Simple electronic instrument modification.

[HarryZhe]

Okay.
I'm no electrician, but that's why I'm posting.
I've been working on modding an electronic keyboard for some friends. Its a really simple little thing, basically a stylophone in a keyboard case, runs off of 4x AA batteries.
Like a child's toy keyboard, really.
Ive cracked it open for them and added a socket for a 6.5mm jack so they can amp it up or run it through an effects pedal. The problem I had when doing this, is, because the circuit is opened and closed with each key press, the amp crackles at the beginning of each note (this is resolved if one holds down the lowest key in the circuit and plays higher ones)
So I figured that, to fix this issue, I could add a "virtual" key at the very end, with a strong resistor, so a note is "always" playing, just too low frequency to hear.
Presuming this fixes the problem, a new issue arises. The thing conserves its battery energy by only having the circuit closed on key presses. Removing this feature would presumably mean more energy consumption.
So to fix this, I though it would be good to add another switch, and a socket, to toggle between a 240v adapter, and battery power, a "portable mode" and "performance mode." (yes, they do use it on stage)

Now the big question is, Would simply hooking up a 6v transformer for the thing work? I don't want to just go hacking away at it because I am not sure the significance of current in this situation.

Any advice on what kind of adapters to use would be great.

 

[dlgoff]

Sounds to me like you have a d.c. offset goning to your effects amp. All you really need to do is capacitor couple the input with a 0.1 mf cap. Put it in series with you connector mod.

 

[HarryZhe]

That seems really simple... Does your method mean i don't have to bridge the end of the key contacts with a strong resistor to close the circut?

 

[dlgoff]

I'm saying you just need to AC couple and do nothing else.

In analog circuits a coupling capacitor is used to connect two circuits such that only the AC signal from the first circuit can pass through to the next while DC is blocked.

http://en.wikipedia.org/wiki/Capacitive_coupling"

 

[HarryZhe]

Ok awesome, i actually had a skim over that article when you first mentioned it but wanted to make absolutely sure first. If i screwed their keyboard up i think i'd have to harikiri or something. Thanks :)

 

[HarryZhe]

On one of the external links from the wikipedia article, there's a diagram of capacitive coupling done with a reistor wired from pos to neg. Theres also a forumla which says that RC="time constant" which seems to be a factor in the output function. Is it necessary to have that resistor in there or would you recommend i simply wire the capacitor in series?

 

[NoTime]

No, you shouldn't need to worry about RC time constants here as this is just pass thru.
Just wire the cap in series and don't use polarized capacitors.

OTOH, I don't know if this will fix your problem since some oscillators do exhibit startup transients.
Try it and see, but you want to keep the cap even if it doesn't cure the complaint.

 

[HarryZhe]

OTOH, I don't know if this will fix your problem since some oscillators do exhibit startup transients.
Try it and see, but you want to keep the cap even if it doesn't cure the complaint.

If it didnt fix the problem, would it be very difficult to remedy it in another way?

 

[NoTime]

If it didnt fix the problem, would it be very difficult to remedy it in another way?

1) Use a real keyboard?
Just about any professional equipment should be able to replicate the sound with little effort.

2) Something like your original idea may be easiest.
I might try something like a diode or hard wire to keep the oscillator running.
Unfortunately, the best way to do that depends on the circuitry.
I doubt it would drain the battery in the length of a few sets.

3) Put in a delay switch to enable the output after the crackle. A CMOS IC and another cap and resistor could do the trick here.

 

[HarryZhe]

Well I think the actual look of the keyboard is important to their performance, they kinda have a strong visual prescence on stage... hard to explain. But yeah ill see how it goes.