A LITTLE FAIRY TALE WHICH NEVER ENDS
Once upon a time there was a land of thermionic valves!
That was my land, these were my valves!
In early 50' and 60' Yugoslavia was one of few countries which manufactured lots of tube gear, of every kind - from little gadgets to industrial and military systems. For instance - Electronic Designers Handbook (McGraw-Hill 1957) was translated, supplemented and published in Belgrade in 1965. - Two years before I was born!
As not so many persons of my age, I was hooked on audio - mainly in contact with pro-audio gear like Studer, Revox & EMT in a local radio-station: in my teen ages.
In the same building, the same crew worked in radio and in a local cinema (a movie theater - for me, it was a movie temple).
You can imagine-durin' 1983 a teenager, symbiotically hooked on pro audio gear and totally tubed cinema gear!
Some history: one of (then) Yugoslav factory producing electronic products was "ISKRA-Zavod za automatizacijo", Ljubljana-Slovenia. They made so many good things. It's not possible to count 'em: projection machines, test equipment of all kinds, amplification and PA systems…
In short - all links in movie reproduction were ISKRA made, incl. audio amplification.
With good insight in all USSR, German and Anglo-American experience, combined with really remarkable own skills, the crew from that factory was capable to produce outstanding products.
ISKRA produced several theater audio systems, which served for any need and combination - one channel "light" sound, 2 and 4 channel magnetic sound, Todd-AO system, big theaters, small ones etc.
First cinemascope movie with 4 channel magnetic sound was demonstrated in 1960. And first Todd-AO (Michael Todd+ American Optical) with 6 channels magnetic sound was demonstrated on17.07.1968, during projection of the Russian movie "Ana Karenina" in Belgrade.
Some of ISKRA ampli systems were:
EK1404-intended for use in medium and smaller movie theaters, completed from next units:
EK1306 - "light" preamp
EK2304 - power amp
EK3104 - control amp
EK4406 - cinema power supply
EK6123 - cinema switch unit
ER 2510 - tape rec-player (re-branded Grundig)
ER1307 - record player
Some characteristics:
power cons. - cca 200 W
power output - cca 25 W/8,16 ohms
Photocell input - 1 mV/390K
phono input - 60 mV/500K
mic. Input - 3mV/1M
low reg-100 Hz+ - 10db
high reg-10KHz+ -10db
nonlinear dist. - 25W, 30-15000Hz, <1%
weight cca 50Kg
ISKRA EK 0720 was most frequently used system, completed from next units:
EK1306 - 1pc normal (or "working")
1pc backup
EK4406 -1pc normal
1pc backup
EK2304 -1pc normal
1pc backup
EK3104 -1pc
EK6123 -1pc
ER2510
ER1307
This system is mono,"light" sound.
Derivation of EK0720 is EK0710, consisted of following units:
EK1104 - 4 channel magnetic sound preamp
EK2304 - 4 pcs
EK4405 - power supply-2 pcs
EK3104
EK6304 - cinema switch unit
ER2510
ER1307
Characteristics of that system were mainly same as EK0720, except for magnetic input, 4x25W out power, and weight of cca 150 kg!
Interestingly, s/n for light sound was 55db and for magnetic sound some 45 db; But, in case of light sound, noise rise with level, in contrast with mag. sound, where noise remains the same.
NOW-EK2304:
This is, in original, beautifully made mono-block, constructed in rack-mounted case; Power and out tranny's are separated on each opposite side of case, in between them, there are tubes located and Elko's and nothing else.
Underneath bottom cover (in rack position this is rear side, so inside is very serviceable) there are small parts-resistors, signal capacitors and little selen rectifier for bias, including few little Elko's for bias and cathode(s)
decoupling…
All parts are first rate quality - bases are ceramic, resistors with security factor of at least 3-5,and nickel-plated parts shine even today!
(Later, for long period, Iskra was supplier of resistors and capacitors for QUAD, England…)
Power x-former is big 350W-like unit; output is 15 sq.cm (like 225W iron). UL taps on out tranny are on 50% and that mode really works well! Sound is same: big, live, but not bigger than life - just music and nothing more or less than that!
In several occasions I tried them in long sessions on K-horns; Man-that's MUSIC!
To be honest-I made some little changes; unnecessary stages were removed, input is 12AT7, phase splitter is long tailed 12AT7, rectifier is 5U4G, signal caps are oilers, feedback is decreased, EL34's are wired in triode, but nothing more (?!). I always think - soul is still here, if iron is same.
All in all, all that just little improved already excellent sound.
As you can see on shmtcs, EK2304 is equipped with internally passive EQ, intented for acoustic matching in specific space.
Loudspeakers: they were based on some ISKRA full-range units for smaller spaces and lower fidelity, and on two-way designs for bigger theaters and higher fidelity.
Full range-type EZ0753 (sorry, no pic) with various drivers-5 or 8W (8"-5 or 15 ohms) and 25 W 12" (5 or 15 ohms) in same reflex box
from 40 to 12000 Hz, in case of most bigger unit.
Two ways - mainly variations (same drivers in different volume reflex boxes) in two type: EZ0701 and KZ11.
In both, there were same ISKRA EZ1606 lf drivers, Altec 811b hf horn, with mounted Goodmans T52a, or similar Altec hf driver. All of them were permanent magnet units.
In some cases x-overs were ISKRA custom units (1KHz; 12/12db, with some 0 to -5 attenuation for hf), in other cases original Westrex 800Hz units. Good choice, don't you think?
In cases where the "light" sound is installed, one or more boxes were behind the screen; in case of 4 to 6 channel magnetic sound - more boxes were behind the screen, few boxes were in space, or auditorium, if you prefer; Somebody mentioned Dolby-surround?!
As you can see from shmtcs, ISKRA products were based on, for the time, modern European pieces like ECC81, ECC82, ECC83, EL34, GZ34 etc. In proper places in cinema supplies were heavy chokes, in conjunction
with Siemens elco's; some of that Elko's are used in my system in present time!
Not only EK2304's (they came to me, when some silicon lover installed junk amps instead) worked in this days, but even original NP21 projection machines work in our cinema and in numerous cinemas across YU and ex-YU.
I am still (sadly-occasionally only) projection man, but like I said before, teenage fever for mentioned gadgets will last forever!
***References:
1. Cinema Technical Handbook-M.Golubovic-Tehnicka knjiga
Belgrade 1971
2.ISKRA EK0720 manual, Ljubljana 1962
Prodanovic Aleksandar
BAC- YU
