In-line Headphone Amplifier




About: I'm Chandra Sekhar, and I live in India. I am interested in electronics, and building small one-off circuits around tiny chips (the electronic kind).

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.

Step 2: The Cabinet

It will need something to hold the circuit board, a battery and the headphone socket. I shall use a AAA cell to power it, and use the metal cladding from two AA cells to fabricate a suitable cover for these.

It is the only way to end up with an assembly little thicker than the battery powering it.

The picture shows the metal cladding being liberated from a couple of dead AA cells.

Step 3: Cutting the Board to Size

The board has been cut and filed to make it about as wide as a AAA cell.

Step 4: The Circuit

The figure shows the circuit, which has been unashamedly copied from the datasheet published by MAXIM for their chip. In addition to the chip and battery, five capacitors need to be fitted.

It would all be very easy, but the chip is intended for surface mounting and very small.

Step 5: Whittle a Circuit Board

A small (jewellers') screwdriver was sharpened into a chisel and a recess dug out of the component side of the circuit board. This board is made of paper laminated with phenolic resin, and it cuts with the chisel very easily. And I like the feel of cutting through it - absolutely unnecessary for the proper functioning of the circuit, I am sure, but it gives me something to do and makes the finished product look as if somebody spent some time on it.

This might be realised, for bulk manufacturing, by laminating laser cut paper and foil.

Step 6: Wiring Scheme

I shall be looking at the underside of the chip, so a scheme of connections is prepared looking at the chip that way.

The data sheets show the connections looking down at the top of the chip, and it is possible to make mistakes if wiring directly from that.

The view below has been labelled "bot view" and the board shows the chip securely inside its recess, and a few holes drilled for the connecting wires.

Step 7: The Other Side

The other side of the board shows the holes, the solder resist scraped away for the wires that connect to ground, and the copper bevelled away for those wires which do not.

Step 8: Soldering Wires

Two wires that connect to ground have been soldered in this picture.

Step 9: Soldering to the Chip

It needs a steady hand, a fine tip on the soldering iron, and good lighting in order to place the wires and solder exactly where they have to go.

The other end of the wires have been soldered to the foil and so they stay put while they are bent over the chip, cut to size and soldered to the pads.

I use a watchmaker's eyeglass for this part of the proceedings in order to see what I am doing.

Step 10: The Capacitors

Two capacitors from old hard disk drives have been liberated to be used in this circuit.

I have drilled a few more holes, and cut a few channels for the wires in this picture.

Both those capacitors are tantalum, surface mount, 1 microfarad.

Step 11: In Position

The capacitors have been soldered in position in this picture.

Step 12: More Capacitors

The capacitor for decoupling the power supply has been connnected up, this time it is on the same side as the chip.

Step 13: A Size Check

I have doubts whether everything will fit in the small volume, so it is time for a size check with the input capacitors and the headphone socket.

The capacitors are OK, but the headphone socket is larger than the thickness of the AAA cell.

Step 14: The Completed Board

The circuit board is complete, with the input coupling capacitors placed on either side of the board.

It is still smaller than a AAA cell.

Step 15: Tinplate Cabinet

This is one shell of a AA cell. Two such shells were soldered together.

Step 16: Ready for Bending

The two AA shells soldered together is ready for cutting and bending to form the AAA cell holder.

Step 17: Wired for Sound

The two wires carry the signal to the headphone socket at one end from the amplifier chip at the other, past the AAA cell held securely in between.

Step 18: Socket for Headphones

The socket for the headphones is to be made of parts from an old one to fit in the diameter of the AAA cell.

Step 19: Tip Connector Fitted

The spring connector contacting the tip of the jack has been fitted.

Step 20: Tip and Ring Contacts Fitted

The tip contact was modified, and the contact for the ring on the jack has been fitted.

Step 21: It Fits!

The jack inserted into the completed socket. It seems to be okay. It has to be inserted with a bit of Tender Loving Care or the thing might come apart.

Step 22: Trial Fit

The AAA cell and socket inside the metal cabinet. The cell looks mangled because I used it as a template to bend the sheet metal.

Step 23: Battery Spring

The spring contact for the battery negative is next soldered in. The picture shows the earphones I am using to test this with.

Step 24: Positive Contact Fitted

The positive contact for the battery is a piece of double sided copper clad board. The side which contacts the battery has clearance cut along the edges so that the battery does not short out to the case. The other side is soldered to the case.

Both the spring and the positive battery contact were taken from a defunct TV remote.

Step 25: It Is Complete

The headphone amplifier is complete, and it works.

The chip I used has a gain of 2 so it needs line levels (around a volt rms) in order to work.

With no signal, the current drawn from the battery was about 2 milliamperes. With the music blasting at a comfortable volume in the earphones shown the current went up to around 12 milliamps.

I should wrap it up in duck tape to make it look nicer, but it looks like the junk it was made from.

I shall try building it into a marker pen body the next time I get the urge to make one.



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    23 Discussions


    Question 3 months ago

    I must ask the crucial question - does it improve the sound over the inbuilt amp in most portable devices?


    lol. You straight up mcguyvered this gadget from nothing. "A small (jewellers') screwdriver was sharpened into a chisel" I was dying. Somebody's tool box is looking a little sparse. Get you a dremel son! Totally slick though. Two thumbs up.


    7 years ago on Introduction

    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?


    8 years ago on Introduction

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


    10 years ago on Step 25

    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!


    11 years ago on Introduction

    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!

    2 replies

    Reply 11 years ago on Introduction

    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


    Reply 10 years ago on Introduction

    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.


    11 years ago on Introduction

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

    1 reply

    Reply 11 years ago on Introduction

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


    11 years ago on Introduction

    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.


    11 years ago on Introduction

    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.

    1 reply

    Reply 11 years ago on Introduction

    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.


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


    11 years ago on Introduction

    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?


    11 years ago on Introduction

    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!


    11 years ago on Introduction

    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...


    11 years ago on Introduction

    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.

    1 reply

    11 years ago on Introduction

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