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.
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.
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.
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.
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......
Here is a link to the design page......
http://headwize.com/projects/showfile.php?file=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.
The earbuds are brand new and there is nothing wrong with them.
Here is the design I followed......
An externally hosted image should be here but it was not working when we last tested it.
Here is a link to the design page......
http://headwize.com/projects/showfile.php?file=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.
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).
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).
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.
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.
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/showfile.php?file=opamp_prj.htm
Have a look at the high current buffers for ideas on modifying your cmoy to add more current...
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/showfile.php?file=opamp_prj.htm
Have a look at the high current buffers for ideas on modifying your cmoy to add more current...
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!
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!
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.
Pit
Pit
Well I just got done talking to the earplug store and they are willing to remake the earbuds with the correct impedance. They offer a 19 ohm version and I think I will go for those. I just have to wait 2 weeks for them to be made.
I did some crude measurements of the amp. I tried the 155 ohm phones + 51R resistor, 155 ohm phones without the 51R resistor, my 35 ohms phones + 51R resistor, and my 35 ohm phones without 51R resistor.
No matter what the load was I could only get around 0.75v output with music before clipping. I could get almost 4.5v output with a 1k sine, but It's hard to tell when a sine is clipping without a scope. I think music is a more realistic test in this case anyway. I tried both the mp3 player and my PC as a source and got the same results.
I have some quad OPA4134UA opamps. Should I try the high current buffer setup shown in Fig. (a) below, or should I try something completely different.
Thanks again for everyones help. It's kind of hard being an audiophile with a hard of hearing girlfriend. I feel guilty that I spend so much time and money on these things and she can barely hear at all. I was hoping to bring her a bit closer to my world by sharing the joy of music through something other than her horrible sounding hearing aids.
To listen to music with her hearing aids on she has to turn them on "telephone" which picks up all sorts of hum and distortion. Awful.
I did some crude measurements of the amp. I tried the 155 ohm phones + 51R resistor, 155 ohm phones without the 51R resistor, my 35 ohms phones + 51R resistor, and my 35 ohm phones without 51R resistor.
No matter what the load was I could only get around 0.75v output with music before clipping. I could get almost 4.5v output with a 1k sine, but It's hard to tell when a sine is clipping without a scope. I think music is a more realistic test in this case anyway. I tried both the mp3 player and my PC as a source and got the same results.
I have some quad OPA4134UA opamps. Should I try the high current buffer setup shown in Fig. (a) below, or should I try something completely different.
An externally hosted image should be here but it was not working when we last tested it.
Thanks again for everyones help. It's kind of hard being an audiophile with a hard of hearing girlfriend. I feel guilty that I spend so much time and money on these things and she can barely hear at all. I was hoping to bring her a bit closer to my world by sharing the joy of music through something other than her horrible sounding hearing aids.
To listen to music with her hearing aids on she has to turn them on "telephone" which picks up all sorts of hum and distortion. Awful.
theAnonymous1 said:
Since you said you don't have a scope I guess it's safe to assume you used a multimeter set to AC volts to measure the 0.75V output while the amp was reproducing the 1khz sine wave. Don't forget that the AC volts setting on a multimeter measures rms voltage. So that 0.75Vrms you measured means you were seeing about a 2Vp-p sine wave from the 2134. That's seems lower than it should be if your load is actually 150ohms. Are you using batteries for the supply rails? If so, are they fully charged?
theAnonymous1 said:
Increasing the load should decrease the current demand and allow the amplifier to swing closer to rails. 40 mA max with 200 ohms should allow the output to swing to about max. +-8 Vpeak. If your signal clips as early as 0,75V even with this load then the circuit is faulty. Period. I still suspect that the 600 ohm load is the minimum for the concerned chip.
Did you feed the sine wave from the same source as the music thus overruling the possibility that there is somekind of an mismatch problem between the source and the amp? How did you measure the output amplitude of music without a scope? Can you trust these measurements to be exact?
You wanted to know if you should built something different. I believe you should. The way I see it, the whole CMOY design is horrible: Current limited opamp that is (in theory) cabable to drive a minimum of 200 ohms load. Simple LM386 chips can deliver higher currents to lower loads, work from single supply rail and likely fit to smaller space as well.
I measured the voltage with a Fluke 111 true RMS meter. The 1k sine and the music tested were both played from my laptop. The 1k sine produced 4.5Vrms from the headphone amp with the laptops output at near full volume. When I played music the amp would clip at anything over 0.75Vrms, which was at an input level far lower than the 1k sines max level. I don't have a scope so when I say it clipped; I mean audible clipping heard through the phones.
The batteries are new and fully charged. Here is a pic of the supply connected......
http://i19.tinypic.com/2afeu1j.jpg
Here is a hi-res close-up......
http://i18.photobucket.com/albums/b118/theAnonymous1/cmoy004-1.jpg
The LM386 looks like it would work well with 2 9v batteries, but it requires large output capacitors and I would like to stay away from that option.
The batteries are new and fully charged. Here is a pic of the supply connected......
http://i19.tinypic.com/2afeu1j.jpg
Here is a hi-res close-up......
http://i18.photobucket.com/albums/b118/theAnonymous1/cmoy004-1.jpg
The LM386 looks like it would work well with 2 9v batteries, but it requires large output capacitors and I would like to stay away from that option.
I'm bussy with the (a) one... just took board from etchant a few minutes ago... it is the datasheet headphone amp circuit for the opa2227... will let you know how it went...
It is basicaly the same, except the resistor to ground on the positive leg is removed...(internal) the 2 resistors on the output side is 200 ohms the gain setting resistors are both 1k.
Depending on the impendance of the source, I would use opa2227 for up to 20k, opa2277 for 20k - 100k and opa2132 for 100k+ sources...
Eagle files....
4cm x 8cm
It is basicaly the same, except the resistor to ground on the positive leg is removed...(internal) the 2 resistors on the output side is 200 ohms the gain setting resistors are both 1k.
Depending on the impendance of the source, I would use opa2227 for up to 20k, opa2277 for 20k - 100k and opa2132 for 100k+ sources...
Eagle files....
4cm x 8cm
You might try looking over at Headwize.com. There are lots of people with a lot of experience in driving headphones. But, the short answer is that I'd use something with higher current capability instead of the OPA. Try a NJR4556 or a LM6172, or try putting a BUF634 (or two in parallel) in the feedback loop of the opamp. The OPA simply has too little current to drive a low impedance load. I'd also remove the output resistor as it is not doing you any good.
If you want a readymade PCB, try the Pimeta at http://tangentsoft.net/shop/ It is not the end all of amplifiers, but it is a step up from a cmoy and will at least give you some ideas on how to drive these phones a little better.
If you want a readymade PCB, try the Pimeta at http://tangentsoft.net/shop/ It is not the end all of amplifiers, but it is a step up from a cmoy and will at least give you some ideas on how to drive these phones a little better.
I think I'll give the current buffer circuit a try after Nordic lets me know how it works for him. I already have some OPA4134UA so the only thing I'll be wasting if it doesn't work out is my time.
This CMOY doesn't sound bad, it's just not loud enough for this application. Maybe I'll throw it in a case and use it myself. It's definitely more powerful than my mp3 players output.
This CMOY doesn't sound bad, it's just not loud enough for this application. Maybe I'll throw it in a case and use it myself. It's definitely more powerful than my mp3 players output.
I would seriously consider using the LM386 or one of its newer cousins. Opamps (except some high power ones) will not drive a 19-ohm load properly. The parallelled version was published at headwize as intended for 68-ohm Grado headphones.theAnonymous1 said:
I've seen the LM386 used for a headphone guitar amp and results with a music source were quite acceptable. Not the ultimate in high-fi but still quite good. Not sure why you have a problem with large output capacitors, is it the physical size? If you can get to the 4 individual wires on the earbuds you could use a BTL arrangement. See, for example
http://www.national.com/pf/LM/LM4950.html
dsavitsk said:
I wanted to reiterate this. It is a simple fix, and I'd bet it will solve the problem. There is no reason to have a resistor in series with the output. People use it to remove some hiss that can occur with some chips, but a little hiss is likely not an issue for you.
Really, any of these chips will drive low impedance phones to literally dangerous volumes. If they don't seem ear splitting to you, then there is something not working right in the circuit, and I think it is that resistor -- or else you set the gain wrong.
Another chip to try is the opa551. It is a high current opamp (~200mA I think) commonly used as a unity gain buffer and is very capable of driving low impedance phones. With a little gain it should work quite well.
Oh, and grados are 32ohm, not 68.
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