Remember Speak n Spell? Well, I have to add an audio amplifier to one to make it louder (and likely add a beefier speaker while I'm at it).
So I crack it open, and find the speaker has not two, but three terminals, and three wires going to it. Strange...
My best guess is that it's a separated push-pull arrangement, allowing a slightly simpler drive topology where both the push and the pull drive a positive voltage above ground i.e. two counter-wound coils sharing a terminal. Placing a scope on the three terminals suggests the same:
Each channel shares what appears to be the common terminal on the speaker.
Quite noisy (and zooming it shows lots of inductive peaks), but notice that each channel seems to be in opposition to the other, while neither goes below ground.
Capturing the two opposing terminals on a single channel appears (skipping the common terminal) continues the suggestion:
What I'd like to do, is remove the 3-terminal speaker, and drive an amplifier with the opposing outputs of the onboard audio amp, but I suspect that the speaker-as-inductor is itself an integral part of the amplifier. Removing the speaker from the circuit and looking at the three different signals, that is in fact what's happening. Each drive (w.r.t. the common) plateaus at about 10V, with widely spaced decaying dips to 0V, while a look at the two drives together as one channel shows a nearly flat trace, with the occasional bump.
This is the kind of thing that really re-enforces the fact that my analog skills are pants :( I'd rather not start hacking into this thing too much, lest I let the magic smoke out. Is there a simple solution I'm overlooking?
I suppose I could leave the original speaker in a void within the enclosure, tie into it for the amp inputs, then feed that to a new beefier speaker installed behind the grille... but that sure seems ghetto.