The Organ Forum

I know that we live in the computer and digital age and I too have dabbled in vitrtual organs the past several years. My question is:

Does anyone know of any company that still sells analog tone generators and filters?

Gary 

 

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Nathan Wilcox

Actually, the components from mid-60s to mid-70s Rodgers analogs are quite straightforward to work with. They were easy-to-understand, elegant designs that can easily be reused, and most of the parts are available from common sources.

I have two Rodgers from that vintage, a 36D and a 660, that I've augmented with racks from other Rodgers that were gutted for either pipe or digital consoles.  A stress-relief hobby from my normal work of embedded hardware/software development.

-- Tom

 

Funny you should ask this as I was wondering the same thing myself a few months ago.  I discovered some schematics for various oscillator circuits out on the web; most of them seem to use something called a 555 chip and various values of tuning capacitors, etc; I know nothing about these but they apparently are a pretty standard issue chip.  Outside of spare boards from old organs, you will most likely have to design and buld your own.  I guess you would have to start out with an oscillator of some kind and build the appropriate shaping circuits from there.  I'd be curious to know if anyone has done a project like this.

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Jimmy Williams
Gulbransen Model D, Leslie 204, hobby organist/technician

Though spare parts for analog organs are still available, I know of no companies still making the generators & filters. Devtronix is now out of business--probably the last company to still offer kits for analog organs. A company from Colorado, Sonic Creations, stopped offering kits sometime in the 90's.

 The 555 is a timer IC, and is still available-it's probably the best option for "rolling your own", though you are limited in waveforms available.

 

Toodles..

The closest offering that I'd heard of in the last few years are some single note tone generators used to replace a missing note in a Hammond mechanical tone generator (maybe caused by an open pickup coil or damaged tone wheel).  As I recall they weren't cheap so building a complete instrument out of these would be darn expensive.

If you were looking to play with analog tone generators, filters and such I'd pick up an old instrument, the schematics and dig in.  In my teen years we had a Thomas Paramount and I loved to play with all sorts of filter mods and "improvements" to that organ.  I've got some ideas for little things I might do with (to?) my Rodgers Trio 321B and I keep telling myself I'm going to see if I can retrofit an ElectroMute type feature into my early Gulbransen Rialto K but it's been tough finding time to "improve" with all the repairs I find myself doing

 

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Gary
(Rodgers 321B, Gulbransen Theatrum, Hammond H-133, Gulbransen Rialto K with 100GK Isomonic Leslie, Rodgers 322 Trio Deluxe,Rialto K 1135 with 102 and 103 Leslie, Allen Theatre Deluxe with Gyrophonics and other speakers.

I don't know if this would interest you, but owner/builders of Schober kit organs often part out their instruments, which are old discrete transistor analog systems.  You might check out http://groups.yahoo.com/group/SchoberOrphans/ if you are interested in that approach.

David

I was considering building a tone generator system to use with some salvaged pedal boards to add bass pedal capability to my portable keyboard but I am pretty clueless as to how this is done. Does one usually have a square wave oscillator which is then filtered to get the desired harmonics? I envision a series of notch filters to reduce the levels of unwanted harmonics. Is this how it is done?

 mike

There are many approaches available, ranging from a single variable-pitch oscillator that is controlled by switching components with the pedal contacts to those with at least one oscillator for each pitch (highly stable).  Some systems have the oscillators running constantly and switch the audio and others switch the power to the oscillators and they only run when switched on by the key contacts.  Some methods allow for multiple tones (polyphony) and others connect the switches to only enable one pitch at a time (in the pedals, obviously).

The only organ I have intimate knowledge of is my Schober Recital Model, which was built from kits in 1964.  It uses the constant-running audio-switched tone generation technology (which is not necessarily the best, although it is the cheapest).  The entire organ (2 manuals and a full AGO pedal set) uses pitches generated by 12 essentially identical circuit boards, each of which has a stable LC oscillator driving 7 (or 8) divide-by-2 circuits; the outputs of the divider multivibrators are run through integrators to produce essentially a sawtooth waveform.  These pitches are sent through the various keyswitching systems and are gathered together into pitch register busses, which are then routed through sets of passive filters to produce the different voices of the instrument.  Although this technology does not begin to achieve the realism possible with sampled tones, it would be a lot easier to build at home, and it is capable of fairly good sound.

If one were to emulate this type of instrument today one would probably begin with a stable oscillator of suitable frequency, running that through a Top Octave Synthesizer (TOS) chip to produce the 12 high-frequency tones of each pitch, and then a series of divide-by-2 circuits to produce enough tones for the keyswitching.  The amount of circuitry involved for use with pedals would be considerably less than that needed for manuals, of course (32 keys instead of 61).  The downside of having phase-locked pitches in the octaves such a scheme produces is probably less detrimental if only used to provide pedal tones, because those pitches would not be locked with the ones in the manuals, anyway, and there are far fewer pitches involved in the first place.

If you are interested in what is involved in the tone generation and filtering process, there are sections of this document

http://launch.groups.yahoo.com/group/SchoberOrphans/files/How%20Schober%20Organs%20Work/

that may be of interest.

Before deciding to embark on such a project, though, I think I'd take the advice above and look for existing circuitry that could do the job.  Perhaps a chat with a friendly organ tech could be helpful.  A search on eBay might also be useful.

David

Thanks for the detailed response David. I have two pedal boards. One is a 13 note from a hammond (non-drawbar) spinet and the other is a 25 note from a Wurlitzer Centura. On the latter I am not sure that all of the magnetic switches work however as not all pedal tones worked when the organ was scrapped.

The larger board is of course nice to have but heavier (less portable). In either case I want polyphonic. What I had envisioned was was one oscillator for each of the twelve semi-tones and then divider circuits to provide the multiple octaves and pitches (16', 8', and maybe 4'). No real need for 32' as the speaker systems for the portable system will be intentionally limited to the mid 30Hz range.

Voicing doesn't need to be complicated. A nice Diapason sound, a soft reed or string and maybe a flute would do anything I need in this application.

 It seemed that the easiest way to do polyphony would be to have each oscillator running continually and just use the pedal contacts to switch the outputs into a mixer circuit. I will check out the document you linked and see if it clarifies things a bit for me.

 Thanks again,

mike

toodles:

 The 555 is a timer IC, and is still available-it's probably the best option for "rolling your own", though you are limited in waveforms available..

Since the frequency of a 555 oscillator is set by RC time constants, depending on components used and frequency there can be some issues keeping things tuned.

I agree with the comment about Rodgers generator ranks, or even easier to deal with, old Allen generator ranks.  Both of these instruments used sine wave oscillators and have very stable tuning.  It's relatively easy to get to a wide variety of organ tone colors from the basic sine waveform using 1/2 wave clipping, full wave clipping, various low and high-pass filtering,  differentiation and formant filters for reeds, etc.

The 1960's book by Richard Dorf of Schober organ fame, "Electronic Musical Instruments" has lots of circuit ideas and methods for doing things.

 

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Grant
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Hauptwerk on a Rodgers 750BE shell.

Dup - deleted

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Grant
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Hauptwerk on a Rodgers 750BE shell.

Mike,

Before you commit yourself to quite a bit of work, you need to give thought to how fussy you are regarding the quality of sound you will find satisfying. To use the pedal contacts simply to switch the tones, will give abrupt attacks and decays that will never sound like a "nice Diapason" etc, although it may pass as a reed. Then to feed those sounds into a set of filters aiming at a variety of tones, ranging from a flute, through to a diapason, a reed, and also a string, will lead to inevitable regulation problems if the filters are truly doing their job. For example, the flute will be too soft at the top of the range, and the string too soft at the bottom of the range.

Although I remain committed to analogue circuitry for home-built organs, there is no doubt in my mind that today, if you want to minimize the amount of work and time involved, the virtual organ is the way to go. I suspect this applies even to a modest project such as what you are planning. You can set up truly satisfying organs on a laptop computer these days (e.g. MyOrgan or jOrgan - both of which are free).

John Reimer

Hi John, that is an interesting approach and may be the most practical as it just occurred to me that the transposition function on the keyboard will be used heavily and so the pedals will also need to be shifted as well. Cost may be the biggest factor as I don't own a laptop. I was hoping that I could implement with a few ICs and some passive components. This could easily escalate into the equivilant of buying a whole new instrument. :)

 mike

Building something from scratch to cover 25 pedals in registers 16', 8', and 4' will require providing pitches from 32.7 Hz (low 16' C) to 523.0 Hz (top 4' C); a top-octave generator chip will produce a high frequency of 523.0 Hz with an input frequency of 125,000 Hz, and each pitch will require 4 stages of divide-by-2 logic (bottom C to B) plus the extra top C the chip also produces.  So you'd need a DC power supply for the chips, some form of stable 125,000 Hz oscillator, a top-octave generator chip, and 12 4-stage divider chips; these would provide 49 pitches and they would be square waves, which have only the odd harmonics.  (However, by adding octave-related pitches together, the even harmonics can be added for those voices that need both odd and even harmonics.)  Square waves are sufficient for producing "stopped" voices and woodwind reeds; the even harmonics are necessary for satisfactory simulation of diapasons, strings, and chorus reeds.

You could go with mechanical switching (3 sets of contacts per pedal), which is what my Recital Model does, or go a better way and use solid-state switches (3 per pedal, or 75 of them)--a big advantage of the solid-state switches is that it is possible to shape the attack envelope somewhat with those.  I know of chips with as many as 4 switches each, so you'd need 19 of those.

Flue pipe (flutes and diapasons) filters are basically low-pass devices that quickly roll off the higher harmonics; as a result, the upper frequencies are louder than the lower ones.  This is countered in my Recital Model by using graduated resistors between the key switches and the filters to attenuate the higher frequencies more.  These resistors are part of the additive network that builds the pitch register busses that feed the filters and prevents the tones from feeding back into each other.  The low-pass design of the filters pretty much eliminates the switching transients, too.  Reed filters include strong resonant circuits to emphasize a particular portion of the frequency spectrum--some resonances are fairly sharp, others are very gentle.  My Recital Model uses a single filter to cover the entire 61 (or 32) note compass of a single stop, but there is no reason it would be difficult to divide the range into 2 or more pieces and provide different filters for each; the output of the more numerous filters would have to be combined, but some of that is already necessary, anyway.  Once the filter outputs have been combined into the composite signal it would be sent through amplification to power the speaker(s).

The above is, of course, a very simplified picture and is intended only to give you an idea of a minimal set of components that might be required to do what you have suggested.  In reality, the number of IC chips involved is not large (especially if you don't use solid-state switching).  If you were to use physical switches, magnetic reed switches tend to be less prone to contact problems over time.

The Dorf book is a jewel, but it has been out of print for a very long time.  Your library may have a copy, and one surfaces from time to time on eBay; some big used book dealers might also have a copy.  Richard Dorf, by the way, was the inventor of the Schober Organ kits.

I'm not an Electrical Engineer, but I did build my kit organ and have some familiarity with what is involved.  If I can be of further service I'd be glad to help.

David