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The Eminent Grand Theatre 2500

Concert Organ - 1975


Blue Book says: nothing.




Tone Generator

Eminent designed their organ with a very intersting tone generator concept. It basically is a "divider organ", i.e. a top octave IC divides the highest 12 notes down from a common high frequency, and the remaining notes are divided down by digital flip flops. This simple principle results in rectangle wave forms, which unfortunately do not sound very sweet by nature. Eminent added a futher differentiator circuit after each 96 tones, and come up with a nice sounding sawtooth generator, and their strings sound have got so fabulous.


However the tone generator supports sawtooth and rectangle waves at the same time. The rects are used for filtered flute drawbars, and piano tabs (which also use some saw footages). Sawtooth is used for strings and principals.

The Clarinet tab is made by subtracting 8" and 4" saws. This results in an interesting tone, a bit different to pure rectangle, because the saw is not symmetrical.


The key has a single switch to activate the several notes electronically.  The keyers are implemented by the IC TDA 0470, containing a 12x transistor array to switch all footages. Many IC of them are used for switching of the saw/rectangle signals, separate for drawbars, principals, piano/percussion and strings.

Image: The Tone Generator: 3 PCBs with the 12 ocatve dividers and the TOS below.


The boasted string-ensemble

Keyers for strings section are even twice: one for sustained strings, one without sustain.

There are 4 modulator circuits that can work in parallel:

  • Chorus: a slow effect
  • Vibrato: Main generator does not use a vibrator oscillator, tuning is constant all the time. Vibrato is realized over the modulators.
  • Multivoice: 3 chorus lines with different speeds, this is the famous Eminent Strings Effect.
  • Orbitone: Leslie speaker simulation.


Image: One keyer section for the flute drawbars.


Looking for the circuit diagrams of your Eminent? This service information is hidden inside your organ! Like with the old TVs, this is Dutch service!


Parts that were defect in my organ: 2xBC170, CD4014, opamp, TDA (TOS)

There are many power resistors, that have broken ceramic bodies, but still work.

The main power transformer was soldered to the 220V connection, I reconnected it to the 240 V pin, to reduce the power consumption a bit with our 230V in Germany.


 The GT2500 has a smaller sister the GT2000, with smaller manuals, but same electronics, which came out 1975 and costed new $36.000!



My personal ranking

****      (4 Stars of 5)


Good sounding Drawbars, Tabs and Strings


Tabs feel cheap

Sound Examples in mp3