In-line headphone amplifier

Powered by a single AAA cell,this amplifier is scarcely thicker than the audio cable and fits between the line out socket of your PC sound card and your headphones. Most sound cards have line level outputs, at high impedance, and will not drive headphones directly at full volume.

It is based on the headphone amplifier chip from Maxim, the MAX9725, which can work at voltages as little as 0.9 volts (range 0.9 to 1.8 volts).

This chip does not need capacitors on its output since it has directly coupled output stages and it generates a negative supply rail by using an internal switchmode power supply stage.

This is a small surface mount chip and can be placed by automatic equipment onto a suitable printed circuit board.

I am using wires on a hand carved board, and am presenting my steps here in the hope that it will inspire somebody out there to try something similiar with the tiny chips those guys insist on manufacturing - great features, but a tiny package.

Step 1 Circuit board

The circuit has to be assembled on a circuit board. I find that a piece broken off a larger board will be sufficient. It has green solder mask on, in fact it is part of a large Y/C panel from an old VCR.

Comments

[bowmaster]

Oh my god. This is perfect for what I wanted, but I have no idea how I'm gonna solder something that small. would this be good for driving a pair of medium-large over ear headphones?

[astrong0]

I love the macro shots, very well and very detailed as well as presented!!

[Bondo]

What a swell instructable! The combination of high-tech electronics, low-tech tools and materials, and funky craftsmanship is great. A great instructable for a novice like me. Thanks for sharing this info!

[horta_12]

I've got two questions: 1. What are the other three capacitors rated at? That would be the two red ones and the black one on the same side as the chip...did I just miss it when I went through the instructions? 2. I looked at Maxims site and there are oodles of different MAX9725 such as the MAX9725CECT or the MAX9725AECT... Which one did you use and would you know what the difference between them is? Thank you!

[neelandan]

The capacitors need to have a minimum voltage rating of 6 V (ie, twice the maximum voltage present in the circuit). This is a minimum rating. You can use capacitors of 16 volt or 25 volt rating, whatever you have available, only the size shall be larger. The tantalum bead type of capacitors I have shown are reputedly very unreliable. I do not mind if my experimental circuits fail after a few days, but if you are constructing these circuits for a friend or for sale that point might be something to bear in mind. Google for "cmoy headphone amplifier" for a thorough discussion on the subject. The various ics differ in gain and packaging. MAX9725A - voltage gain 2 (power gain 4) (the one I used) MAX9725B - voltage gain 1.5 (power gain 2) MAX9725C - voltage gain 1 (power gain 1) MAX9725D - voltage gain 4 (power gain 16) For experimental use, the difference between these is not very significant, though I would rather not deal with the D version with relatively high gain. The next letter in the type designation is E, denoting extended temperature range. The next two letters are TC, denoting a twelve pin DFN package. I would not recommend anyone trying to grapple with the other chip scale package - the one I used is small already. The chip I used had the full part number of MAX9725AETC

[neolife]

I used the MAX 9725AETC too but I also ordered a the chip-scale pack (MAX9725AEBC+)to see how small it was it is about 1 mm long and .5 mm wide. The 12 pins on the bottom must be very small(I didn't open its packing didn't want to loose em). The MAX9725AETC is also very small about 3mm by 3mm I haven't soldered them yet (got em in about an hour ago) but the project is not for you if your new with soldering.

[ZeroTruths]

Disregard this if it's already been mentioned. Won't encasing your project in metal, short connections?

[n0ukf]

It's called shielding. If you think it'll short out, you put a layer of tape between the connections and the shell

[Jalakahops]

0_0 No one has commented on that awsome soldering? I had a terrible time redoing the connections to the audio jack in my mp3 player which is easily twice the size of that chip! Maybe I need a new iron or just lots and lots more practice. The only problem with putting it in a pen body is it seems like you would have to rebuild it everytime the battery went dead, unlike the easily reconstructed battery shell.

[Ora]

I have the chip, and one 1uF capacitor, what else do I need to do this? I'm stuck, and would like to finish this for halloween.

[neelandan]

You need five capacitors - three can be 1 uf or greater, electrolytic, and two have to be low leakage .47 uf or so. Plastic film or ceramic types should work. A battery holder and stereo earphone socket to suit your plugs would be nice, too.

[RichardBronosky]

Just don't try taking this on an airplane. If TSA stumbles across it, you will never explain it.

[srmousse]

I think this is great! just the other day I was wishing I had a very small amp to boost the levels for some cheep speakers I have...I think I'll make one! =)

I would love to see something like this powered by a usb port. You could mount a small speaker wire to plug from the usb to the lineout and plug the speakers directly into the usb... What would you have to do differently as far as cap's are concerned?

[fegundez1]

how do you know witch caps to use?also where could one get the chip,as Maxim will be wondering about the sudden interest in free demo chips!

[phatso]

Also, electrolytic capacitors can actually take about 1/2 volt reversed polarity. In the circuit diagram (step 4) I notice there are left and right headphone connections but no ground return. Might be good to correct that. Belleye - doesn't your computer have a mic input? A line input won't work with a mic; it's 25-35db less sensitive than the line intput. Also,condenser mics need DC to power them which appears on the mic input but not on the line input. This particular chip won't work well for a mic preamp but I seem to remember there was one on here somewhere...

[Belleye]

Nice job, can you do the same with a microphone and plug it into the "line in" on a PC? I can never get decent volume out of my mic/s.

[wongsta]

I'm wondering about this too, might need a mic amp some time...

[bhunter736]

Looks great! I want to make one. Where did you liberate the chip from? Or did you buy it somewhere?

[neelandan]

I asked Maxim for a sample.

[thereza]

You've got a big problem with your circuit.. You said you used tantalum caps -- which are polarized. In the circuit, their use is to AC couple the audio signal to the amp. You would need a non-polarized capacitor to make it work A)better and B) not blow up (though at those voltages it probably will not blow up). You want to find a capacitor and check it's capacitance with an LCR meter, or just spend the 50 cents and order the right part from mouser or something.

[cooperised]

@thereza: It's not a problem. You're right that the tantalum caps are polarised, but the circuit is running off a single rail supply; you therefore can be sure that the input pins will be DC biased to around half the supply voltage (so 0.45V-0.9V), while the signal from the soundcard will have no DC bias. So as long as the caps are orientated so that the positive end faces the chip, they will always be biased the correct way round and the AC signal will pass through them just fine. HTH...