Ultra portable IPOD headphone amp..

[Buriedcode]

Hi,

A neighbour has asked if he could use an amp to boost the volume on the ipod docking stations at his local gym - he says its just too quiet and likes stupid volumes. Being the geek I am, he came to me to see if I could come up with a relatively simple, and very small headphone amplifier, for in-ear earphones (so we're not talking 16-ohm over-the-head hpones).

Even though I'm a perfectionist, I do not see the point in trying to re-invent the wheel and using expensive opamps, so I am not after audiophile grade, but it would be nice if it was pretty transparent. The specs are:

-Small, portable, powered by one or two AAA batteries. (requires boost).
-10dB gain max
-No bells and whistles with EQ, just in, volume, and out.

This is purely something to boost the volume output capability of the docking stations at his gym, with a volume control. 3.5mm jack in, and 3.5mm Jack out.

Before I go ordering a bunch of expensive parts for testing, I have a lot of TS922 ( http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001188.pdf )

and AD8531 ( http://www.analog.com/static/imported-files/data_sheets/AD8531_8532_8534.pdf ) laying around. Both are high current output opamps, geared for buffers, rather than amps (5-10dB gain sohuld be ok though). Has anyone had experience of these as headphone drivers? The THD on both doesn't look too great to me, but the low current consuption and low voltage operation frankly, is a winner.

I also have a number of low voltage DC-DC boost converters so getting 3.3V from a single celll, or 5V form two cells is no problem - although it'll require some filtering/layout considrations to stop any noise. Even though it has to be small (I'm tihnking, sometihng the size of 2x AAA cells side by side) I work with SMT (QFN- 0603) all the time, so that shouldn't be a problem.

As its not paid work, just a favour, despite this forums reputation for audiohpile grade projects, I am not going to spend too much money on this, I just thought I would run this by you guys in case a similar project has popped up that I didn't find on my search. The C-moy amp is pretty popular, but the use of a 9V battery (I ahte those things) as well as the current consuption, is jst a bit much for this - I know I will have to sacrifice at least some quality because of that.

Any pointers? Just hoping it would save me hours of protoyping and listening tests if someone could give advice/experiences

BuriedCode

[dfuhr7gm]

hello,
i'm just encountering my last problems with my miniature op-amp headphone amplifier ...
i found a TCL272 at home, so i can power it from a mobile cell battery.
when i finished the whole thing and erased its last problem (the bass is ringing and crackling), i'll give you my project files. please just send me a pm, otherwise i'd forget :$
ps: i'm using it to drive akg k271mkII from my ipod nano with that stupid EU volume limit ...

[FREDtheDOG]

@Buriedcode
I've had great success with the LM4880/4881 family, very small, runs all day on 3Volts, draws very low current, only a few external parts, can recharge useing 3.6V NiMh battery off a usb port

[dfuhr7gm]

@FREDtheDOG:
could you possibilly be so kind as to send me that schematic? :$
as my experiment with the tlc272 was a fail ...

[sgrossklass]

Looking at the TLC272 datasheet, it appears its output voltage swing ranges from VSS to (VDD - 1.2 V) maximum, even with light loading. On a 3.6 V supply, that gives you a 1.2 Vpp (420 mVrms) AC voltage swing, abs max. As per Figure 13, this would dwindle away to nothing at only 5 mA.

In other words, this chip is pretty much entirely unsuited for driving low-impedance loads of the hungrier variety (which these AKGs are) on a low-voltage supply. The bass was distorting as it clipped badly.

You need a beefier RRIO (or at least RRO) amp for such a task, the LM488x family being just one example (small headphone drivers are available from multiple companies).

In order to save the TLC272 project, you'd have to go capacitor-coupled and shift input (and thus output) DC levels away from midpoint until symmetrical output clipping is obtained under load. It probably still wouldn't sound great but it would go a lot louder than it does now. If you've got multiple chips floating around, an Apheared-47 style buffer arrangement would do a lot about the remaining crappiness.

At least this kind of exercise gives one a lot more respect for little headphone amps like the FiiO E6 or the AS3543 (e.g. Clip+) headphone driver.

[Avro Arrow]

Originally Posted by dfuhr7gm

@FREDtheDOG:
could you possibilly be so kind as to send me that schematic? :$
as my experiment with the tlc272 was a fail ...

All the information you need to know is here.

[dfuhr7gm]

@sgrossklass:

wow, thank you!