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Old 5th February 2007, 02:15 AM   #1
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Question DIY headphone amp not loud enough

Before I ask my questions, let me explain that I built this amp for my girlfriend who is hearing impaired. The amp was built to drive a pair of custom molded earbuds.

http://earplugstore.stores.yahoo.net/chmisearph.html

Specs: 25 ohm 115db/1mw (they don't seem that sensitive to me though)

The amp is the basic CMOY circuit using an OPA2134UA and +-9v supply. All components are SMD.

Pic

The amp works perfectly, but I am disappointed with the output level. I was hoping that with a +-9v supply and 25 ohm phones that I would get some serious output, but that just isn't the case. It plays loud, but not uncomfortably loud; which is what I was shooting for. I tried one of her hearing aids on and they get uncomfortably loud, so I know the amp needs more output.

So I guess my question is what can I do? Is there something that can be optimized in the circuit, or do I just simply need more voltage? This amp needs to be as small as possible so trying to get close to the +-18v limit of the opamp with batteries gets REALLY bulky.

Any thoughts or suggestions is greatly appreciated.
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Old 5th February 2007, 02:54 AM   #2
teemuk is offline teemuk  Finland
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For a somekind of earphone system the power to 25 ohms obtainable from a +-9V supply should be more than enough. Have you measured how much power the amplifier really gives out? With that supply voltage an ideal amplifier should give out about 1,62 watts to 25 ohms. I'm not familiar with the chip you used but I gather that it's not actually a low power opamp. I checked out the datasheet quickly and it seems that 600 ohms is the lowest load that the concerned chip can cope with.

I know from experience that LM386 low voltage power amplifier chip running from +9V rail gives out about half watt of power to 8 ohms. With sensitive earpiece this would be enough to cause a series hearing loss for an average person. Same chip running from +16V rail would give about 1W to 32 ohms. The issue is definitely not about the rail voltage. How much power does your amplifier really put out? Does it collapse under a small load such as 25 ohms?

Anyway, for an earpiece system even less than hundred mW should be plenty.

If the amplifier is fine and can drive a 25 ohm load then I would suspect that the earpiece is faulty.
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Old 5th February 2007, 02:59 AM   #3
BWRX is offline BWRX  United States
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What is the maximum level of the input signal you're using and what is the gain of each channel?

The 2134 can do 35-40mA of output current, so it should be able to swing about 1Vp-p with that 25ohm load before giving up. At those levels distortion will probably be higher than you'd like.
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Old 5th February 2007, 03:28 AM   #4
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The amp technically sees a 76R load, as there is a 51R resistor in series with the output.

The earbuds are brand new and there is nothing wrong with them.

Here is the design I followed......

Click the image to open in full size.

Here is a link to the design page......

http://headwize.com/projects/showfil...=cmoy2_prj.htm

I used the same values as above except R5 is 51R, C1 is 2uf, and I omitted R1. The gain of the amp is 11.

I'm not sure of the output level of the mp3 player I used, but I'm sure it's the same as the majority of players on the market. I can make the output of the amp clip, so input signal and gain seem plenty high enough.

This amp is definitely louder than an amp I recently built based on a TPA6111A2DR amp IC powered by a single 3.7v lithium cell. That amp was louder than the mp3 player, but not by much.

You have to remember that what you would consider excessive volume is what my gf would consider acceptable volume.

I don't understand how that little hearing aid of hers is so darn loud from that little tiny battery.

EDIT: Ok, I just measured the impedance of her phones and they are 155 ohm, not 25 ohm like the specs say. That is kind of frustrating, as I probably wouldn't have bought them if I knew that before hand. I will definitely be contacting the earplug store and asking why there is such a discrepancy in the specs.
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Old 5th February 2007, 04:34 AM   #5
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I'm not sure, but I don't think DC impedance (measured with a DMM) will match the impedance for AC (your audio signal) 1 for 1. And the 25ohms will probably be between certain frequencies.

I'm suprised that with a gain of 11 you're not getting loud, whether it's into 25ohms or 115ohms.

Try removing R5 (it's optional anyhow) and seeing what happens?
Try some other headphones (i.e. 32ohm earbuds).
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Old 5th February 2007, 04:37 AM   #6
teemuk is offline teemuk  Finland
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If something is loud that is usually due to efficiency of the speaker system not the power of the amplifier.

The chip you use has internal current limit of 40 mA so your amplifier can barely drive the 155 ohm earpiece. You don't need to have a load that is any lower value than this - your amp couldn't drive it.

To load of 155+51 ohms the amplifier's output power should be about 155 mW RMS. Have you verified this? If the earpiece's efficiency is indeed 115 dB @ 1mW (I doubt it) this should be uncomfortably loud. Note that an average human has a high risk of getting permanent hearing damage when exposed to sound pressures greater than 85 dB.

With all the math considered, something in your system does not work the way you want it to... Trying another earpiece might give you better results. I suspect that the SPL ratings of the one you tried and bought are bloated.
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Old 5th February 2007, 06:35 AM   #7
Nordic is offline Nordic  South Africa
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What source are you useing... the output impendance may also affect the output, but I think more likely you woud do good to add another 9V battery, and turn up the gain a bit..

I mention the impendance, because with that circuit, I have a portable FM radio that plays at lower volume trough it than without... Also an Ipod shuffle... healthy volume gain... Ipod nano smaller volume gain...

Those chips can do about 40mA output per channel (I doubt at 9V though)....

Rod Elliot has a nice headphone amp design with crossfeed... now that plays plenty loud.....

http://headwize.com/projects/showfil...=opamp_prj.htm

Have a look at the high current buffers for ideas on modifying your cmoy to add more current...
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Old 5th February 2007, 07:08 AM   #8
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I have used this circuit before to drive a pair of 16 ohm shure earbuds I have and I also had the same problem.
Do not remove R5,the circuit will go into oscillation with the low impedence phones(I used 47 ohms).
Replace R3 with a 1k trimpot(10 turn works best) so you can adjust the gain.When you find the gain you need,measure the trimpot and replace with a fixed value.
Hope this helps!
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Old 5th February 2007, 07:34 AM   #9
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Sounds sensible to me. Carrying a bleeping rucksack full of batteries might get her off music. And Anonymous - stand by your girl! Having people think she is stupid, not realizing that she just can't hear them...sheer hell. But ain't life hell for you, too? Kitten used to shout at me, and before I could shout back she took her hearies out.

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Old 5th February 2007, 01:53 PM   #10
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Wow, whats goin on with the forum lately. This is the second time I have lost a post from the forum being restored from before I posted. I don't really feel llike typing it all over again.
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