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lm386 smd earphone power

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rompelstilchen

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lm386 smd earphone power ?

Hello,

I want to amplify 2 guitar pickups with a lm386 so I could hear them in a regular (smartphone like) earphone

I dug a bit and it seems a single coil guitar pickup is around 50–100 mV output (doubled since I use humbuckers)

let's say, max 400 mV since I use 2 humbuckers

I have some smd 386 amp

I think they are .25W (cant find the info anywhere, but I read it a while ago)

are they powerfull enough ?

circuit I found (probably drawn straight from datasheet) : https://www.gadgetronicx.com/headphone-amplifier-circuit/

regards
 
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In the datasheet of the OPAmp TI recommend it as an AM radio amplifier with 8Ohm speaker. So I think it can drive a headphope, and output power depends on your supply voltage. With the 10uF capacitor between pin 1 and 8 the gain is 46dB, so 50mV can be gained to 10V, it should be enough.
 

ok....sheers, I thought it would be more complicated than that..I mean impedance, wattage depending on the load, and stuffs like that

I am not really good at analog electronics :)

I'll etch it and see how it performs
 

An electric guitar pickup needs a preamp with an input resistance at least 20 times higher than the input resistance of an LM386. The high frequencies will be cut if you use it.
an old vacuum tubes amplifier had an input resistance of 1 million ohms or more (the LM386 has 50 thousand ohms) and today a Jfet is used as a high input resistance preamp.

The LM386 output has enough power to destroy your hearing and destroy an earphone. An attenuator is needed.
 

Hello rompelstilchen,
Check the schematic below:

Guitar Jammer

The above is a Guitar Jammer designed by Silicon Chip magazine here in Australia.
I've made some modifications, but essentially its Silicon Chip's idea.
The project was designed to connect your guitar with your CD player or any other
low signal audio source (eg. PC Sound Card etc.), so you can jam with any of your
preferred music.
Each has its own separate volume control so you can get the equalisation between
the two inputs the way you like it.
Its good for 8 to 32 ohm headphones.
It has the option of using a 9 volt battery or a 9 volt plugpak. Its also reverse polarity
protected.
I hope the above might suit your needs.
Regards,
Relayer

All rights to Silicon Chip Magazine.
Taken from their October 2000 issue...
 

The Guitar jammer is missing the high input impedance of a vacuum tube or Jfet so it will muffle high frequencies. Oh, maybe old people or people who abused their hearing do not hear the high frequencies anyway, except old people like me with hearing aids that play high frequencies.
The guitar jammer has a speaker output level that is much too high to feed headphones without attenuation.
 

Hello Audioguru,

except old people like me

Same this way, though I'm not needing a hearing aid just yet... :razz:
It shouldn't be much longer though. Listening to AC/DC live in concert
at the local high school before they got famous. I used to stand in front
of one of their main speakers and just soaked it all in... :clap:

The guitar jammer has a speaker output level that is much too high to feed headphones without attenuation.

According to Silicon Chip, the output power is only 120mW into 8Ω, less into 32Ω. I doubt that would overdrive a set of headphones.
I have already built this many moons ago and it works extremely well. :thumbsup: Though at full volume, it can be a bit much on the
ears. But you wouldn't use it like that. You only want the guitar volume to be slightly higher than the music source, so you can hear your
own lead breaks, and not what the lead guitarist is doing.
Regards,
Relayer
 

I simplified the schematic for my needs

guitar jammer.jpg

- the smd package is 0.6w instead of 1.25w, I guess it wont fry the headphone
- the gain should be 90 (**broken link removed**)

questions :

- why a low pass filter (green) as input ? it seem to go up to 100kHz (graph : **broken link removed**)

- so I guess the RC filter (blue) is the impedance filter right ? I cant find any graph generator

- why has the 220µF (datasheet) been replaced by another RC filter at the end (orange) ?

- what is the point of the bypass capacitor (brown) and why this value differs from the 10µF (datasheets)

please enlight me ... :)
 

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1) The datasheet of the LM386 shows an output power of 700mW into 8 ohms when powered from 9V and the distortion is 10%. Way too much for a headphone that is extremely loud at 100mW.
2) The maximum allowed dissipation (amount of heating, not output power) is 1.25W for a through holes package or 600mW for a surface mount package.
3) The impedance of a magnetic guitar pickup is high then the preamp must have a very high input resistance and not have a 3nF capacitor to ground like in the input of the Guitar Jammer. The 50k input resistance of an LM386 is too low.
The output level of the pickup is about 0.4V then an amplifier gain of 90 times will cause severe clipping distortion because a volume control is not shown on the schematic (maybe it is on the guitar?).

Most electric guitar preamps have an input resistance of at least 1M to allow the pickup to resonate which produces a peak at about 5kHz. A preamp input resistance of only 50k like the LM386 amplifier is shown on a graph to remove the peak and cut frequencies above 3kHz.
Here is a typical guitar preamp circuit:
 

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