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How I can make a headphone amplifier


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Hi Jesus,
1) The pickup of a guitar needs a 1M load. The input of your preamp is only 10k.
The 741 opamp is old and has poor performance. If a much better TL072 dual audio opamp is used then one opamp of it could be an input buffer with a 1M input that drives the preamp circuit that you have.

2) Add a 0.1uF coupling capacitor between the output of the equalizer and the input of the LM386 amp. The 741 preamp has a DC voltage gain of 11 so its output could have an offset voltage of 66mV. Then the LM386 amp has a DC voltage gain of 20 so its output could be offset 1.32V which reduces the headroom before clipping without a coupling capacitor.

3) The LM386 needs a series RC zobel network at its ouput to prevent it from oscillating. It is shown on its datasheet.

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1. The 1M load needs to be in parallel or in series to the input?

If you connect a 1M resistor in parallel with the input then the guitar will have a load of slightly less than 10k. Its load should be 1M.
If you connect a 1M resistor in series with the 10k input of your preamp then the voltage divider will reduce the level to 1/100th.
Your preamp needs to have a buffer opamp in front that has a 1M input resistance to ground. Many guitar circuits use a jFET instead of an opamp buffer for some extra fuzz distortion that sounds like the distortion from a vacuum tube.

2. If there is not capacitor between the 741 and 386 what happens?
the speaker will sound like if i am applying voltage to the speaker?
The output coupling capacitor of the LM386 blocks DC. Its DC output voltage is supposed to be at half the supply voltage so it can swing up the same amount as it can swing down. If the 741 opamp has its max allowed amount of offset voltage then the 741 and the LM386 amplify it to up to 1.32VDC which causes clipping distortion if you don't turn down the volume. A coupling capacitor will block any DC offset voltage from the preamp.
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In the preamp section, 4.5V is derived from the potential divider of two 4k7 resistors. On the diagram, this is connected to ground. You must not connect that 4.5V point to the ground connectors of the other sections, because for those other sections ground is the negative supply rail. In other words, the ground symbol in your preamp schematic shouldn't be there.

The preamp in this design has an ouput offset of 4.5V (assuming a supply of 9V). That will play merry hell with the 386,  so you must couple with a capacitor from the preamp to the next stage.

The specs for your preamp should be: 1) Input impedance 1M, 2) Ground should be common to all sections. In my schematic I've shifted the volume control to after the op-amp, and I've decoupled the 4.5V mid-point with a capacitor in the feedback loop. This frees me to offset the input by +4.5V (so the amp can operate from a single supply) and create the 1M input impedance with only two resistors. Put this preamp as the first stage, then the equaliser.

post-20531-14279143227297_thumb.png

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Hi Cabwood,
The original preamp schematic shows a positive supply and a negative supply. So I think its output is at 0V plus or minus the amplified input offset voltage of the opamp.

You increased the input impedance but the tone controls section must be driven from a low impedance opamp output, not a volume control pot.

I think an opamp with a 1M input impedance should be added to the input and it feeds the volume control at the input of the preamp. Then the volume can be turned down to stop the preamp from clipping if the guitar is being played hard.

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Audioguru,

I considered the pot's effect, and decided (subjectively) that the equaliser wouldn't suffer much from the 5k output impedance of the volume control. You are right though, there would be attenuation, and shift of band freqencies. I haven't figured out how much.

I made an assumption - that the supply was a single 9V battery. The original design would be OK if supplied from dual rails. I assume that there is only a single supply because of the 0V centre point being derived from 2 resistors. The ground symbol is visible in all three circuits, and my guess is that Jesus's circuit doesn't work because he's connected them all together. That is sure to break the preamp because of the fake ground.

My design is to address the single supply issue, remove any ambiguity regarding ground points, and provide 1M input impedance, all with a single op-amp. Perfection, as you state, would require another op-amp to buffer the output from the volume pot.

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Hi Cabwood,
I don't have an electric geetar but all their preamps on the web have a 1M input impedance. This one is only 9.1k which is nearly a dead short to a high impedance pickup. I think that is why it doesn't work.

Maybe the resistors across the supplies are to hold down the unloaded voltage of a cheap power supply. Maybe the 0V from the supplies is already connected to ground.

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Hi Jesus,
A 741 is a pretty old opamp and is lousy for audio. I don't know if it works with only a single 9V supply.
I think it is important to have the volume control ahead of the fixed gain preamp so you can avoid overload.

Your schematic shows a positive supply and a negative supply for the preamp which is good. Remove the two 4.7k resistors and connect the commons of the supplies to ground.

Add to the input a non-inverting opamp with a gain of 1 and with a 1M input resistor to ground. It feeds the volume control of your preamp.

Add a 0.1uF coupling capacitor from the tone controls circuit to the input of the LM386 amp.

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