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I used LM386 to amplify but its minimum 20x gain is to much for me use because my signal is maximum +-0.5v ac , riding 1V dc . Imgur

this is my circuit. I am seeing a signal on scope but I cant hear anything when I connect the speaker [I am measuring without it]. Moreover the scope image is the same as the input image ,no gain ! I can't see what I missed here

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    \$\begingroup\$ The 741 is (a very old design) op amp and is not an audio amp like the lm386 - it cannot drive enough current to operate an 8 ohm speaker. If the LM 386 gain is too much add a volume control (potentiometer) before the amp to reduce the input signal. \$\endgroup\$ – JIm Dearden Oct 5 '16 at 11:19
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    \$\begingroup\$ Just put an attenuator ahead of the LM386 input. \$\endgroup\$ – Brian Drummond Oct 5 '16 at 11:21
  • \$\begingroup\$ 2 questions . 1. what makes an audio amp -an audio amp[output resistance?] , as in why lm386 work and this one not 2. I am using an IC that has an output of SP+ and SP- . both of them are inputs to my lm386 [sp- to - of 386 , sp+ to + of 386] . How can I put volume control then? [ in the examples of lm386 it shows only when one input is used] \$\endgroup\$ – Maor Oct 5 '16 at 11:23
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    \$\begingroup\$ Can't see the circuit. Broken imgur link? \$\endgroup\$ – Rohat Kılıç Oct 5 '16 at 11:24
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    \$\begingroup\$ @Maor: Read the datasheets. It's the power handling ability of the output transistors. The 741 can only output a few mA. \$\endgroup\$ – Transistor Oct 5 '16 at 11:26
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You haven't asked a question. Am assuming that your question is: why is no audio heard from the 8-ohm speaker?
Although JimDearden has given a decent proposal, here you'll find why your's may have failed...
The LM741 spec sheet from Texas Instruments has a key spec: internal current limiters allow no more than 25 mA to flow in or out of the pin 6 output pin. This puts a severe limit on how much signal can be delivered to that very small 8-ohm load. If your input signal is riding on a 1v DC signal, then the 741 output will try to amplify this 1v to yield -5v at the output pin. Such a large output will invoke the internal current limiter to put a steady 0.2v DC across that 8-ohm load.
Even if you were to reduce that DC offset to zero volts, for any input signal voltages larger than +/- 40 mV, those same current limiters will kick in. The result will be rather low-amplitude output from the 741 - you might be able to hear it, but not easily except in a very quiet room. If you turn up the amplitude, the current limiters will clip, causing annoying distortion. Some op-amps can deliver more current, but they're generally not designed for this kind of service into such low-impedance loads.

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Several problems with the circuit. First off, LM741 has common mode range of up to +/- 3V less than power supply so in other words your usable voltage range is only +/- 2V. +/- 15V voltage range is recommended. You have +1V DC bias on the input so straight away the poor amplifier is trying to push -5V to the output, which it's incapable of reaching. Usable output voltage is also 3v less than rails so +/- 2V also.

So in order to get any kind of reasonable operation, add reasonably large capacitor in series with the audio signal. Reasonable in this context would be e.g. 47uF (plus towards audio source!) which will affect amplification by about 6% in the low frequency range if you use 1k/5k resistors. Increase those resistors by factor of 10 and this sinks to <1%.

You also need to add power stage because that opamp is incapable of driving the speaker. To do so, you need to change the circuit to non-inverting because you need to get a feedback from the output stage.

Here's a circuit that will work with a bit of modifications: http://www.deeptronic.com/small-audio-power-amplifier-using-op-amp-and-two-transistors/ What you need to change is obviously replacing +6V with +5V. VR1 and speaker are connected to actual GND and other ground points in that schematic go to -5V. You probably don't want that 100k / 5k voltage divider either, for 5x amplification you'd want 39k and 10k (4.9x).

Finally, you can drop the capacitors from speaker and feedback resistor.

As a parting shot, I'd either get +/- 12V power supply or if that's not practical, replace LM741 with for example TLV272 (dual) or TLV271 (single) which is a rail-to-rail output but can't do more than +/-8V (or 0 to 16V).

You could also hook the TLV272 to a 12 volt supply and use the circuit as-is. Single supply audio amplifier

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How to get a 5 times gain with an LM386.

enter image description here

C1 decouples the DC signal from the AC allowing only the AC through.

R1 and VR1 reduce the AC signal by about 1/4. (10k/(10k +27k))

VR1 can further reduce the signal if needed (volume control)

The LM386 is set up for a gain of 20 so the overall maximum gain between the input signal and the output will be 0.25 x 20 = 5

Using a source with bridged outputs.(re. your additional comment/question)

The + and - signs of a bridged output refer to the phase of the output signal. Both outputs operate around the same DC level (usually half the supply voltage) but are 180 degrees out of phase (when one goes up, the other other goes down). This enables the circuit to give the voltage maximum swing for any given single supply.

It is important that neither output is connected to ground as it would short circuit the output.

To pass on the signal you only need one of the outputs (either will do).

You will lose half of the amplitude by using only one output. If you need to compensate for this change the value of R1 to 10k, giving the LM386 amplifier a voltage gain of 10 rather than 5.

enter image description here

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  • \$\begingroup\$ I do have SP+ and SP- inputs (I think this is what you called bridged output ) to the AMP. as mentiond before I did used the lm386 but alwayes connected both SP+ to + and SP- to - becasue when grounding one of them I didnt get any signal (short).I dont understand why this happen and because of that I thoguht I cant use only one of the 386 inputs. havent thought about doing voltage divider to one of the entrance I will try it (and comment on the results if anyone interested ). a I want to say thank you to you and everybody in this post who spent their time to answer my question. \$\endgroup\$ – Maor Oct 6 '16 at 16:33

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