MP3 headphone amplifier circuit

I would like to make a headphone amplifier using transistors (mostly bipolar junction and if required FET) instead of Op-Amps.

The specification of the amplifier is that it should:
Provide an improved sound quality over the un-amplified output of the MP3 player.

Have an input impedance of at least 100kΩ.

Have an output impedance of less than 100Ω.

Have a frequency response of at least 20Hz – 20kHz (this means that the lower limit
should be <20Hz and the upper limit >20kHz).

Have a volume control.

Operate from a PP3 battery

Have a tone control or cross-feed.

The amplifier is intended for small signal, AC, mid range frequency.
I would like to build either a class A or class AB type amplifier.

Any help with the circuit design would be greatly appreciated.

What will the new amplifier do?? How can another amplifier "improve the sound quality"??
If your MP3 player cannot play headphones clearly then there is something wrong with it and adding another amplifier will not fix it.

I would aim for an output impedance of <0.1 Ohms, not 100 Ohms!

Brian.

betwixt said:

I would aim for an output impedance of <0.1 Ohms, not 100 Ohms!

Brian.

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Many power amplifiers feed headphones through series 270 ohm resistors. Maybe headphones produce better bass when they are allowed to resonate a little.

Only because the speaker cones wobble a bit :grin:
I would prefer the higher damping effect of a low output impedance and play with the frequency response electronically.

Brian.

Speaking of wobbling, the woofer on the speaker system I have wobbles sideways VERY MUCH. I thought it is a 5" woofer but it is just a Drone Cone with no coil and no magnet. The real woofer is completely inside the vented enclosure and is only 3" or less.

Here is its ad. It is rated at 150 WHATS and the small power transformer inside is labelled 9VAC/1.1A (9.9 watts). So the real output is about 6W. It sounds pretty good.

Audioguru said:

What will the new amplifier do?? How can another amplifier "improve the sound quality"??
If your MP3 player cannot play headphones clearly then there is something wrong with it and adding another amplifier will not fix it.

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The specification for the project says 'improved sound quality', I think improved is defined as higher distortion as the amplifier can't improve sound quality of the original audio source it can only make it louder.

Audioguru said:

Many power amplifiers feed headphones through series 270 ohm resistors. Maybe headphones produce better bass when they are allowed to resonate a little.

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That may give a louder bass but not necessarily better. If it's resonating then wouldn't the bass tend to be somewhat one-note and boomy?

crutschow said:

That may give a louder bass but not necessarily better. If it's resonating then wouldn't the bass tend to be somewhat one-note and boomy?

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Big speakers have heavy cones and loose suspension causing "one note bass" and a boomy sound unless they are damped by the extremely low output impedance of a modern power amplifier. The tiny lightweight drivers with stiff suspension in headphones barely resonate so feeding them through a series resistor is commonly done.