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Electronics -> USB powered headphone amplifier

Last modified on Tue, 1st Dec 2009 at 22:39 GMT by zipplet
This is a stereo USB powered headphone amplifier based on the TDA2822M. The schematic is similiar to the example one in the datasheet with minor changes so performance should be equivalent to that described in the datasheet. Unfortunately I do not own an oscilloscope so I cannot meter the performance of my unit. All I can say is it sounds good and there is very little noise :) Smilie

The desire to build this headphone amplifier came about from my frustration at the low volume from my computers headphone socket even when set to maximum volume. Most people may have a set of amplified speakers with a headphone socket on the front but I don't and I did not want to replace my speakers. The amplifier also gives me a hardware volume control which I very much prefer to software volume controls - which have been known to stop working at the most inconvenient time during a game! It also doubles as a convenient connection point for a set of headphones at your desk if your PC is on the floor.

Input impedance is about 10K ohm. Because the impedance is very high, amplifiers in devices that normally have very poor bass response such as the Nintendo DS will sound great through this amplifier, as those devices will have no problem driving such a high impedance load well. That is a trend that annoys me with some modern miniaturised devices, it comes about because they want to use smaller and smaller capacitors in the audio amplifier to save space and money.

/!\ WARNING /!\ This amplifier can be screamingly loud - it can destroy a cheap pair of in-ear headphones and/or damage your hearing if not used sensibly or if you set the maximum gain too high. You have been warned! If built as described here, the maximum gain is too loud for normal listening but will allow you to drive a set of beefy full-size headphones at a good volume, or listen to quiet sources. Changing the feedback resistors is not a good idea!

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Schematic diagram


Click here to download a full size printable schematic

Power section
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The fuse is optional but I recommend it just in case you make a mistake. You do not want a fried USB port, do you? Some motherboard USB ports have surface mount non-resettable fuses.

Main section
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My schematic software does not have jack socket symbols so I used a header symbol. The 3.5mm jack pinout is:

Component list


Construction


I built mine inside a semi-translucent blue project enclosure - part 30-1849 from rapid-electronics, on a piece of matrix board. Try to keep wire lengths to the minimum, I admit I failed at this because I built mine in a hurry. At the very least keep your left and right audio paths on opposite sides of the PCB to reduce crosstalk - I did do that. I fitted my power LED onto the PCB and aimed it at the front of the enclosure which makes it glow blue when switched on.

R3, R4 and R5, R6 form voltage dividers for the feedback (gain). I strongly recommend leaving these alone but you can play with them if you want to alter the gain. As-is, the gain is set to about 10 (if I remember correctly). Setting the gain too high will cause severe distortion. There is some headroom for a higher gain but I do not recommend increasing the gain because the amplifier is loud as-is.

C7, R7 and C8, R8 form a high pass filter with 2 purposes - firstly, unwanted high frequencies that your headphones can't reproduce are sinked away to ground and secondly they provide a load to the amplifier for those frequencies. I may be slightly wrong here, email me if so, thank you.

Standard grade electrolytics are fine in this project but if you are a purist you may want to use low ESR electrolytics. I don't think the difference will be audible however. Miniature electrolytics are fine if you want to save space. I used tantalum capacitors for C1 and C2 to save space.

I fitted a clip-on ferrite bead to the power cord. This may help to reduce noise/interference. I'm not sure if it really helped.

Please note that during my tests, I have found that this amplifier sounded poor on one of my powered USB hubs. The culprit was the power supply for the hub which had a lot of ripple causing an audible mains hum. You could improve the circuit to reject this ripple if you want to use a rubbish powered hub - or just plug it into your PC USB ports which will be clean.