Tiny Headphone Amps!

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Alright, I have seen a lot of headphone amps out and about, and I have decided to make my own. I was wondering if anybody can point me in the correct direction. I have a few requests of any designs anyone can give me that are:

Small- both in overall size and amount of parts
Cheap- not necessarily cheap as in bad sounding and poorly made
No PCB- either point to point, or a predrilled board

If anyone has any suggestions it would be quiet nice! I plan on using this to drive my 64 ohm Behringer HPX2000, which are 100mv max. I would like a design that could be used with lower ohm loads also, maybe a selector for 64 or 32 ohm loads? Thanks a bunch for your time and suggestions!
Relax said:
thank you very much for those two links. I also want to mention again, that I want to build this amp! hehe, I don't think i mentioned it before.

Just make a chu moy ;)
Here is one I made:
An externally hosted image should be here but it was not working when we last tested it.

So they can be really small. Just use a single dual opamp and make it on proto board. Easy:

Lots of good info here on Tangents fantastic site:
I decided to just build the cMoy on tangents site. Ordered parts from digikey. Enough to make two, that way if the first one has major issues I can rebuild, or if it works out perfectly, I can build another and sell it! should get the parts soon, since I am about 40 minutes away form Digikey, hopefuly tomorrow or wed. latest. I will post again soon. ANyone got any other ideas anyways?
That one on Ebay looks pretty tricked out, the power switch is pretty neat too, the whole multiple times do different things. I am surprised at how expensive these little amps go for, they can't be half that in parts and time. But hopefully I will start soon! thanks for your help.
Relax said:
BTW I am using the dual opa132 or OPA2132 by Burr-Brown. I can't make any sense of that datasheet either! Will the cMoy drive my 64 ohm Behringers well without exceeding 100mv max?

Yes it will drive them fine, dont worry about exceeding 100mv. You will probably not be able to have them over your ears at that output anyway, that is why it doesn't matter. I've never heard of anyone damaging headphones by over powering them.
Works great!

Except its only in one channel, the right to be exact, that I hear anything. The sound coming from the right is loud and pretty clean for awhile until its too loud, but I hear nothing form the left! Whats wrong with me? I looked at the soder and everything looks good, resistors are fine, caps, well, I don't know how to test those yet. could having the LED not in do this? ANY help would be greatly appreciated. Its not the amp either, I switched it out with a new one and same problem. Thanks for your time in advance.


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> output specs for the cMoy

The dead-simple Cmoy uses a 9V battery and an ordinary opamp chip.

Therefore the maximum output voltage is 4.5V peak, and really only 3V peak 2V RMS in hi-Z loads.

The maximum output current is 20mA to 60mA peak depending on the chip used.

I once prepared a chart showing estimated maximum power for many different headphone amplifiers. Cmoy-like amps plot like curve "b", with 10mW-20mW from 32 to 300 ohms. Three 9V batteries with a high-current opamp chip will plot near line "d", 300mW-600mW into common headphone impedances. Two 9V battery Cmoys are popular for hi-Z phones and fall somewhere between curves "b" and "d".

For comparison, the "*" dots are rated power for a wide variety of headphones. Curve "e" is my suggestion for a "universal" headphone driver, one that will put ample power into any common headphone without mush risk of gross over-drive.

> 64 ohm Behringer HPX2000, which are 100mv max.

No, the HPX2000 is rated 100 milli WATTS, not milli Volts. Makes a difference.

In 64 ohms, 100mW is 2.5 Volts RMS, or 7V peak-peak, which is not a bad match for a 9V battery with some losses in the chip.

Actually, 100mW is not high; some similar phones are rated 3,000mW. With speech/music signals, you can probably run transient peaks 10 times higher than the thermal rating. In fact I'm sure this headphone is made to plug into B-ringer mixers that run +/-15V on the headphone-amp with about 30 ohm series resistance, almost 7VRMS and 700mW of transient power peaks.

Do the B-ringer HPX2K need big power? The specs say "110dB@1KHz" which, by itself, is utterly meaningless. It is traditional to measure headphones in dB SPL at 1 mW. But 110dB SPL at 1mW is absurdly high. I suspect they mean 110dB SPL at 100mW, the rated power. With a 9V Cmoy you would get about 103dB SPL peaks, say 88dB SPL average music level at the edge of clipping. Probably ample for most users.

> I've never heard of anyone damaging headphones by over powering them.

This is very stupid, but I have many burnt-up headphones. Some come from the Student Labs: I hate to think how much ear-damage those kids have. And personally I have left headphones in a PA system but off my head, and found them blown after the show.

> could having the LED not in do this?


One channel good, one channel bad, means you have a mistake. Take it out in the bright summer sun and check the solder joints, part values, and the what-goes-to-what again. Don't overlook your input and output jacks. Check jack pinout, some are strange. If you used the low-price P-sonic stereo volume control, note that it has a very unexpected pinout.

> OPA2132 by Burr-Brown. I can't make any sense of that datasheet either!

OPA2132 datasheet, page 6, bottom, output voltage versus current. You have to make some sly guesses and know some about how this chip will clip, but for a single 9V battery I read it as 30mA 2V peak, or 28 milliWatts in 64 ohms.
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