# Help figuring out an op amp circuit with two loudspeaker outputs

I have to design a circuit that amplifies a sine wave with either a AD8616 or a LTC6256 (currently using the AD8616) to a 50 ohm loudspeaker and a rated power of 0.5 W. I'm a novice when it comes to analog electronics and I'm getting very confused trying to design a circuit for my needs. I'm not a native English speaker too, so please correct me when I use the wrong terms.

The circuit:

I am really trying to figure out if this design is correct and what it actually does. I got the gist of how non-inverting configurations work, but I'm not sure how the input sine signal is transmitted.

First there's the input voltage at 3.3 V that gets divided evenly, so at Pin 3 (non-inverting input) there should be 1.6 V. I read that through a feedback loop the voltage at both inputs (inverting/Pin 2 and non-inverting/Pin 3) should be the same (1.6 V). Therefore there should be a gain of 11 (with R3 and R4) at output 1 right? I am able to measure the 1.6 V with a multimeter, but at the loudspeaker outputs the voltage seems to drop with time (probably because of the capacitors?).

I'm slightly more confused at what's going on at pin 5-7 because I don't understand how the sine signal is transmitted. I don't know what voltage there is at the non-inverting Pin 5, is it the voltage from the sine signal? Shouldn't it also get 3.3 V like Pin 3? When I try to measure it with a multimeter, there's little to no voltage and I can't seem to power the loudspeaker. I think it's because the sine wave measures only about 150 mV.

The op amps:

Currently I'm using the AD8616 because of its high output current. The data sheet reads 150 mA at 5 V, so I'm thinking of switching the 3.3 V input to 5 V because of the loudspeakers high impedance of 50 ohms, so it can get more power.

I'm sorry if I'm all over the place, I'm stuck at what I should do or switch up to get the loudspeaker to work.

Edit:

simulate this circuit – Schematic created using CircuitLab

• The circuit: I am really trying to figure out if this design is correct Did you use a circuit simulator and check if it works? Also explain why you feel the need to "design your own" when ready-to-use ICs and modules exist that can drive a speaker (for example: LM386) while you (obviously) lack experience. Why this (difficult) 2-opamp design? Is it a bridge output? 2 speakers? The 100 nF output caps are quite small, I think you need 100 uF or more. Commented Feb 22, 2021 at 12:39
• I tried to build the circuit in LTSpice and it worked normally, not sure if I did a mistake there since it's not working as great on the breadboard (plugboard?). The need to design it on my own comes from it being part of the structure for my thesis, I'm trying to oscillate a small parallel-plate capacitor at a certain frequency. I more or less got a rough layout of what I have to do from my "supervisor" (not sure of the term) and was told to figure out the design for the op amp. I'm sorry, I meant one loudspeaker with two outputs! I agree, I don't have any experience so I'm struggling.
Commented Feb 22, 2021 at 14:27
• You are asking too much of the AD8616 in this application. At 100mA load current, you might get within about1v of each DC supply rail...with a DC supply of 3.3V, your speaker load is starved. The 150mA output current is likely short-circuit current, and not to be used as a valid operating current. It is likely to overheat at 5V DC supply, and you won't get much more power out than at 3.3V Commented Feb 22, 2021 at 15:30
• 50 ohm is a strange value for a speaker, are you sure that's right? Commented Feb 22, 2021 at 16:21
• Your speaker is similar to headphones: try searching for headphone amplifier amongst various manufacturers (Analog Devices, Texas Instruments for example). Watch out for package type: you want one easily bread-boarded. Commented Feb 22, 2021 at 17:05

If you redraw your schematic using the normal triangle op-amp symbols it will be easier to understand your circuit.

simulate this circuit – Schematic created using CircuitLab

Figure 1. Upper circuit redrawn.

The idea of a schematic diagram is to show the schema of the circuit. Yours is a very bad example as it it quite difficult to identify the various parts. In Figure 1 we can see clearly the power supply the voltage divider and the inverting op-amp.

.. at Pin 3 (non-inverting input) there should be 1.6 V.

Correct. Because you have a single supply we need to bias the audio signal mid-way between V+ and GND.

I read that through a feedback loop the voltage at both inputs (inverting/Pin 2 and non-inverting/Pin 3) should be the same (1.6 V). Therefore there should be a gain of 11 (with R3 and R4) at output 1 right?

No, for an inverting amplifier the gain is given by $$\ A = - \frac {R_f}{R_i} = -10 \$$.

I'm slightly more confused at what's going on at pin 5-7 ...

The second amplifier is a non-inverting amplifier. Its gain is given by $$\ A = 1 + \frac {R_f}{R_i} = 1 +\frac {90k}{10k} = 10 \$$.

The idea here is that since your supply voltage is so low we drive the speaker in "bridge" mode. While one output goes up, the other goes down and vice versa. Now you will get double the voltage on your speaker and four times the power (since $$\ P = \frac {V^2}R \$$.

I'm thinking of switching the 3.3 V input to 5 V because of the loudspeakers high impedance of 50 ohms, so it can get more power.

Good idea.

Fault finding

With no music signal the output of U1A and U1B should be the same voltage as the junction of R5-R6.

C3 and C4 are too small and you will get no bass. Try 100 μF or something around that size.

Help figuring out an op amp circuit with two loudspeaker inputs

The diagram shows two outputs to one loudspeaker.

From the comments: What do the capacitors do?

Figure 2. OP's redrawn circuit.

• R5 and R6 are the mid-supply reference voltage, discussed above. C5 holds that voltage steady by filtering out any noise on the 3.3 V supply.
• C1 is DC blocking. It allows the audio signal in but prevents R4 being grounded by whatever is driving the input. e.g. A guitar pickup is just a coil of wire and would provide a DC path which would affect the outputs.
• C2 does the same job for U1B and prevents its DC operating point (remember that you want the music signal to oscillate around 3.3/2 V) from being upset.
• C3 and C4 remove the 3.3/2 V DC from the output. These would normally be there for audio outputs to another stage. Since you are connecting your load across the two outputs you can omit those with very little chance of damage. When the input is silent those two outputs should be at the same 3.3/2 V and therefore no current will flow in your "speaker". This is done in car audio systems, for example, and results in a much better bass response as there are no capacitors (which would act as high-pass filters otherwise).
• Thank you so much for taking the time to help me. I created the full schematic and will insert that in the original post. It helped me understand the concept of the bridged amplifiers better. I read up on that and if I understand correctly, if for example the output of UA1 is -10V, the output of UB1 will be 10V. The output voltages are polar opposites because one opamp is inverting while the other is not. I think I got that! Technically this circuit should work. I have another question, what purpose do the capacitors have? I read that it's filtering noise from the circuit