How to switch the polarity of an ordinary sustain pedal

[ITGITC]

Major points for trying though, Sue.

[gangsu]

Major points for trying though, Sue.

 

Sure. Thanks, Tom.

 

I think I finally get the drift of this whole discussion. "Normally Open" means that physically speaking those little black contacts aren't "in contact with each other" until you press down on the pedal. Which, come to think of it, is the way it should be. Why hang a circuit board from the ceiling? Anyway..It's not just a matter of changing the circuitry. I tried that. What this calls for is redesigning the entire pedal.

 

just so you know I hadn't given up, I replaced the Casio bubble with the lower profile one. Very carefully cut the little posts off and crazy glued it into place. Now it works perfectly again. It's not ALWAYS sustaining... only when the pedal is UP.

 

GN

 

PS If I'm still wrong about this, I don't want to know.

[MoodyBluesKeys]

You are NOT still wrong - your analysis is quite correct.

 

Reasons go all the way back to the earliest electronic keyboards (and guitar amps) that used pedals for control of any parameter. In the case of keyboards, sustain was the most common parameter. The manufacturer wanted to also cover the situation (most common) where the purchaser did NOT add a pedal. Larger manufacturers have cost analysis staff that go through every circuit looking for ways to obtain the functionality cheaper. Because of differing circuitry, for some companies, this resulted in using normally open pedals; and other companes used normally closed pedals. Over time, each company used one design, because of the cost of tooling a different design (and "we've always done it this way" played a part also).

 

In the early days of analog circuits, there was no such thing as the board being able to sense what was attached and automatically set its logic. After digital circuits were well-established, that became a possibility (but a lot of the larger companies did not bother because it would cost a bit more and "waditw".

 

Putting a SPDT (single pole, double throw) switch in the pedal and adding an additional switch to select "polarity" was done only when third party companies began to sell pedals - and the ability for a music store to stock one universal product outweighted the incremental increase in price.

 

On the other matter of "half-pedaling," there are a couple of circuits in common use for those instruments which support it. An acoustic piano is an analog device - there are theoretically infinite number of positions in which the sustain pedal can be pressed, resulting in an infinite possible separation of the dampters from the strings. Digital devices use the concepts of calculus (one can approximate any waveform with enough discrete digital steps).

 

The lower cost approach is to provide a pair of switches, one of which actuates when the pedal is pressed about half-way, the other actuates when the pedal is fully depressed. This results in a trinary state rather than binary - and the internal logic of the machine is designed for this. This is probably adequate for the present state of the art in sampled instruments, where one is unlikely to have multiple sets of samples at different points of "half-pedaling."

 

The other approach used some form of potentiometer or other linear voltage divider that does provide more than trinary control. The internal logic in those machines will have more than three states (but still not an infinite number of states).

 

It is believed that, even with a master pianist playing late Romantic music, less than 20 steps would be enough to surpass the present state of the art in reproduction.

 

Of course, the whole half-pedaling thing is only useful when one is performing music that uses such delicate nuances. If one is playing keyboard in a rock band, fuggetaboutit - there are only two states for the keyboardist - covered up by the drummer and guitarists, or with enough power and a patch designed to be LEAD, and over the top of the others. When playing baroque music, no need for half-pedaling there, same for country, gospel, pop, R&B, funk, etc.

 

[gangsu]

Thanks so much Jim. Hah, I left off the crazy face in my previous post. Figured my own ridiculous logic was already obvious.

 

Just some meandering thoughts in case you're curious how I was looking at this.

 

I figured "contact A + circuit B = NO momentary switch". Or, contact A + circuit C = NC momentary switch". That's the only reason I destroyed the Casio pedal. I was looking for the illusive circuit. They looked different, I figured they were different.

 

So, why didn't anybody tell me they're in fact "the exact same thing". The only difference is the Casio has it mounted on the floor, and the GEM has it mounted on the ceiling, where the upspring keeps it connected.

 

So, a SPDT circuit is something like a gear shift? You travel though both points to get where you're going? And where is that exactly?

 

I better stop. I need learjeff to pull my "sense switch".

 

- I do think you don't have to be Horowitz to appreciate half-damping.

[MoodyBluesKeys]

A home three-way light switch is an example of SPDT. SPDT = Single Pole, Double Throw. A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as 'on-off-on'.

 

I tried to find a clear picture and diagram on the web of the type switch used in a pedal, but could only come up with the generic toggle switch about 1/3 down the page on this link:

http://www.kpsec.freeuk.com/components/switch.htm

The SPDT switch has one contact that will change from the second contact to the third contact when the switch is operated. Kind of like having both a normally open and normally closed switch in the same housing, with one common terminal.

 

I completely agree on half-damping, but some genres of music would have much importance, others it would not be used.

 

[Joe Muscara]

But I'm not quite sure if it's still a switch or a potentiometer in the pedal that they are using.

On my RD, whatever it is, it seems pretty good. I've only tried with my foot, so I'm not sure if its truly continuous, or three discrete positions, but I can go between no sustain, full sustain, and partial sustain as I choose, and the notes ring out or don't like I suspect. Lifting off the pedal makes it sound like I've muted the strings, and I can "catch" the notes with it too.

I plugged the sustain pedal from my RD into the "control pedal" input (the one used for expression pedal on organ) on my Electro, and the sustain pedal behaved just like the expression pedal except for a shorter throw. Similarly, plugging my expression pedal into the "Damper" jack on the RD made the expression pedal behave like the sustain pedal does.

 

It appears that the sustain pedal is a continuous controller in this case.

[gangsu]

It appears that the sustain pedal is a continuous controller in this case.

 

Very cool! Point for Roland.

 

This whole topic is so interesting to me. Shaking my head...

 

Jim, thanks for the webpage! And for taking the time to stick it out here. I can't wait to have an M-Audio pedal (or similar switchable pedal) just to look inside.

 

I'm curious about MC's pedal, the roland that he was able to successfully rewire. Where was circuitry housed, and is there a real advantage to having it overhead? I can see that possibly a normally closed switch might be more reliable (a foot not being as even tempered as a spring).

 

 

[gangsu]

I completely agree on half-damping, but some genres of music would have much importance, others it would not be used.

 

You know where I miss half-damping the most? Onstage. I'll play piano with no reverb and that sudden cut-off kills me. Here at home it doesn't bug me so much, obviously because I don't have to kill all the effects in a small room.

 

Besides, I'm not a purist when it comes to music genres. A pedal can also be an extra finger whenever you need one.

[gangsu]

And to wrap this up, I'd like to share an interesting conversation I had last night in a music store with the resident pro.

 

Me: Have you got any sustain pedals?

 

- Sorry, no.

 

Me: Too bad. I just wrecked one trying to use it's circuit board to convert another brand.

 

- Some brands are the opposite polarity. All you had to do is swap the wires.

 

 

 

 

I'm very (very) grateful for this forum.

[gangsu]

And one LAST note.

 

It works! thanks to Throbert in particular for posting those pictures with the note that Fatar makes both types of piano style pedals ...knowing there's no way they'd waste money on a different chassis for each ....looked for the spot I could screw the original board onto, found it, soldered the wires back where I found them, blah blah blah...

 

Plugged it in et voila.

 

Hey, it may not last, in fact it probably won't, but I'm right now I'm there.

 

.

[MonksDream]

Well done, Sue!!

[gangsu]

Well thank you so much, MonksDream.

 

The only question remaining is, why the heck didn't Welcome Bill just give it to me straight? All the job required was a Philips screwdriver. (I suspect it's because he didn't know )