Using Devtronix generator-keyers on Schober organs

Cleaning keyboards
Adding connectors
Devtronix generators


Schober organs and many other electronic organs of that era use "subtractive synthesis" to imitate the sounds of organ pipes.  Starting with a waveform such as a sawtooth wave which contains a rich set of overtones (and sounds rather like an unmusical buzz), some overtones are removed or reduced to produce a musical sound.  This is exactly analogous to such instruments as the violin, where the bow and string produce a sawtooth wave and the resonances in the body of the instrument reduce some overtones while preserving others.

In Schober organs, sawtooth waves, which contain both even and odd numbered harmonics, are the basis for sounds such as diapasons, open flutes, strings, and most reed stops.  Square waves, which lack even harmonics, are used as the basis for Bourdons, other stopped flutes, and some reed stops such as the Clarinet.  The stop filters control the strength of the harmonics so that a Diapason sounds different from a Trumpet, and Bourdon from a Clarinet, even though the original sawtooth or square waves are the same in each case.

Schober tone generators either made sawtooth waveforms directly or by adding up to seven square waves to make a very good approximation.  Square waves were produced by subtracting the even harmonics from a sawtooth wave.  To compensate for the generally "low pass" nature of the stop filters, the resistors in the keyboards were proportioned to pre-emphasize higher pitched notes.  After filtering, the loudness of the notes was approximately equal across the whole keyboard.

In the original Schober circuit, the keyboard contacts directly controlled the switching of sawtooth waves for each note.  This always introduces a small (but tolerable) click at the beginning of a note, but the click became very objectionable and the action erratic if the key contacts were oxidized or damaged. 

Although the electronic keying in the Devtronix tone generators will often solve keying problems on Schober organs, they also change the sound of the organ.  For the most part, I don't like the sound with Devtronix generators as much as the original Schober sound.

Why the sound is different with Devtronix generators

There are four main differences: speed of attack, overtone structure, scaling, and tuning.  For flutes and diapasons, the slower attack of the Devtronix keying removes a certain crispness and even a suggestion of Baroque "chiff" from the sound.  The sound also doesn't contain as many overtones as the original Schober generators (four octaves of even harmonics at 8 foot pitch instead of as many as seven), which makes it sound less natural, less full, and more "electronic", especially on string stops such as the Gamba.   Also, the scaling (loudness at different pitches) is wrong for flutes and diapasons, which are much too loud in the bass and too soft in the treble.

In some respect, the Devtronix tone generators are more suitable for reeds.  Reed pipes have a relatively slow attack, similar to the Devtronix keyers.  The reduced overtones don't seem to have as much effect on the reeds, perhaps because reed formant frequencies are typically within four octaves of the fundamental.  Since Devtronix scaling does not pre-emphasize the treble, it actually works better with the "bandpass" filters for reeds than with  the "low pass" filters for flutes.

I haven't yet tried a Devtronix generator for the Pedal clavier.  I would expect fewer problems with scaling, since many Recital organs already have Pedal scaling similar to the Devtronix.  Lack of harmonics may still be a problem, though at 16 foot pitch the Devtronix generator has five octaves of even harmonics.  It would be interesting to see how much the difference is noticeable.

The other major annoyance, at least for me, is that the Devtronix generator does not actually produce an equal tempered scale.  This can easily be demonstrated by first playing C and G together, then playing D and A together.  Beats can be clearly heard on the C to G fifth, but there are none on the D to A fifth!  Many pipe organs are tuned in unequal but "well tempered" temperaments such as Werkmeister, which favor commonly used keys over more remote keys.  Unfortunately, the top octave generator used in the Devtronix boards mostly does the opposite; it's a sort of "reverse well" temperament.  Its best major keys are E, B flat, and B; the worst are C, F, F# and G.  I may try running the Devtronix keyers from the top octave of the Schober generators or building a better digital top octave generator, and there may even be some way to change the scaling, but those are future projects.

Advantages of the Devtronix generators

To be fair, the Devtronix generators do have several advantages.  There is virtually no leakage through the keyers, so there is very little background noise.  The keying is much less sensitive to contact resistance, and if desired the attack can be speeded up by substituting smaller capacitors in the keying circuit.  Tuning is also easily accomplished since there is only one adjustment (but they will always be somewhat out of tune due to the faulty temperament).

Perhaps the best thing about having more than one set of generators is chorus effect due to slight differences in tuning.  This sounds more like a pipe organ, and probably works best if each division plays through a seperate speaker.


To get the best sound from a Devtronix generator, the output level controls need to be set properly.  If you are using only Devtronix generators, you can probably just set all the output controls to maximum.  However, if that produces distortion or if you are still using the Schober generators for anything besides 2 2/3', you will need to adjust the Devtronix generator or generators for the best level and the best square waves.  I will describe the process for the case where one keyboard has a Devtronix generator and the other does not.

Start by turning on the Swell to Great and Great to Swell couplers.  Turn on only the Contra Violone 16' (or whatever 16' stop you have on the Great).  Play middle C on each keyboard and adjust the 16' output control on the Devtronix generator until both keyboards play at the same loudness.  Now turn off the Contra Violone and turn on the Swell 16' Bourdon, which uses square wave ("WL") tone.  Play middle C and adjust the 8' output control up and down to find the place where there is an abrupt change in tone quality.  Compare the tone of middle C played from each keyboard, and adjust the 8' control carefully for the best match in tone quality between the two keyboards.  This should produce a good hollow-sounding "stopped pipe" tone, and the loudness should be the same on both keyboards.  Now turn off the Bourdon and select a stop such as the Gamba or Dulciana which uses regular 8' tone, and make sure the loudness of middle C is the same when played from both keyboards.  If not, adjust the 8' control make the loudness the same, then go back and re-check the tone on the Bourdon.  There should be a setting which is nearly optimal for both.

Now turn on only an 8' stopped flute, and adjust the 4' output control while playing middle C until you detect the sudden change in tone quality; then adjust until the tone quality matches on both keyboards.  Turn off the 8' stop and check the loudness of a Swell 4' flute by playing middle C from each keyboard.  If necessary, adjust the 4' output control for the best tradeoff between the tone quality of the 8' stopped flute and the loudness of the Swell 4' flute.

Now turn on only a stopped flute on the Great (for example, the 4' Chimney Flute) and adjust the 2' level control as above for best tone, always using middle C.  Then check the loudness of any 2' stop and re-adjust if necessary.

Note that because of the difference in scaling between the Devtronix and Schober generators, this procedure will give a different result depending on which octave you use to compare the generators.  I've specified middle C because I think that gives the best tradeoff between the two, but you may decide otherwise after trying it.


On the balance, I think the best thing to do is to clean the keyboard contacts and use the original Schober generators.  For keyboards that have already been modified for Devtronix generators, this will require replacing any keyboard resistors that were removed during the modification.

At the moment however I'm still using a Devtronix generator on the Swell.  But I'd rather keep the Recital sound as close to 100% Schober as I can, so I may change it back later.

Last updated May 3, 2007