Transistor-Based Class A/AB MP3 Headphone Amplifier Design with Tone & Volume Control

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#1 21670478 24 Nov 2013 14:26

Ash Tatpati Ash Tatpati

Anonymous

Post #1
21670478 24 Nov 2013 14:26

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.
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#2 21670479 24 Nov 2013 17:44

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #2
21670479 24 Nov 2013 17:44

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.
#3 21670480 25 Nov 2013 10:25

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #3
21670480 25 Nov 2013 10:25

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.
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#4 21670481 25 Nov 2013 16:08

Ash Tatpati Ash Tatpati

Anonymous

Post #4
21670481 25 Nov 2013 16:08

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
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#5 21670482 26 Nov 2013 11:23

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #5
21670482 26 Nov 2013 11:23

A BSS123 N-Channel MOSFET can be used as shown in the attached schematic. This amp is singled-ended and AC coupled.
#6 21670483 02 Jul 2019 10:48

Tina Mildred Tina Mildred

Anonymous

Post #6
21670483 02 Jul 2019 10:48

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
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#7 21670484 04 Jul 2019 10:50

Tina Mildred Tina Mildred

Anonymous

Post #7
21670484 04 Jul 2019 10:50

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 ?
#8 21670485 04 Jul 2019 12:34

Conrad Mannering Conrad Mannering

Anonymous

Post #8
21670485 04 Jul 2019 12:34

This link appears to lead to a feedback site and I would suggest that the link is spammed.
#9 21670486 08 Jul 2019 20:33

Max Maxfield Max Maxfield

Anonymous

Post #9
21670486 08 Jul 2019 20:33

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!
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Topic summary

The discussion focuses on designing a transistor-based Class A or Class AB headphone amplifier with tone and volume control, powered by a PP3 battery, targeting improved sound quality over an unamplified MP3 player output. Key design specifications include an input impedance of at least 100 kΩ, output impedance below 100 Ω, frequency response covering 20 Hz to 20 kHz, and inclusion of volume and tone or cross-feed controls. A suggested approach involves a multi-stage amplifier with a JFET or MOSFET input stage to achieve the high input impedance, followed by bipolar transistor output stages configured as emitter followers or push-pull for efficiency and low distortion. A shared schematic operates at approximately 7.5 V (compatible with 6–9 V PP3 battery range), featuring an input impedance of 800 Ω (not meeting the 100 kΩ target), output impedance around 30 Ω, voltage gain between 4 and 7 depending on load, frequency response from 18 Hz to 10 MHz at -3 dB, and a quiescent current of 11 mA. The amplifier is single-ended and AC coupled. The BSS123 N-channel MOSFET is recommended for the input stage to meet the input impedance requirement. It is noted that while the amplifier can increase volume, it cannot inherently improve the original audio quality beyond amplification. Some responses included spam links unrelated to the technical discussion.
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Transistor-Based Class A/AB MP3 Headphone Amplifier Design with Tone & Volume Control

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