Creating line-level output from speaker output

You have probably connected your trimpot in series with the signal, that is the reason why it doesn't work as it should.

Use a 100 Ohm pot instead, connect the input signal between the ends, and the output signal between one end and the wiper. That is how you connect a volume control.

If you dont have exactly a 100 Ohm pot you can use values between 50 and 1000 Ohm.

I have this electronic talking/musical educational toy that only has a built-in speaker for audio output, but I'd like a line or headphone output for it as well.

So I did the logical thing: disconnected one of the wires to the speaker, connected it through a mini-jack's "switch", then connected the other output wire to the 3rd pin of the jack. In other words: when a jack plug is connected to the device, the speaker is cut off. If the jack plug is disconnected, audio is heard through the speaker.

So far so good, but the signal is way to loud for a headphone and line-levels, so I experimented with a trimpot to see which value I would need to get a decent level and ended up with approx 30K Ohms which I found as a regular resistor and connected this between one of the signal wires and the jack-plug.

OK, the line level has been taken care of, but the sound is crap!!! It sounds like almost all high-level audio signals are filtered out. The toy isn't exactly hi-fi I'm sure, but it's got to be better than this! So am I doing something wrong? If so, what should I do to get a decent line or headphone level output when the device only has a speaker?

Yes, I measured the speaker to 8 Ohms.

So the two resistors should (ideally) be 4 Ohms then, but since there are no 4 Ohm resistors easily available, could I use something more common like two 3.3 Ohm or 4.7 Ohm resistors? Of the two, it sounds like you're saying 4.7 Ohms would be better.

Your diagram helps, but I'm still a little confused. Are you saying that I should leave one of the existing output pins unused, while the other one is connected to R1 etc., creating one of the new line-level output pins, while ground (- of the battery connector) should be connected to the other line-level output pin?

I assumed that one of the speaker pins would already be connected directly to - of the batteries, but that's not the case.

Here's what I'm talking about as a picture explains things so much better (as for the speaker, I think it'll make things a whole lot easier if I disconnect it altogether for now).

o-----, existing | audio o .-. output | | R1 (4.7 Ohms) | | '-' | +------------+ | | .-. | | | R2 (4.7 | | | Ohms) +----o '-' New line-level output | GND o------------+-----------------o

like

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No.

Oh. I also assumed that. It's a little trickier, if the speaker isn't grounded. You might try connecting a resistor (4.7, 5.6, 8.2 or even higher) between the output terminals, and then use a capacitvely coupled voltage divider with somewhat larger resistors (an order of magnitude).

If that doesn't work, you'll have to use a transformer.

Oops. That should be: 8.2, 10, 12 ohms or higher.

It's doubtful that you'll ruin anything playing around like this. What you've done with the cap is to allow the resistance to appear smaller as the frequency goes up. The reactance of the cap is 1/(2*PI*F*C)...that reactance (in Ohms) is in parallel with the resistor. The effective resistance will be less as the frequency goes up so more amplitude at higher frequencies. You have given a slope to the frequency response.

This was not done originally because it costs! Cheap is the action word.

frequencies,

given

word.

As the frequency goes down, the caps impedance goes up. Looking at the Xc equation, you will see that there is an inverse relationship. Well, anyway, the cap finally appears to be an open circuit and that would leave only the resistor to modify the amplitude rather than the resistor in parallel with Xc.

You can get the reverse effect by placing the cap between the signal source and ground. This would be called a low pass filter. The low frequencies would be passed more so than high frequencies....(high frequencies would be shunted to ground)

I haven't quite managed to work it out, but there's this other talking toy *with* a headphone socket, only that it's way too loud for a line output (the computer used for sampling it has the input level way down and it's still too loud), so I figured out by opening up the toy that there was a resistor along the ground line of the headphone socket.

I can't remember the value, but it was pretty low. Under 1 K Ohm or something. I put a 30 K Ohm resistor in place of it and audio output is more in the acceptable level. I found out something very interesting regarding audio quality... The voice doesn't have much of an high-end. It's mostly low frequencies, but playing around I figured that by placing a capacitor alongside (in parallel with the resistor) I suddenly got a crispier, more high-end voice, just as if I was to turn up the treble!

Now, I don't know much about electronics theory, but like to build stuff and so on, so I don't know what's "right" or "wrong" here, but to me, the audio quality suddenly became a lot better and easier to comprehend, so can someone tell me why the producers of the (educational) toy didn't bother to add a capacitor like this in the design? Is there a downside to this that I haven't noticed yet? Could I end up damaging the device by modifying it like this?

Thanks for explaining! Does this mean that the more amplitude higher frequencies get, the more lower frequencies have their amplitude lowered? Or are the lower frequencies unaffected while the higher ones are just raised? (higher capacitance means more amplitude to higher frequencies I think is what I figured out).

What companies do to save a few cents.... And what little I have to spend in order to improve the toy! ;-)

That's a good explanation! Might come in handy some time I need to lower the high frequencies.

Wow! Not really knowing what I was doing and not making sense of the circuitry (not having a common grounding for the speaker output among other things) I experimented with a lot of things, among others finding an 8 Ohm resistor and placing it across the speaker output terminals

-and it helped!!! The sound actually became a little better. But I can see now that the sound source in itself wasn't very good, and you can't improve on what's bad to start with ;-)