loudspeaker design and construction history, so
can only give some general comments.

So far as I know, the general configuration of a cone mounted on a large
stamped spider with a cylindrical voice coil operating in a magnetic
field appeared commercially around 1930. The earliest speaker in this
configuration that I know of were made by Magnavox, and had field coils
to produce the magnetic field, complete with a separate field coil power
supply mounted on the speaker base.

I believe that nickel-cobalt-iron alloys, including “Alnico,” were
developed in the 1925-35 period. The first PM speakers I am aware of
were used in “midget” radios in the mid-1930’s, and they were very
common by the time that the 35Z5/50L6 All-American Five was introduced
in 1939-40.

For the design engineer working in the 1935-50 period, many of the
choices in power supply component selection were cost-related rather
than performance-related. A power supply that includes a large filter
choke can use small filter capacitors, and achieve excellent ripple
control without significant voltage drop through the filter.
Additionally, using a small capacitor or choke input to the filter
allows use of a smaller power transformer because rectifier current
flows for a longer part of the AC cycle and produces less heating.

Additionally, in the 1940-46 period, and again in 1950-53, nickel and
cobalt were not readily available for use in non-military applications.
The design engineer faced both a cost factor (the total cost of power
supply components plus loudspeaker) and, during the WWII and Korean War
periods, availability factor.

Cost and availability, not technical performance of loudspeaker designs,
were the key issues in selecting loudspeakers in the 1935-55 period.
Quality loudspeakers with permanent magnet fields were available in the
late thirties, but a quality system also wanted ripple control in the
power supply. If a PM speaker and a field coil speaker with equal
performance were available at the same price, the design engineer needed
to consider the added cost of a filter choke if the PM speaker were
used, or of large filter capacitors and added dissipation in an RC
filter vs. an LC filter section. It was not until the mid-1950’s that
the total cost of large filter caps, an RC filter, and a PM speaker was
definitely less than that of small caps and an LC filter using a dynamic
field speaker.

I’d have to go look at my 1949 Magnavox console speaker layout, which
has one large speaker with a field coil, and three small ones, with a
3-way crossover network. I think that two speakers have field coils and
two are PM. While not common, except in high-end consoles,
multi-speaker layouts with crossover networks were used in the 30’s and
40’s, and I know of several that used a PM speaker in conjunction with an
electrodynamic speaker. This choice seems to have been clearly
dictated by the desirability of an LC power supply filter for the first
section and an unavailability of adequate DC current for a second
speaker unless it were connected as a parasitic load across the power
supply.

The power that went to a loudspeaker voice coil in a console radio of
the thirties and forties was almost always less than 10 watts, the power
available from push-pull 6V6’s (and 42’s or 6F6’s). The power available
from a pair of 45’s or 47’s in push-pull was substantially less.
Additionally, the audio response expected from most of these radios was
from about 100 Hz. to 6-7Khz at the 3db. down points, so a single 12
inch speaker was very common in consoles large enough to mount the
speaker.

AC-DC sets with half-wave rectifiers, both the 300 ma. designs using a
25Z5 or 25Z6, and the 150 ma. AA5, generally used PM speakers. In a
half-wave supply, the ripple from the rectifier is much higher (120% of
supply voltage, as I recall) than with full-wave supplies (I’d have to
look, but I think it works out to 30 or 40% of supply voltage), which
was too much ripple for a dynamic speaker to swallow without producing
extra hum.

The loudspeaker manufacturer names that come to mind from the thirties
are Magnavox, Jensen, and Electro-Voice, and the name I associate with
AA5 speakers is Quam. I think that all of these manufacturers also
built diaphragm drivers for use with horn systems.

The setups used in movie theaters needed to put plenty of audio power
into the hall, and it can be surprising to see a pair of 2A3’s driving a
large exponential horn setup. This shows what can be done with low
power. The horn technology derived from technological developments in
acoustic phonos. Indeed, an Orthophonic phono horn driven by a
diaphragm driver makes a very respectable speaker setup.

The first real adventure into “high fidelity” audio that I know of was
the sound track for the Walt Disney movie “Fantasia.” The Disney
studios bought Hewlett-Packard’s first audio oscillators, in 1939, for
work with this movie, and theaters showing the film were required to
upgrade their sound systems before showing it. As I recall, the high
frequencies were 10Khz., and one can find 10Khz. as the high frequency
cutoff for “high fidelity” in the literature of the forties. My
Hallicrafters S-36A, a 1940 design modified in 1943, and built in 1945,
claims 10 Khz. “high fidelity” performance on FM. The actual measured
audio performance in the unit is 3db down at 80 Hz. and 17 Khz. at 5
watts (Hallicrafters claimed 3, but the output is a pair of 6V6’s). I
have a cheap bass reflex speaker and a PA 70 volt transformer on the
box, and it is amazing how well the thing does. I think that this
points out the focus on speaker efficiency using large acoustic boxes in
very early “high fidelity.”

So far as the use of permanent magnets in loudspeakers goes, the first
loudspeakers I know of used headphone technology with a U-magnet driving
a small horn, and many loudspeakers were built using a rocking armature
and a horseshoe magnet to drive either a free-mounted cone structure or
a spider-mounted cone.

The design of loudspeaker transducers follows, more or less, the design
of D’Arsonval-based meter movements after the mid-1920’s. The use of an
electromagnet rather than a permanent magnet seems to be an innovation
that accompanied the introduction of cylindrical voice coil speakers.
The use of an electromagnet to develop a field flux in meters goes back
at least to the turn of the century, and this configuration is found in
wattmeter movements and other movements that measure a volt-amp product.


===================================================================
Hank van CleefThe Union Institute
E-mail vanclee–(at)–etcom.com or vanclee–(at)–mn.com
===================================================================

 

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