Adding a line out to a Casio battery operated keyboard

[gooseEL34]

Hey all.

I have a friend who knows I am handy with a soldering iron.  He has a Casio PT-10 29-key keyboard and wants to add a line out, so he can send the output to a DAW/multitrack for recording or use headphones.
Item has a mono 2 1/2" speaker as its output.   

I figured I could just take a 3.5mm female jack and wire it to the terminals of the speaker (using a voltage divider if the signal is too big for earphones.  That would allow for use of the speaker AND headphones and if I wanted to kill the speaker, I could put a SPST make break to turn the speaker off.

I did the following and the volume is ok, but it has developed a clicking noise.  This unit had a feature which would allow you to record up to 100 notes in record mode and then play them back, one at a time, by pressing a button (called 1-play) in any rhythm you chose.

Any idea if adding the jack alone would have caused the click, or did I disturb something.  Also, maybe there is an issue with using 250 ohm Seinnheiser phones  

I am out of my element here as I normal work on tube amps and rarely vary from the schematics....

Am I screwing something up by adding a speaker load in parallel with the OEM speaker?   Should I use a switching jack to kill the main speaker when using headphones

Is there a better way to add a line out to one of these cheapo keyboards that have no out other than an internal speaker?   

Thanks for any help you can provide.

Tim

[AKADriver]

Under what conditions does it click? Does it click with each note or is it random? Does it click if there's nothing connected to the line-out jack? Does it click only through the headphones or through the speakers (or both)?

A random click in a piece of audio gear usually means a bad connection, like a bad solder joint or loose connector. This is the likely culprit also if it clicks at all times and not just with the headphones connected.

The high impedance of the headphones is actually a good thing. The internal speaker is probably something like 4 ohm or 8 ohm and adding a 250 ohm load in parallel won't change the total impedance much.

However if the Sennheisers have some active circuitry inside I could see this causing some undesirable non-linear effect. Just need some more info to categorize the problem.

A switching jack is usually what you want for this kind of application anyway.

[Ian.M]

If its a bridge tied load output stage, (not uncommon in low voltage battery powered kit) directly tapping the signal at the speaker is *NOT* a valid option.  Grounding one side of the speaker via a line out cable forces the rest of the circuit's 0v rail to dance like a yo-yo!

Check if the volume control carries the signal (rather than simply providing a control voltage to the chipset).  If it does, the signal at the top end of the volume pot, with an AC coupled low voltage rail-to-rail OPAMP buffer stage scabbed together on a scrap of perfboard may be the best option.

[gooseEL34]

Thanks for the help....

The volume "pot" is actually one of those sliding plastic pieces with tiny conductive metal "fingers" that slide along a straight track.   I will see what is inside.

The click is there whether you are playing a note or not.  This keyboard has a sampling function and the problem is worse with the record function.

As far as grounding the speaker, I do not think that is it.  The case is plastic, so I am not grounding through a chassis, I merely added a female jack in parallel with the speaker leads.   

I will try to post pics down the road

[AKADriver]

You are grounding the speaker if the line-out is connected to an external line-in and not to a set of headphones.