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Need help designing custom Cmoy for bone conduction

post #1 of 3
Thread Starter 

I'm planning to design a Cmoy of my own (possibly using SMD parts) to power some Bluetooth bone conduction headphones that I'm designing. I've chosen the transducers to use and tested one at 1.1V line level from my iPod 5G, and it's only audible in a very quiet room, so I need amplification. The specs are as follows:
*Up to 30V P-P power handling

*300 ohms

*Small and very cheap, so I can use 2 transducers each side in parallel to bring it down to 150 ohms.

I want to match the power output of the bone conduction speakers in Google Glass, which consume 415mW. (so lets assume 100% efficiency to be on the safe side). That's 200mW RMS per side. So if P(200) = V^2 / R(150), then the voltage must be 5.5V RMS (~7.7v P-P).

Since I'll be running Bluetooth A2DP, quality is no real issue (that's what my Sennheiser's are for). I'll be going for a custom made Lithium Polymer battery since the power source will be hanging off my face, so I can go for 3.7, 7.4, 11.1 or 14.4 volts. SMD parts would be preferable since I want to reduce size to hide the entire assembly behind my ears (battery one side, amp the other side).


So:

*Am I right in thinking I'll need 7.7V output to output 200mW from a 150 ohm load?
*Is there any way to do it with a lower voltage?
*Would an opamp handle 400mW?
*What would be the minimum battery voltage required (a small drop in power is ok if I can use a smaller battery)?
*How many mAh would you recommend to be able to run for 24 hours of constant playback at full volume?

Thanks for the help everyone.


Edited by Blufires - 6/11/14 at 11:29am
post #2 of 3
Quote:
Originally Posted by Blufires View Post
 

I'm planning to design a Cmoy of my own (possibly using SMD parts) to power some Bluetooth bone conduction headphones that I'm designing. I've chosen the transducers to use and tested one at 1.1V line level from my iPod 5G, and it's only audible in a very quiet room, so I need amplification. The specs are as follows:
*Up to 30V P-P power handling

*300 ohms

*Small and very cheap, so I can use 2 transducers each side in parallel to bring it down to 150 ohms.

I want to match the power output of the bone conduction speakers in Google Glass, which consume 415mW. (so lets assume 100% efficiency to be on the safe side). That's 200mW RMS per side. So if P(200) = V^2 / R(150), then the voltage must be 5.5V RMS (~7.7v P-P).

Since I'll be running Bluetooth A2DP, quality is no real issue (that's what my Sennheiser's are for). I'll be going for a custom made Lithium Polymer battery since the power source will be hanging off my face, so I can go for 3.7, 7.4, 11.1 or 14.4 volts. SMD parts would be preferable since I want to reduce size to hide the entire assembly behind my ears (battery one side, amp the other side).


So:

*Am I right in thinking I'll need 7.7V output to output 200mW from a 150 ohm load?
*Is there any way to do it with a lower voltage?
*Would an opamp handle 400mW?
*What would be the minimum battery voltage required (a small drop in power is ok if I can use a smaller battery)?
*How many mAh would you recommend to be able to run for 24 hours of constant playback at full volume?

Thanks for the help everyone.

 

subscribed. 

post #3 of 3

You have already answered many of your questions.

 

An op amp on is own would not be enough...but and op amp

and buffer would be no problem.

 

An op amp and buffer will not fit behind your ear.

 

Lets assume your rig draws 500mW (probably conservative).

To run for 24 hours, you would need a 12 AH battery.

That will not fit behind your other ear.

 

Unless you use some kind of boost converter to increase

your voltage, you battery will need to provide the peak to peak

voltage plus a little headroom.

Look at your 11 or 14 volt options.

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