All-new analog organ!

[Firepowerforfreedom]

Hammond has the music industry. Allen has the church market cornered. Lowery... well, they've got something. And Roland... can you really call their organs "organs"? What's missing—inexpensive, portable organs that still have the classic analog sound.

I think it's high time an analog organ hit the market again.

I'm no electronics expert, but I have the schematics for a Conn Caprice, a passion for learning, and, most importantly, some circuit sims so I can fiddle with the design before I actually build it. However, I know I can't do it alone, so I figured I'd search for some help here.

My ideal design is an individual oscillator design (with the oscillators mostly in ICs—Hartleys would require external inductors, however) that's roughly the same dimensions or slightly larger than a Casio keyboard. It would be solid-state except for the amp, which would have a single preamplifier tube to give it a vintage warmth. How many voices, how many keys, and how to put everything together is up in the air. If you're interested in weighing in on the design or contributing your knowledge or skills to this project, reply to this post!

Thanks,
FFF

[Firepowerforfreedom]

So far, I'm thinking of basing the system on Hartley or Armstrong oscillators.

[toasterDude]

Great idea, I've often thought the same myself. We're not the first to have this brainwave though. Check out this discussion group:

http://launch.groups.yahoo.com/group...guid=309683625

It's gone idle now, but there are about 500 archived posts about many different potential design aspects.

I guess the main question is whether you're talking about a one-off, custom project for your own amazement, or something that's ultimately meant to be marketable. The big problem with trying to make one marketable is the cost factor... not sure how many people would be up for shelling out big $$$ for essentially a one-trick pony, when you consider the competition in terms of MIDI clonewheel boards, synths, workstations etc. Plus you have to factor in the mfr. cost of a set of cool chrome-plated legs!

But I'm definitely with you in principle.

cheers,
td

[Firepowerforfreedom]

Great idea, I've often thought the same myself. We're not the first to have this brainwave though. Check out this discussion group:

http://launch.groups.yahoo.com/group...guid=309683625

It's gone idle now, but there are about 500 archived posts about many different potential design aspects.

I guess the main question is whether you're talking about a one-off, custom project for your own amazement, or something that's ultimately meant to be marketable. The big problem with trying to make one marketable is the cost factor... not sure how many people would be up for shelling out big $$$ for essentially a one-trick pony, when you consider the competition in terms of MIDI clonewheel boards, synths, workstations etc. Plus you have to factor in the mfr. cost of a set of cool chrome-plated legs!

But I'm definitely with you in principle.

cheers,
td

Well, about the one-trick pony... I'm intending to have it be customizeable—different voices, different octaves, based on what set of oscillator cards you load. I was thinking one octave per card, to make changing it out easy. Also, the voices section is swappable with other voice section modules.

[jimmywilliams]

If you mean individual oscillators for each individual pitch, that is quite expensive in labor/parts, and takes upa lot of space. I think Seville organs had a similar design years ago - these huge boards with individual pitch generators if I remember correctly. A Hartley oscillator will be bulky becuase of the inductors - and they get larger with the lower notes - see the old Gulbransens with individual tibia/sine oscillators to get an idea. You may also want to research Schober kit organs which I THINK had different options for generators/filters?? No point in redesigning what has already been designed - better to improve upon a foundation that already exists.

If you want individual oscillators for each pitch AND VOICE, instead of making a single set of triangle wave oscillators and then applying the voicing filters to them, that is even more expensive.

You may want to consider maybe two sets of oscillators - one for sine waves (flutes/tibia/"tonewheel") and another for more complex voices that can be filtered as you wish. Good flute voices are an important factor to distinguish good sounding organs from crappy sounding organs.

Becuase you said individual oscillators, I take it that dividers are out of the question then.

I would be curious to see how this goes.

[Firepowerforfreedom]

If you mean individual oscillators for each individual pitch, that is quite expensive in labor/parts, and takes upa lot of space. I think Seville organs had a similar design years ago - these huge boards with individual pitch generators if I remember correctly. A Hartley oscillator will be bulky becuase of the inductors - and they get larger with the lower notes - see the old Gulbransens with individual tibia/sine oscillators to get an idea. You may also want to research Schober kit organs which I THINK had different options for generators/filters?? No point in redesigning what has already been designed - better to improve upon a foundation that already exists.

If you want individual oscillators for each pitch AND VOICE, instead of making a single set of triangle wave oscillators and then applying the voicing filters to them, that is even more expensive.

You may want to consider maybe two sets of oscillators - one for sine waves (flutes/tibia/"tonewheel") and another for more complex voices that can be filtered as you wish. Good flute voices are an important factor to distinguish good sounding organs from crappy sounding organs.

Becuase you said individual oscillators, I take it that dividers are out of the question then.

I would be curious to see how this goes.

My plan was to improve on the original Conn design, but if it would be cheaper to use a divider circuit while still maintaining the sound, that'd probably be better. The Conns used two oscillators per note: one sine osc for flutes/tibia, the other a filtered pulse for almost everything else. Any other ideas for oscillator design (other than Hartley or Armstrong)?

[jimmywilliams]

My plan was to improve on the original Conn design, but if it would be cheaper to use a divider circuit while still maintaining the sound, that'd probably be better. The Conns used two oscillators per note: one sine osc for flutes/tibia, the other a filtered pulse for almost everything else. Any other ideas for oscillator design (other than Hartley or Armstrong)?

OK - sounds like you already have a good foundation to start with. Filtered pulse? Or were they filtered triangle or staircase waves?
If the Conn tibia set was anything like the Gulbransen set (modified Hartleys) you have to investigate the keying mechanism. I.e. are the generators "always on" or are they activated by a keying voltage like the Gulbransens were? In my Gulbransen the main generators (dividers) were always on. So you had two different keying mechanisms there.
You would want to simplify your keying design I presume so maybe you can have the tibias always on. Does the Conn work this way?
I don't know enough about the various types of sine generators other than the Hartleys that need those large inductors. I researched more modern sine oscillator circuits a while ago but if I understand them right most of them are just sine filters applied to a square or triangle wave - IIRC the main chip was called a 555??.
Gulbransen used filtered sine waves in their later organs - they called it the "flute filter." Sorry my knowledge is very Gulbransen-centric but those are the generators I am most familiar with. There were some flute filter issues with higher frequencies. And those were dividers too. You may want to get hold of some of those later Gulbransen schematics (or even a cheap/free later Gully organ like a Pacemaker) to examine how you can do the flutes with dividers. Personally I think the individual oscillators are the way to go but cost will be a factor.
I am curious now to read that other thread. No time tonight but will look into it over the next few days.

[Firepowerforfreedom]

If by always on, you mean the oscillators are always going, regardless of whether they key is depressed or not, then I believe so. I've modeled the Conn oscillator (which has a ground, bias/tremolo in, pulse out, sine out) in an app of mine and can see that the oscillations continue long after the key has been pressed. They just don't make any sound until they get enough voltage. The oscillators get 75V each time a key is pressed.

The shape of the pulse waves is difficult to describe. They're certainly not pure pulses, but whether they're triangle or something else, I'm not sure. I'll upload the schematics so you can see.

[jimmywilliams]

If by always on, you mean the oscillators are always going, regardless of whether they key is depressed or not, then I believe so. I've modeled the Conn oscillator (which has a ground, bias/tremolo in, pulse out, sine out) in an app of mine and can see that the oscillations continue long after the key has been pressed. They just don't make any sound until they get enough voltage. The oscillators get 75V each time a key is pressed.

I see ... very high keying voltage there ... you would probably want to design yours so that they run at a lower voltage, are always running, and get keyed to ground to complete the circuit.

[twnelson]

The original Conn oscillators from their first tube models were voltage keyed, via the plate circuit, no less! If I was forgetful about the vintage, such an organ would quickly remind me of this when it zapped me with 75VDC while trying to diagnose a key contact problem.. Tube models built from around the early '60s onward and all transistor models were continuous running oscillators with either transistor gate keyers or the notorious vinyl keying rods. They were all Hartleys.

The independent oscillator Conns derived all the different base waveforms from each single oscillator (although the old plate-keyed models and the 900 Customs had separate osc sets for each division): the sine wave for the flutes/tibias came off one point in the circuit, the 'pulse' waveform with its high harmonic content came off another, and on the tube oscillators, a sawtooth waveform with mostly even harmonics from a third. On transistor models, the pulse output was filtered appropriately for all voices except the flutes/tibias. On tube classical instruments, the sawtooth was used for the diapasons via transistor keyers.

The last generation of Rodgers analog organs (like my 760) used dual op-amps with R/C networks (no coils) to create very stable sine wave oscillators, both continuously running and keyed, with sine and pulse outputs. I'm sure that this approach would still be feasible and could be made quite compact with a bit of effort and the latest surface-mount components.

BTW, the pulse output from all of these oscillator types is the feedback impulse that keeps the oscillator running. On the Hartleys, it is taken from the coil center-tap and has an asymmetric, sort of damped-decay waveform. Very 'analog', definitely not a square pulse or triangle.

--- Tom