Observations on the M-100 Series Percussion

[stefanv]

I am in the midst of several projects with my M-111, one of which is an extended preset system. In designing it, I decided I'd like to add the ability to include percussion in presets, so I started to study the percussion wiring on the schematic. I'm sharing my findings here in case they are of any use to anyone.

Unlike other Hammond models such as the B-3 or M-3, the M-100 series does not have the so-called "percussion volume drop" that applies to all the harmonics. It does have volume drop for the selected harmonic (which the other models also have, in addition to the overall volume drop). This is achieved by disconnecting the selected harmonic's busbar from the corresponding drawbar, and routing a tap from an intermediate stage of the percussion preamp, through a 22 Ohm resistor, back to the drawbar instead.

The M-100 series also lets you select both harmonics (2nd and 3rd) at the same time. An interesting side effect of the way this is wired is that when you do this, the 2nd and 3rd harmonic busbars are shorted together, the mixed 2nd/3rd signal goes into the percussion circuit, and the tap provides the same volume-reduced signal back to both the 2nd and 3rd harmonic drawbars. So instead of these drawbars controlling those harmonics separately, you have two drawbars that each control the volume of both harmonics. For example, it means that the drawbar settings 00 8000 000 and 00 0800 00 sound exactly the same when 2nd and 3rd percussion are selected. Odd that they did it that way.

[enor]

Hm. There should be no noticeable volume drop for the selected harmonic after "returning to the drawbar" from the perc amp! Maybe something is wrong with your amp?

- - - Updated - - -

Oh, and regarding the percussion oddity with the two buses shorted together:

I agree it is kind of a strange decision to manufacture them this way, but it's the only way to do it without redesigning the amp - which I take it they weren't keen on. Multiple percussion voices at the time, though, must have been a feature people asked for so the decision to implement it in this somewhat half-assed way might have caused a bit of a debate in the design department... oh, the things I wish I could have been around to hear! ;)

[stefanv]

Hm. There should be no noticeable volume drop for the selected harmonic after "returning to the drawbar" from the perc amp! Maybe something is wrong with your amp?

You're absolutely right. I had falsely assumed this caused a volume drop without even trying it. There is no perceptible volume difference in the steady-state tone with or without corresponding the percussion enabled. Actually, I discovered on my organ this is true only for the 3rd harmonic; the 2nd has no steady state tone at all when 2nd percussion is turned on. Must be a dirty switch contact.

This makes me wonder though why they used this complicated switching arrangement for the percussion. If you're not going to mess with the corresponding steady-state tone, why not just tap it off of the bus-to-drawbar signal, instead of interrupting and rerouting it?