[sdiy] Note volume compression in the Hammond organ

Tom Wiltshire tom at electricdruid.net
Fri Jul 3 11:22:56 CEST 2020

The tonewheel signals from the pickups are fed to the keys via low value resistors. These are between 100R and 10R on the big console organs (B3 and similar) and are all 16R on the smaller Spinet organs.
So as you press more keys, these resistors are effectively put in parallel, reducing their value. The output of this passive mix then goes to the transformer (and which transformer tap is decided by the drawbars, but that isn’t relevant here).

I’m still not clear what’s going on though, Ben. Are you saying that the reduction in resistance loads the pick-up thereby reducing the level, or that the reduction in resistance reduces the level in the passive mix? I’m not seeing where the effect occurs. 

> On 3 Jul 2020, at 02:09, Ben Bradley <ben.pi.bradley at gmail.com> wrote:
> I recall that Hammond key busbars go to the drawbars which switch taps
> on a transformer that sums all the audio together (designed back when
> passive summing with a transformer was cheaper than using a vacuum
> tube as an active device for makeup gain). I hadn't noticed the "voice
> robbing" effect on Hammonds, but it may not be the worst in this
> regard ...
> As a teen in the 1970s I DID notice this in my Farfisa MiniCompact,
> and I knew enough electronics to see what was doing it. Each generator
> output went to the keyswitch board that had a 47k resistor between the
> generator output and the switch. The summing resistors for each of the
> three busses were also (IIRC) 47k, so each key's signal really dropped
> as more keys were played. I replaced the summing resistor with a 4.7k
> which of course reduced the signal a lot, but the keyboard sure
> sounded and felt different, more dynamic as each note held its volume
> as more keys were played. The original was like a compression effect.
> So yes, it appears this happens to any organ whose busbars don't
> connect to a very low impedance. Each added key lowers the total
> impedance seen from other keys, lowering the resultant signal level. I
> hadn't really thought of this in a lot of years (okay, decades). It
> never occurred to me to "simulate" or duplicate it, as I always
> considered it a defect.
> I've heard in recent years of old professional Hammond players going
> into the showroom where the new Hammond-Suzuki electronic models are
> sold, playing one for a minute or two and saying "It's just not the
> same." I've wondered what all the subtle differences might be, perhaps
> the tone pitches vary slightly due to slop in the gears or something.
> Surely this "voice robbing" is also part of it.
> On Thu, Jul 2, 2020 at 6:16 PM Michael E Caloroso
> <mec.forumreader at gmail.com> wrote:
>> "Voice robbing" is a convenient imperfection in Hammond organs.  It is
>> caused by the loading of the pickups in each tonewheel.  There are
>> zero buffers for the tonewheels.  As multiple keys using the same
>> tonewheel are pressed, the pickup is loaded heavier thus its output
>> level drops.
>> The level drop is subtle enough that the ear doesn't detect it.  The
>> net effect is as bigger chords are played relative to single notes,
>> the RMS level remains consistent.
>> The same effect exists in pipe organs that are wind driven.  As more
>> pipes are activated, the wind pressure drops.  Yet it is not a fault.
>> That's a not-very-well-known behavior that few clonewheel organs duplicate.
>> MC
>> On 7/2/20, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>>> Maybe it has something to do with how all of those tones get mixed together.
>>> These days we'd probably do it by summing all of the signals into a "virtual
>>> earth" mixing node, so that each signal source doesn't "load" down or
>>> otherwise interfere with the others. Maybe they cut corners in the original
>>> Hammond design and it leads to the behaviour you mentioned?
>>> Sorry for the speculative answer, will be interested to see what is said by
>>> those in the know about these beasts.
>>> -Richie,
>>> Sent from my Xperia SP on O2
>>> ---- Tom Wiltshire wrote ----
>>>> Hi All,
>>>> I’ve got a question about the Hammond organ (B3/C3, but others too) and I’m
>>>> struggling to find an answer, so I thought I’d ask here to see if anyone
>>>> knows.
>>>> I’ve heard it said that the way the harmonic volumes add up in the Hammond
>>>> organ is not linear, and that there is effectively some compression of the
>>>> sound as more notes are added. I’ve never been clear how exactly this
>>>> works, and if this is true or simply a way of expressing the fact that
>>>> doubling the power of a waveform doesn’t double its amplitude. So does the
>>>> Hammond harmonic addition add power or amplitude?
>>>> This is important since all the tones in the organ come from the same 91
>>>> pitches of the tone wheel generator. This means that a given ‘C’ note will
>>>> appear as the fundamental (8’ pitch) or one note, the second harmonic (4’
>>>> pitch) of the note an octave lower, and the sub harmonic (16’ pitch) of
>>>> the note an octave higher, plus being re-used as the 3rd, 5th, etc of
>>>> other notes. If you hold down a bunch of notes, you could easily get 4 or
>>>> 5 copies of the same tone being fed to the output. How do they add up?
>>>> I’ve tried to find some reference that deals with this, but the only really
>>>> complete document is Hammond’s original patent from 1934. I’ve tried
>>>> digging through that too, but it’s 37 page long and written like a patent,
>>>> not an explanation (obviously).
>>>> If anyone knows, or knows where I should look to find out without having to
>>>> read 37 pages of Patentese, I’d be grateful!
>>>> Thanks,
>>>> Tom
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