MP3 Headphone Amplifier Circuit

I want to make a headphone amplifier using transistors instead of Op-Amps.

The specification of the amplifier is that it should:
* Provide an improved sound quality over the un-amplified output of your MP3 player.
* Have an input impedance of at least 100 kΩ.
* Have an output impedance of less than 100 Ω.
* Have a frequency response of at least 20 Hz – 20 kHz (this means that the lower limit should be 20 kHz).
* 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 voltage gain do you want your amplifier to have? For a voltage gain of 10 a single gain stage can be used followed by the output stage. A driver stage may be needed for the output stage giving a total of three stages.

The amp can be differential and DC or AC coupled or it can be single ended and AC coupled. Global feedback can be used or just local feedback. It depends on how much distortion you want; global feedback for very low distortion or local feedback and single-ended for that even harmonic sound that some people find to be "musical."

To meet the >100k ohm input impedance the first stage should use a JFET or a MOSFET. For the output stage I would use a Bipolar transistor configured as an emitter follower.

I can post a schematic minus the tone control. I never use tone controls in my audio systems but we should be able to find a circuit that can be added.

Attached is a schematic for a headphone amp. I designed it for a nominal 7.5 volts so it would work well from 6 to 9 volts which is the range of a PP3 battery. The spec it does not meet is the 100k ohm input impedance. It is 800 ohms and is high enough for an MP3 player to drive. To achieve 100k ohms the input stage must be a FET. The output stage is a push-pull for efficiency; to obtain lower battery drain.

Specs

Input R = 800 ohms
Output R = 30 ohms
Frequency response = 18 Hz to 10 MHz at -3 dB
Voltage gain into 30 ohms = 4
Voltage gain into 300 ohms = 7
Output voltage into 300 ohms = 1.4 V RMS
THD into 300 ohms and 1.4 V RMS = approximately 3%
Power = PP3 battery
Quiescent current = 11 mA
Improved sound quality? As to improved sound quality, unless one defines improved as higher distortion an amplifier cannot improve the sound quality of the original audio source. It can make it louder though.

I will build this circuit and test what the results are like. In order to attain at least 100k ohms input impedance which of the bipolar junction transistors would I need to replace with an FET?
Is this amplifier single ended and AC coupled?

Thanks for your help David

A BSS123 N-Channel MOSFET can be used as shown in the attached schematic. This amp is singled-ended and AC coupled.

Attached is a schematic for a headphone amp. I designed it for a nominal 7.5 volts so it would work well from 6 to 9 volts which is the range of a PP3 battery. The spec it does not meet is the 100k ohm input impedance. It is 800 ohms and is high enough for an MP3 player to drive. To achieve 100k ohms the input stage must be a FET. The output stage is a push-pull for efficiency; to obtain lower battery drain. Specs Input R = 800 ohms Output R = 30 ohms Frequency response = 18 Hz to 10 MHz at -3 dB Voltage gain into 30 ohms = 4 Voltage gain into 300 ohms = 7 Output voltage into 300 ohms = 1.4 V RMS THD into 300 ohms and 1.4 V RMS = approximately 3% Power = PP3 battery Quiescent current = 11 mA Improved sound quality? As to improved sound quality, unless one defines improved as higher distortion an amplifier cannot improve the sound quality of the original audio source. It can make it louder though.Hi, this is something you can get help with in the official

Attached is a schematic for a headphone amp. I designed it for a nominal 7.5 volts so it would work well from 6 to 9 volts which is the range of a PP3 battery. The spec it does not meet is the 100k ohm input impedance. It is 800 ohms and is high enough for an MP3 player to drive. To achieve 100k ohms the input stage must be a FET. The output stage is a push-pull for efficiency; to obtain lower battery drain. Specs Input R = 800 ohms Output R = 30 ohms Frequency response = 18 Hz to 10 MHz at -3 dB Voltage gain into 30 ohms = 4 Voltage gain into 300 ohms = 7 Output voltage into 300 ohms = 1.4 V RMS THD into 300 ohms and 1.4 V RMS = approximately 3% Power = PP3 battery Quiescent current = 11 mA Improved sound quality? As to improved sound quality, unless one defines improved as higher distortion an amplifier cannot improve the sound quality of the original audio source. It can make it louder though.Hi, this is something you can get help with in the officialAny update for this comment ?

This link appears to lead to a feedback site and I would suggest that the link is spammed.

Good catch -- the link was to a feedback form relating to Kroger -- I marked the comment as spam and suspended the user -- I have no idea what these folks hope to achieve by posting this sort of dross -- give me strength!