# Coupling Audio signal to speaker

I'm trying to couple an audio signal from an given Circuit and Output it to a speaker. I have no experience with analog circuits and the circuit I already have is the result of searching the web.

I tried both circuits and I can see on an oscilloscope that there is analog output with both circuits. But as soon as I connect the 8Ω resistor there is no more signal and I can't hear anything.

Maybe someone can explain what I`m doing wrong and it would be very kind if someone explains why it is wrong.

EDIT

Thank you all for your help, all of your answers got me closer to a solution. I changed C4 to a higher value and got rid of the potentiometer and replaced it with a 20R resistor. Also i took a different speaker (from a phone) with 46R. Now I can hear the sound that is carried over the circuit (it is really quiet but it is ok since the speaker is just for testing purposes, it will be connected to the mic-in of a SoC later)

If someone has more information for me I'm thankful for any kind of help :)

• Where is the 8R resistor? Mar 13, 2017 at 9:21
• How many amps are you trying to put through the dinky little potentiometer that you are likely using? Mar 13, 2017 at 9:34
• I need an extra resistor? I thought the Speaker has the 8R... The Potentiometer was supposed to be a voltage Regulator Mar 13, 2017 at 9:45

You apparently want to "hear" a voltage signal someplace, and want to be able to adjust the volume for some reason. Your implementation has some problems:

1. Putting a capacitor in series with the speaker is a good idea. However, you used a polarized capacitor. That by itself isn't wrong, but it does mean you have to be careful with the polarity when you connect this circuit to the signal you want to listen to. Since your capacitor is only 1 µF, there is no need for it to be electrolytic, and therefore polarized. 1 µF ceramic caps are readily available.

2. 1 µF in series with a 8 Ω speaker is way too little. 1 µF and 8 Ω have a rolloff frequency of 20 kHz. Everything below that will get attenuated inversely proportional to frequency. You're a factor of 100 down at 200 Hz, for example. You'll mostly hear high frequency components of the signal.

A few 100 µF would be a better match for the 8 Ω speaker. However, then you're back to using a electrolytic capacitor, so watch the polarity.

3. A 22 kΩ potentiometer as a volume adjustment for a 8 Ω speaker is a gross mismatch. It will essentially act like a volume switch, not a fader. Consider the case of the pot only being set 1% down from full volume. The top part of the resistor would be 220 Ω. That will attenuate by a factor of about 30 just by being in series with the speaker.

So what to do? Go buy a "powered speaker". A cheap set of "PC speakers" will do fine. These come with a amplifier and volume control built in, and usually have a isolated wall wart supply so that you can float the speakers to whatever signal reference exists in the circuit you want to listen to the signal in.

R2 is a potentiometer and it forms a potential divider when it is not loaded down by the 8 ohm speaker .If R2 was say 20 ohms then it would function more like a potentiometer .C3 is not needed and can go .C4 needs to be much larger to pass the full audio .Try 1000 microfarad for the cap .

• Ok I'l try a bigger cap thanks, but what do you mean by "R2 is a potentiometer..." and "If R2 was... like a potentiometer"? Mar 13, 2017 at 9:37

22 kOhm is awfully much when compared with 8 Ohm, that loads the voltage nearly unnoticeable. Less than 1% is present when the pot is at the halfway. 22 Ohm or 50 Ohm should work better, if there's no other problems.

If there's nothing when the pot is turned to max volume, then your audio output is not made for loads as low as 8 Ohm.

The 1 uF capacitor shaves off all bass. Have at least 500 uF for 8 Ohm. There's no need to have any capasitors, if your output is a proper audio output with no DC.