Here is the circuit I am working from with an LM741. Yeah, I know, and if you have a better suggestion please do.

I'm using a Technics SB-S25 8Ω 50W speaker and on the input an Amazon Echo/Alexa. Looking at the output @ D I get the following scope shot and sound from the speaker.

And when I plug the scope into point E I get the same thing. Until I plug the speaker into point E and then this.

The signal is completely attenuated and increasing the input to the Op Amp has no effect on the output with the speaker attached. So just what am I doing wrong here? Is the speaker impedence too high? Do I need to add another amplifier? I did change R2 on the Op Amp to 5.1kΩ to increase the gain but it had no effect.

 

That tells me the input impedance of the power amp is very low or there is a short.
What type input are you using on the Alexa?

 

What is in the "Power Amplifier" block?
You can't connect the output of your circuit directly to the speaker, if that's what you are doing.

It looks like it is oscillating.
Are there decoupling capacitors from the op amp power pins to ground?

 

... Hi ...
A look at the 741 datasheet PDF indicates that minimum 741 supply voltage is +/- 10 volts.
So maybe change that and observe any difference.
... It seems that +/- 15 volts supply would be the nominally recommended amount.

 

What type input are using on the Alexa?

I typically have it connected to a pair of Bose desk speakers from it's stereo out socket. I fabbed a stereo jack to a mono pigtail to use as the input for the Op Amp.

You can't connect the output of your circuit directly to the speaker, if that's what you are doing.

OK, and why not? It worked fine until I added the low pass filter to the circuit. Audio is new to me so I'm on shaky ground here. What should I use for an output amp, LM386? That was left out of the exercise instructions.

Are there decoupling capacitors from the op amp power pins to ground?

Yes 0.1uF on both supply rails and supplying @ +/- 15V.

 

Do the math. Signal goes through 5.1K resistor to 8 ohm speaker.

 

Scrap the LM741 opamp. TL071 will work with +5V and -5V supply voltages, and even lower.

 

You don't need the preamp. The phone output of the Alexa can drive a power amp directly .

 

I'm, confused
Is the output going to a speaker or a speaker amplifier.

It worked fine until I added the low pass filter to the circuit.

Then there appears to be a problem with that filter (what's its purpose?).
Double check the filter connections and the component values.

 

Actually, the filter is using a 15.1kΩ resistor as the 16k is a bit of an oddball. So yeah, there is a big resistor between the op amp and the speaker. K, then let me put a LM380 or 386 after the filter and try it. Thx.

TL071 will work with +5V and -5V supply voltages, and even lower.

K, I have some so I'll give them a shot. I am using +/-15V instead of the +/-5V. I already ran inot that earlier and stopped trying to do it at that low a supply voltage.

You don't need the preamp. The phone output of the Alexa can drive a power amp directly .

Well, that kinda defeats the entire basis for the exercise. I'm trying to learn a bit about audio.

 

You SAY the circuit is trying to drive an 8 ohm speaker but your schematic SHOWS it driving an audio amplifier that usually has a high input resistance.
You show a lowpass filter that cuts most audio frequencies (1kHz is at -3dB and higher frequencies are reduced to 1/10th the level each octave). Many of us can hear to 20kHz. Less than 1kHz are only vowels and boom boom sounds.

I agree that a lousy old 741 opamp should never be used anymore.

 

I'm, confused
Is the output going to a speaker or a speaker amplifier.
Then there appears to be a problem with that filter (what's its purpose?).
Double check the filter connections and the component values.

The speaker is connected to point D and works fine. As per the exercise, I added the filter and connected the speaker to point E on the circuit and when I did it completely attenuated the output signal. Without the speaker attached the scope shows the amplified output on point E until I attach the speaker then goes flatline. The only reason for the filter was to understand its effect on the output signal. This is a lab exercise from a book I'm working with. I have double-checked the circuit and components.

 

You SAY the circuit is trying to drive an 8 ohm speaker but your schematic SHOWS it driving an audio amplifier that usually has a high input resistance.
You show a lowpass filter that cuts most audio frequencies (1kHz is at -3dB and higher frequencies are reduced to 1/10th the level each octave). Many of us can hear to 20kHz. Less than 1kHz are only vowels and boom boom sounds.

I agree that a lousy old 741 opamp should never be used anymore.

I am not using the unspecified power amp shown. Only connecting to points D and E on the circuit. Yes the filter attenuates below ~1300hZ. I know how much you love the good ole antique 741 Chips suggested the TL071 but I am not using the +/-5V supply shown on the schematic. Already learned to use +/-15 on the 741s instead. I do have some NE5532P or OPA134PA if those would be better? They are certainly a lot quieter.

 

I'm trying to learn a bit about audio.

OK that's good. The low pass filter you show will only work with a very high input impedance amp. The LM386 as an example has an input impedance of 50K so you need to design the input filter with that in mine.
Or you could position the filter in front of the op amp used as a buffer.

 

Or you could position the filter in front of the op amp used as a buffer.

Yeah, that makes sense, really no need for it after unless the op amp was making lo freq noise... In fact, I'll try it both ways. Thanks for the suggestion.

 

LM741 will work with +15V and -15V.
You cannot drive an 8Ω loudspeaker with an opamp.
LM386 is a lousy power amp,
For the moment if you don't want quality, just get it working first. You can go ahead and use the LM741 and LM386. Then upgrade later.

 

You cannot drive an 8Ω loudspeaker with an opamp.

Actually you can even with the old 741 if you only need a few milliwatts, but as SamR stated it's just an exercise.

 

does that make sense Sam? You need lower impedance filter or a buffer, better yet make it an active filter. The 741 has cutoff built in so it shouldn't be necessary. I like the TL071 also but the 741 should work fine for your exercise.

 

Actually you can even with the old 741 if you only need a few milliwatts, but as SamR stated it's just an exercise.

Then I would have to crank up my hearing aids by about +80dB.

 

The TS stated it was working fine until he connected the filter. It must be putting out enough power to hear, but then again maybe his hearing is better then most.