Toy solar system adding line out - Harder then expected

de:Beer · Joined: Jan 30, 2020 Posts: 3 Location: Eindhoven

Hey all

Last week I have been working on a new toy. Its a toy that provides information about our solar system and has quite a lot of nice sounds.
I added some bend points to it like: pitch shift, repeat, touch points and a go crazy noise switch to it.

Now I felt like adding a line out to it, which I thought would be very easy. Because it usually is.
But the when I replaced the speaker with a jack the volume was super low and noisy when amplified.

So after looking into the circuit it looks like its a simple class A one transistor amplifier. (Schematic below).

If I replace the speaker with a jack like I did in the first try there would be no current on the collector therefore there would be no amplification.

So now I think I should have a resistor from the collector to 3v3.
and then add a jack between the resistor and the collector. (schematic below)

But I just don't know what kind of value I should use for this resistor.
Would (3.3v / 2) / 0.005A = 330 ohm be the correct value?

I'm not good with calculating resistor values so please tell me if I'm all wrong or thinking in the right direction.

Also my suggested schematic is also a guess so I would also love to know if I'm all wrong or thinking in the right direction.

Thank you for reading, and I'm curious for you reply! Very Happy

Circuit bend solarsystem.png

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From left to right. First is how it is. Second is how I think I should add a jack out to it. I don't know what kind of value to use for r2.

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JovianPyx · Posted: Thu Jan 30, 2020 6:20 am Post subject:

That is a standard common emitter amplifier. As such, the collector resistor controls the gain. The larger the resistor's value, the more gain. I would try 4.7K to start with and see what happens - this will be a cut-and-try experiment because there is only cursory information about the circuit.

Also R1 seems quite low and could be shunting a lot of the input signal to ground. I'd work with R2 first, but if acceptable results are not achieved, try raising the value of R1, I'd start at 1K and see if it gets more amplitude (and still sounds correct).

PHOBoS · Posted: Thu Jan 30, 2020 12:29 pm Post subject:

I had some varying results with adding line-outs to toys and the like. Often it sounds better through the speaker which has mostly to do
with the limited bandwidth and response. A line out usually creates a much harsher sound. It can help to leave the speaker in place,
not sure if that is because of the inductance. Also replacing it with a low value resistor can sometimes improve the sound quality a bit.
At least it just has a single switched output, sometimes they have two outputs directly from a chip which can be a bit to tricky to interface with.

de:Beer · Joined: Jan 30, 2020 Posts: 3 Location: Eindhoven

Thank you JovianPyx and PHOBoS for the reply! Very Happy

I tryed a few resistors and it only produced a stable sound around 25 ohm.
Wich definitly a value I did not expect. But if it works it works.
So that is what I have put in place now and it sounds glorious, in a circuit bended way ofcourse.

Once everything is done I will do my best to post some photo's of the device and some sound files.

Thanks again for the help!

JovianPyx · Posted: Thu Jan 30, 2020 3:19 pm Post subject:

Once I saw PHOBoS's reply, I had thought the same, since it's a toy it might want a smaller resistance due to being designed to drive a speaker. Good that it works now.