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Chip Amps Amplifiers based on integrated circuits

Lost on LM386
Lost on LM386
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Old 26th May 2018, 01:51 PM   #21
Mooly is offline Mooly  United Kingdom
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Lost on LM386
Do you mean R2 in the diagram ? If so then that is part of a 'Zobel' network and required to keep the amp stable. Most amps have such a network.

Headphones really need a series resistor of perhaps 47 or 100 ohm (for a low power amp like the LM386). Whether you can do that depends on the socket. If you use the socket to disconnect the speaker automatically then you might be able.
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Old 26th May 2018, 02:23 PM   #22
Pontiac51 is offline Pontiac51  Austria
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This resistor is not in the diagramm, because I first built without headphone out.

The Headphone socket is switched and the switched tip is connect over to the speaker out. Non switched tip and ring on the headphone socket are shorted to get stereo signal. And the headphone plug negative has a 120R and this is connected with its own cable to the negative leg of the 470uF.

So far, so good?
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Old 26th May 2018, 07:42 PM   #23
Mooly is offline Mooly  United Kingdom
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Fair enough

Changing the series ground resistor to the headphones should be dramatically altering their overall loudness. The higher the resistor the quieter they get. I don't know if that happening or not.

The LM386 should easily drive headphones to any possible level needed and so if its distorting past a certain volume setting then that suggests the chip has reached its limits of voltage swing on the output... but that should be way over what any low impedance (are they low impedance ?) headphone should require.
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Old 26th May 2018, 10:00 PM   #24
Pontiac51 is offline Pontiac51  Austria
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I'm using headphones with 24 and 38 Ohms.
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Old 27th May 2018, 06:46 AM   #25
Mooly is offline Mooly  United Kingdom
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Those are low impedance and so changing your resistor from 10 ohm to 1k should reduce the sound level very dramatically. If it doesn't, then you haven't got it wired up in the way that perhaps you think it is.
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Old 27th May 2018, 08:16 AM   #26
Pontiac51 is offline Pontiac51  Austria
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So, this is a tradeoff then? I have to find the right value for the headphones not to crackle and not to loose too much volume.

Or can I improve this by going from 9V to 12V?

IMHO this looks way more chaotic now, but HOORAY, it works. I'll trim the cables when moving it to a new housing.
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Old 27th May 2018, 08:48 AM   #27
Mooly is offline Mooly  United Kingdom
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Without further tests it is all guesswork.

Ideally you should monitor the output on an oscilloscope to see just how much voltage swing is present at the output. In other words, is the amp clipping. That said, an running off 9 volts should absolutely be able to blow your ears off with headphones.

Going to a higher supply voltage (within the limits of the chip) is usually a good option.

Another thought is that you are feeding the amp very low bass signals that would cause distortion anyway in a headphone driver. As an experiment you could try adding a small cap in series with the input as shown here. This would create a high pass filter limiting extreme bass (that probably isn't audible anyway via headphones or small speakers)

Try a 0.1uF (assuming the pot is 10k) and see if that helps. If it is to bass light then increase the cap to a 0.22uF and so on. A 0.1uF will give a -3db point of 160Hz, a 0.22uF would give -3db at 72Hz.

The cap must be added to the input side of the pot in order to maintain the DC path from pin 3 to ground. Worth trying.
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Old 27th May 2018, 03:28 PM   #28
Fast Eddie D is offline Fast Eddie D  United States
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Many headphone amplifiers have been built with the 386. Musicians love them because you can (or could) buy everything at Radio Shack and stick wire them on perfboard.

On 9 volts, it can provide more than enough power to blow your eardrums with low impedance cans. It's also noisy, high distortion, not very precise. But it's cheap and easy.

Raising supply voltage will increase linearity a little but shouldn't be necessary. Check the max rating on your chip (some are 12 volts and some 18 volts).

The datasheet is always worth a look. http://www.ti.com/lit/ds/symlink/lm386.pdf Figure 10 is fine and dandy for headphone amp, with a resistor in series with the output. Try values from 10-50 ohms. You can even use an array of resistors and a selector switch. Find the values that work best with the headphones you are going to use. You can have a different resistor for every headphone in the house. Old school 600 ohm cans might need 50 to 120 ohms. You can also put a capacitor on the input. Try 4.7 uF for f3 = about 3 Hz.
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Old 27th May 2018, 03:35 PM   #29
Fast Eddie D is offline Fast Eddie D  United States
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I see what Mooly's talking about. You can put the dominant pole on the input for lower distortion. 0.47 uF gives 34 Hz.

Once you calculate your pole for one capacitor value, then you can scale it up and down and avoid calculating the pole for every value.

Then you have to look at your output pole. Make it lower than the input pole by at least a factor of 2.

If you sort it out like that, then you will have made that turd as shiny as it can get. Careful consideration of values will make a difference!
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Old 27th May 2018, 04:57 PM   #30
Pontiac51 is offline Pontiac51  Austria
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That did not work. I have added a 100nF cap to the input as marked by Mooly. Then I tried a 100nF and a 47nF in paralell (thats 147nF, right?). No change. Then I tried a 1uF, also no change.

Finally I tried what Fast Eddie suggested. A 100nf on the IN and a 47nF on the OUT of the volume poti. No change. Then a 2.2uF on IN and a 1uf on OUT. Nothing.

@Fast Eddie: Where is F3 in Figure 10? I can see only two caps?
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