To the left, I have the DAC0 and GND from the Arduino. To the right, I have the output to an 8ohm speaker. I've lost the source to this image. I don't think the page even explained in details how the circuit works. But I think I have a clue. I guess the BC547 gets a voltage of 5 V, when no audio output, because of the 100k and 120k resistors. And the 5 V would be important if the logic HIGH level is 5 V? With 3.3 V, I should change those two resistors to others to get 3.3 V, right? Is the ratio between the 12k and the 10k resistors similarly important?
I don't have 100n caps, but I have 470n caps. How would that harm the circuit?
Where would a potentiometer go to control the volume?
I have no particular reason to use 9V. I guess 6V could work as well. On the other hand, the Giga can use anything up to 24 V. Maybe the Giga could run on same 9V as the audio circuit. It's a tiny speaker, not meant to fill the room with black metal, just some robot speech.
Interestingly, a google image search turns up 100's of copies of that circuit diagram.
People have just used it to pad out their blogs. But anyway, here are just a few:
It will be fine. In fact the 470n is a better choice for the output cap, it gives a cut off around 40Hz, with 100n it would be around 200Hz.
From here:
The value of the pot isn't critial. One in the range 10k to 100k should work. Ideally it should be an 'A' type, with a logarithmic taper. If you only have a 'B' type, that won't do any harm, but it won't give a linear control of the volume.
LTSpice simulation suggests that you will be very disappointed with that circuit. The gain is reportedly about -40 dB. Let us know what it does in real life!
This simulation has 200 mV 1kHz input swing, output swing 1.5 mV (-42 dB).
There are LOTs of easy-to-use audio amplifier chips. Its an easier and more foolproof way to build an amplifier.
And all of them are class A/B (or class-D) which s better and more efficient than your class-A (1) transistor circuit.
LM386 is super popular for these "little amplifier" designs.
Decades ago I used the LM380 for several little amps.
Or you can buy amplifier boards if you don't want to build it yourself.
Most of these chips have more voltage gain than you need so you may want to add a voltage divider (2 more resistors) so your volume control isn't always near the minimum. (You do need the power & current gain to drive a speaker.)
(1) It's not class-A because it's better. it's class-A because it was first back in the tube days and it was cheaper and common for a long time because it uses one less tube (or transistor). With ICs and modern cheap semiconductors it virtually costs nothing for a better and more complex design.
The diagram in post#1 is a pre-amp with small-signal transostors.
Not suitable to drive an 8 Ohm load.
Nowadays we use class-D amplifiers, because of higher efficiency. like this one.
Leo..
