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Old 26th January 2007, 09:00 PM   #1
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Default Portable headphone amp.

I listen to my mobile's FM radio and MP3-type player while working and walking.

The volume is not loud enough for standard cheap ear-bud 'phones. (90dB(ish) 32ohm)

I need a simple amplifier running off 2 x AAA rechargable cells which I can package in a "matchbox".

Is it better to look for an efficient voltage switching converter and use (say) a 5V amplifier, or are there good IC amps running off 2.4V directly?

Where to look for parts? I am woefully out o date - +/- 15V opamps I can do ........

TIA
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Old 26th January 2007, 09:19 PM   #2
lineup is offline lineup  Sweden
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Default Re: Portable headphone amp.

Quote:
Originally posted by cliff
standard cheap ear-bud 'phones. (90dB(ish) 32ohm)

I need a simple amplifier running off 2 x AAA rechargable cells which I can package in a "matchbox".
.

You could try this one, for 2 batteries = 3 Volt:

Quote:
Portable Headphone Amplifier
3V Battery powered
High Performance unit


Technical data:
Current drain:
35mA per channel with 32 Ohm impedance headphones.
Much less with higher impedance loads

Output voltage: Above 2V peak-to-peak on all loads
Sensitivity: 90mV RMS input for 2V peak-to-peak output
Frequency response: Flat from 30Hz to 20KHz

It uses only 3 small transistors:
Q1____________BC239C 25V 100mA NPN High-gain Low-noise Transistor
Q2____________BC337 45V 800mA NPN Transistor
Q3____________BC327 45V 800mA PNP Transistor


I would replace Q1 with another input transistor.
Like BC550C, for example.
BC337 and BC327 are very good transistors for this job.


More description of this project at this link
of this Little Headphone portable amplifier,
that also is a good 'first diy audio' thing to build for anyone:

http://www.redcircuits.com/
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Old 26th January 2007, 09:39 PM   #3
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That is very helpful and I could cope with that!

My circuit analysis skills hybernated! Some questions:

1) Is the voltage gain R3/R2 or R3/R1 or (as I suspect) something else!

2) I presume C2 just bootstraps Q1's Vcc

3) To add a volume control I could just add a 1K pot between C1/R1 and "ground", since the source will be low impedance. C1 may then have to increase - or be removed since this will totally float from the source.

4) Where to find PCB mountable AAA cell clips?


Thank you so much for your quick response.
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Old 27th January 2007, 03:19 AM   #4
Nordic is offline Nordic  South Africa
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Mini Ipod headphone amp DIY project
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Old 27th January 2007, 02:15 PM   #5
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I just finished a headphone amp for my girlfriend that uses a TPA6111A2. The TPA6111A2 runs off a single 2.5v-5.5v supply but requires output capacitors. I have it powered by a 3.7v 900maH lithium cell.

I am also building a micro CMOY amp for myself that will use a SOIC OPA2134. It will be powered by two 3.7v 140maH button cells in a symmetrical supply config. I know 140maH isn't much capacity but I want this thing small and I only need it for a few hours a day.

Here is a pic of the finished TPA6111A2 amp and the board for the micro CMOY.

http://i18.tinypic.com/48fpxz7.jpg
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