Line-out replacement for piezo speaker

[Arild P.]

I have this electronic talking toy with a "ceramic speaker" (or
whatever it's called) and want to create a line out socket so I can
connect it to an audio system, but I'm having problems getting a good
sound.
The "speaker" looks similar to this picture I found by doing a google
search: http://www.mcld.co.uk/oddmu/tincanbass/piezotransducer.jpg

and is marked as follows: Murata VSB41D25-07AR

I've searched on the web for it, trying to get some specs, but to no
avail (I did find Murata.com, but that part is most likely outdated and
not available any longer since the toy is from the early 80's).
I've tried measuring the resistance across the "speaker", but nothing
comes up on my multimeter! Must have an infinite resistance or
something strange is going on ;-)

Then I disconnected the speaker from the toy's circuit board and
connected a jack socket in its place. The signal was way too loud, but
I got the right level by connecting 440K Ohms worth of resistance (two
220K Ohm resistors) along one of the signal paths. Still, the sound
isn't quite too my liking. I believe I need to filter out some bass. I
might possibly also need to raise the treble a bit, but I'm not sure
about this yet. But I definitely have to remove some lower frequencies.
I know that a filter involves resistors and capacitors, but other than
that I'm clueless and have spent a lot of time playing around with
different combinations, but not really getting anywhere, so what do I
do?
This is what I've done so far:

ORIGINAL CIRCUIT:

----------------->------
audio |
out O piezo-ceramic speaker

|
----------------->-------


NEW CIRCUIT:

----------------->-------------------------------------------------->

audio line
out out

----------------->---------------o---xxxx----o---xxxx-------->
220K 220K

 

[Roger]

I have this electronic talking toy with a "ceramic speaker" (or
whatever it's called) and want to create a line out socket so I can
connect it to an audio system, but I'm having problems getting a good
sound.

Your best bet would probably be to use an op-amp 'integrator' circuit.
Take a conventional inverting op amp circuit and place a capacitor in
paralell with the feedback resistor. the input resistor should be
connected to the speaker wire via a capacitor (use +/- supplies on the
op amp. Use 100K for the input resistor and vary the feedback resistor
to get the desired gain. The integrating capacitor should be such that
C * Rf is appox 0.0001

Once you have the gain about right by juggling the resistors (should be
1V p) you can juggle the capacitors. A smaller coupling capacitor gives
less bass. A bigger feedback capacitor gives less treble.

 

[[email protected]]

----------------->-------------------------------------------------->

audio line
out out

----------------->---------------o---xxxx----o---xxxx-------->
220K 220K

Problem with that, it relies on the impedance of the input of your amp
along with your series resistors to drop the voltage down. Plug it into
a different amp or other input, and the volume level may be different.

High side 220K 10uF
----------------->---------------o---xxxx----o o--||----->
| |
50K x |
audio Audio x<----| line
out Taper x out
|
----------------->-------------------------------o----------------->
Ground side


Try that before trying to alter the frequency response.

I'm having a hard time getting that to display correctly. Signal side,
to 220K resistor, to one side of a 50K audio taper potentiometer. Other
side of the potentiometer to ground/common. Center of potentiometer to
10uF capacitor, other side of 10uF cap to amplifier input.

Look up piezo speaker on Google for more info. DC resistance is
infinite, it acts a bit like a lossy capacitor.

 

[Arild P.]

Problem with that, it relies on the impedance of the input of your amp
along with your series resistors to drop the voltage down. Plug it into
a different amp or other input, and the volume level may be different.

OK, I've redone the circuitry, but one question regarding what you
said: will the output volume change only if I connect it to a different
input/amp, or will the volume sometimes change even when it's connected
to the same input?
The sole purpose for adding this output is for sampling (recording) the
toy's audio with my computer.
Anyway, I've followed your example and also played around with
capacitors to change the sound's bass/treble. This is what it looks
like now:


High side 220K 10nF
----------------->---------------o---xxxx----o o--||----->
| |
50K x |
audio Audio x<----| line
out Taper x out
|
----------------->-------------------------------o----------------->
Ground side

Note that I'm using a 10nF capacitor instead of 10uF as you suggested.
At least I think that's what it is, if I haven't misinterpreted my
digital multimeter. The capacitor is marked "103".

I tried out many different values, but found something between 10 to
15nF to give the sound I wanted.
Well, very close anyway. If I used say a 6nF capacitor (or lower) the
sound became very "thin", filtering away almost all the bass. If I used
a 40nF capacitor (or higher) the sound became too "bassy" and "harsh".
But if I want to do this really well I also want to filter out some of
the digital noise from the toy. If I use a higher value capator this
goes away, but that'll also change the overall sound giving more bass,
which I don't want.
Are there other ways I can create the filter? I would like to try
something like a 12 or 13nF capacitor, but I don't have any with those
values. Besides, based on my experimentation with all sorts of
capacitors I doubt it'll remove the digital noise without removing the
frequencies I want as well.
Is it possible to create a "narrow" filter of some sort in addition to
the above filter?