Stern Radio L4055, P554 and other speakers

Elliptical Loudspeaker Stern Radio Leipzig L4055 mod 1956 manufactured  June 1959

 Stern Radio speaker L4055 -2Stern Radio speaker L4055

Wide range speakers were step ahead in development of so called „concert speakers” which already delivered quality of sound adequate to reproduce music. Concert speakers were capable to reproduce lower tones than standard ones used in radios of postwar era. In post II WW era radio had mostly it’s informative function and this was main expectation of users. Since music got it’s popularity in nineteen fifties radios were adopted to it by use of stronger amplifiers, bigger speakers capable for lower tones and effective tone balance controls – linear and switched programmed . Much effort was put to loudness correction what resulted in use of volume potentiometers having even 4 control connection points. Then idea of smooth middle and sopranos came to resolution. This resolution was so called „Wide range speaker” With development of Hi Fi FM radio transmission producers provided  auxiliary tweeters usually located at sides of radio for spacious impression. .

Unfortunately concept of radio receiver did not change, and those significant changes of it’s internal technology had no reflection in overall concept of enclosure being obviously also acoustic system. Thermal requirements for cooling of many tubes forced concept of perfect ventilation rather than perfect acoustic result. The more tubes , the worse perspectives for acoustic :) . In authors belief this speakers and electronic are underrated in their capability and all what was needed it was to separate them.

.The big advantages of this speakers are 1) relatively light cone resulting in good reproduction for middle and sopranos, 2) big surface lowering movement of coil required for bass ( small magnetic field born distortions, small Doeppler effect) and 3) small signal phase shifts in whole reproduced range of frequencies  comparing to standard few way crossover systems.

Some constant  phase shift may be  existing by parallel radiation of common frequencies for small and main cone which are positioned i different distance from reception point.

Advantage (in authors belief) of such wide range speaker is that upper range and lower range is radiated by same concept of diaphragms – cones and it assures both similar radiation characteristic in wide angle shift from symmetric position to speaker. It means when listener in not in optimum direction to speaker the timbre if sound is preserved in way more natural than with mixed technologies. It’s no direct need to stay in center point to achieve great impression.

As example I present below wide range speaker L 4055 designed and produced by  Stern Radio Liepzig in 1956 – 1959. Power of cone may look today not much impressive – it’s 6 Watt. But assembled in very good acoustic system with amplifier 7 Watt they do not show disadvantage of loudness to my 100 Watt modern three way system MATCH with amplifier 40W – 90W . Impedance of L4055 is 7 Ohms.L4055 was designed and  produced for Stern  tube radio „Juvel 2″ and was driven by only one EL84 power pentode. For sopranos  producer provided auxiliary two  located on sides  tweeters P554 in serial separated from low frequencies by serial capacity 1 uF.

 L4055 with P554 were not the last word of  producer.

 Next improvements came with systems used in radio named  „Beethoven 2″ where standard round one cone „concert” speaker was supported by 3 tweeters driven from separate tweeter amplifier. That solution allowed elimination of phase distortion created by speaker’s crossover circuitry and was up to author’s knowledge unique. Bad acoustic was main reason diminishing possible result.

 

Another unique project was provided in radio named „Stradivari 2″

To assure the elimination of crossover distortions in basic hearing range, the „concert” speaker was spared with Stern Radio 8 Watt low toner, and this low toner was supplemented by two middle toners having double (similar to wide range) cones each!!. Low  and middle speakers were connected in serial and  not separated electricallydue to frequency at all!!!. Only by their unique frequency characteristics Low toner had impedance 3 Ohms, parallel connected middle toners 4 Ohm, all together in serial resulting in 5 Ohm. Thus reproduction was created by total 5 cones (one mono channel) without any phase shift depending on frequency!! Again to obtain sopranos Stern Radio Leipzig assembled their own design 1 auxiliary high class tweeter P554 connected with capacity 2 uF in serial. Although it was exceptional quality tweeter in author’s opinion this capacity was the only (except bad acoustic of radio ) small disadvantage of that system.

Cone Tweeters P554

 

By the way there is presented also an idea of modern tweeter with spherical diaphragm. Although it is most popular and trusty it is worthy to notice it’s significant difference according to old cone type. Spherical type has one mechanical soft and elastic suspension while good class cone type had two suspensions – one small diameter soft fixed with thread in center and other was outer shape hard glued to metal body. It may be possible that those two suspensions having of course different capabilities, stiffness and resonance frequencies allowed more comfortable conditions to basic cone  than one low tuned spherical. In authors experience this type cone tweeter happened to have better lower range than used by him different types of modern tweeters. For this reason and for direction characteristic original P554 cone type was chosen as the best instead of modern spherical in vintage set..

Oval shape of cones?

Another interesting problem is reason of use the elliptical shape of cone on old type speakers. One obvious explanation is that oval shape of speaker was practical to place rather big dimension speaker in desired by customers radio shape of limited height.

In author’s opinion this shape poses also positive futures regarding balance and management of mechanical resonances. The same cone has different masses in two perpendicular directions and different stiffness what results in mechanical loading the diaphragm in their perpendicular resonances. Today round shape is common even in high class drivers. Why not if modern speaker’s columns are just to be high and slim? And of course it’s convenient in serial production? The resonance is common and whole cone is engaged and finally making it harder to get rid of -.. Specially due to desired today high stiffness of big power cones. Mostly that resonance is engaged to produce bass in cooperation with resonant acoustic system For the non resonant type acoustic in authors opinion, (on a reason that tested by him oval speakers were always distinguishing with friendly, beautiful and surprisingly natural sound) the positive impact of oval shape could be better researched.

Today rare but tested by author elliptical; speakers :

Stern Radio L4055,

Stern Radio P540,

Tesla ARE 668,

Tesla ARE 462,

TONSIL GDS 31-26.

2015 May 01

Stern wide-range 2

Frequency characteristic starting at 11Hz and ending at 22kHz  for made by author speaker box with L4055 and P554 shows visible main resonances of enclosure. In presented frequency characteristic are visible also  three waves which by author are considered as 3 resonances at 3 kHz, 4kHz and 5kHz of space between two mentioned above  diaphragms. That is the price paid for other advantages.  Performance below 43 Hz looks in graph not believable but personal impression in practice comply to picture.

  For comparison characteristic of based on modern speakers, MATCH filters and  transmission line enclosure also developed by author

MATCH  transmission line

In author’s opinion after the tests, sound of first one is more clear, spectacular and may be example how visible results of technical measurements  may   not  comply to   personal  impression. Good linearity of MATCH frequency management system and little higher enclosure resonances  are visible.