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Topic: What is this amplifier doing, exactly? (Read 7 times) previous topic - next topic

Nick Gammon

I found a 220 uF cap lying around and that gives this:



I presume I put the + side on the Arduino side? That's the more positive side of it, right?




As for the 100 ohm between emitter and ground, that made it much softer:



Did you mean to replace the 1K with 100 ohm between base and ground?

oric_dan

The 220 uF looks much better than 0.33 uF. The quick decay is gone, and you'll
not have any dc-currents through the speaker.

I thought the 100R in the emitter would help improve the low-freq response, but
it also kills the gain too much. So you might go back to tying the emitter to gnd,
and use a 10K in series with the 220 uF cap on the base. I might also use a larger R
on the base to gnd, eg go back to the 100K.

I'm not sure why you're getting the overshoot on the leading edge. Do you have the
scope probe ground lead tied close to the same point as the speaker gnd, or right
at the emitter?

Nick Gammon

The Arduino Gnd pin, so there was a bit of a cable run. I moved it to next to the emitter, but no real change on the display.

By a coincidence I got these in the mail today from eBay:



2 Channels 3W PAM8403 Class D Audio Amplifier Board 5V

$US 2.84 each.

Wired one up to my iPhone and those old speakers. Quite nice sound out of it.

Nick Gammon


So you might go back to tying the emitter to gnd, and use a 10K in series with the 220 uF cap on the base.


Nah, that just killed the sound altogether (the 10K resistor).

MisterResistor

Your question on the 10 ohm resistor was good. It is exactly for current limiting. Remember the 6-8 ohm speaker is an impedance (Xl = 2*pi*f*l) usually measured at 1000 HZ. So with no ac signal, the dc value of that speaker is basically a short circuit.

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