Stereo to mono, signal mixer works on Orcad not on breadboard

Thread Starter

Atsea

Joined Jun 23, 2017
5
Hi

I'm building an small Bluetooth speaker. I got an stereo Bluetooth amplifier and since I have just one speaker I'm trying to built a mixer as in the picture attached. I simulated the inputs with AC random signal with VDC offsets etc to see how the circuit reacts. It actually works in Orcad but not when I built it. It actually through a very faint signal to the speaker that can't be barely hear.
R+ goes into R1
R- Ground
L+ R2
L- Ground
At the speaker
Vo (purple probe) into the positive terminal. Negative terminal to ground. If I plug R+ or L+ into the speaker's positive terminal I get sound but nothing when I get Vo which is supposed to be Vo=-(V1+V2)

I have two 15V power supply so I connected +VCC positive to pin 7 negative to ground and -VCC negative to pin 4 positive to ground.

Any help will be very welcome. Thank you in advance for your support.

Untitled.jpg
 

Attachments

Sensacell

Joined Jun 19, 2012
2,893
Your speaker is probably 4 or 8 ohms, a 741 is not a power amp, it cannot drive a speaker directly.

Try adding an 8 ohm load to your simulation- see what happens.
 

crutschow

Joined Mar 14, 2008
27,764
Your speaker is probably 4 or 8 ohms, a 741 is not a power amp, it cannot drive a speaker directly.

Try adding an 8 ohm load to your simulation- see what happens.
He says he has a stereo amp, which I assume is what he is connecting to the mixer output.
 

Thread Starter

Atsea

Joined Jun 23, 2017
5
Your speaker is probably 4 or 8 ohms, a 741 is not a power amp, it cannot drive a speaker directly.

Try adding an 8 ohm load to your simulation- see what happens.
Yeah, flat, the signal goes to 0v when adding a 8 Ohmns resistor. So then assuming that I have both signals added, I should make a power amplifier right after the op amp right?
 

Thread Starter

Atsea

Joined Jun 23, 2017
5
He says he has a stereo amp, which I assume is what he is connecting to the mixer output.
It is correct, the stereo amp is connected to the mixed. In that case how could I create a mixer that can handle a 4ohmns speaker?. Any idea that I can start looking into_

Thank you in advance for your help
 

Thread Starter

Atsea

Joined Jun 23, 2017
5
If you already have an amp the can drive the speakers why do you want the mixer to drive 4 ohms? :confused:
The reason is because the Bluetooth amplifier is stereo and I need mono that is why I wanted the mixer so I could combine both channels into one. If not, since I just have one speaker, I would have to feed either R or L channel into the speaker. Is there any other way that with passive components I can mix both signals?.
 

crutschow

Joined Mar 14, 2008
27,764
Just mix the two stereo signals together into one channel of the amp and use that to drive your speaker.
You can do that with a 5kΩ or so resistor at the output of each stereo signal (2 resistors total) and combine the two resistors into one of the stereo amp inputs.
upload_2017-7-7_23-42-37.png
 

Thread Starter

Atsea

Joined Jun 23, 2017
5
Just mix the two stereo signals together into one channel of the amp and use that to drive your speaker.
You can do that with a 5kΩ or so resistor at the output of each stereo signal (2 resistors total) and combine the two resistors into one of the stereo amp inputs.
View attachment 130578
Thanks crutschow. I tried that and worked but volume dropped to half. I'm thinking in having if right after the mixer a LM386 amplifier or use a transistor as amplifier. What do you think a bout that?. I will do some simulations and see what happens. Thank you again for you help.
 
Top