I was working on my little radio last night and trying to improve the output of the LM386. Two changes I made seem to have had a huge effect on the output volume. The first was to use a small audio transformer between the output (pin 5) and my speaker (not shown in schematic.) I understand why that helped. But, I also found a schematic online that shows a 1 nf capacitor going from pin 3 (input) to ground. Using that capacitor gave a large boost in output as well. Nearly all the other schematics I've seen online do not show this small change, but it sure seems important (at least in my circuit.) So, what does it do and why did it help so much? I understand bypass capacitors in a regular amp prevent degenerative feedback, but this doesn't seem to be the same thing. I've included the schematic.
Hello Stellar,
I have built many LM386 amplifiers and I can assure you that something is wrong in this case.
In a correctly operating LM386, that 1nF capacitor across the input can do nothing but attenuate the input signal at higher frequencies. The 6dB (half voltage) point drop in gain will be affected by the position of the volume control and the source impedance of your detector output. But with the potentometer near the center of its travel, compared to the signal voltage at 1kHz, the voltage into the amp will be half (-6dB) at 66 Khz. At other positions of the potentiometer the half voltage point will be higher, so any attenuation caused by the capacitor will be well above the audio band- as it should be.
It sounds very much like the amp is oscillating, which would not be surprising as there is no high frequency decoupling on the supply line.
To see if this is correct place a 100nf or bigger ceramic capacitor directly across pins 6 and 4 of the LM386. If that does not work add a 10uF upwards aluminum electrolytic capacitor across the same pins in parallel with the 100nF capacitor (positive connection to pin 6). If that does not work you may need to look at the physical layout of the circuit.
The 1nF capacitor is there either as a botch to stop the amp oscillating or more likely to prevent radio signals from entering the amplifier. What can happen is that when AM signals get into the input of an amplifier the protection and parasitic diodes, fabricated into the chip, rectify the AM (detect it) and you get the radio station audio at the output. To do a proper job of the input filtering you should put a 1K resistor between the input potentiometer wiper and the capacitor.
You say that you have connected a 10uF capacitor between the detector and the amplifier. That will almost certainly be an aluminum electrolytic capacitor and will probably be insufficiently biased. There is already a 1uF isolating capacitor in series with the input so just remove the 10uf capacitor and replace it with a link (the existing 1 uF capacitor should be a non electrolytic, solid type).
The other thing you should do is to put a 470uF or larger capacitor across the battery. Can you say what type of battery it is because if it is small or badly discharged that will also cause problems due to the high source impedance. Can you measure the actual battery voltage with a meter while the amp is producing sound?
A final area of concern is the detector circuit on your radio: are you sure you are taking the signal to the amp from across the integrating capacitor in the detector. The other thing is that the detector source impedance may be sufficiently high that the input impedance of the amp is swamping it.
Moving on to the transformer that you have added to increase the volume. I am intrigued by this. Can you give some more details; the impedance of the loudspeaker would be helpful.