Mono Audio Amplifier

[auto_218]

Hello All! I am currently working on building an audio amplifier. A little background information first. I maintain communication systems on aircraft. I fly a lot and constantly have a set of David Clark headsets on my head. When we are flying for 5+ hours obviously we have to stay entertained and watch a lot of movies and listen to a lot of music. Sticking a pair of ear buds in and putting the David Clarks over them starts to hurt after ohhhh 5 seconds! Being a hobbyist I don't want to go pay $300 for a pair of headphones at the store! I can make a simple in line patch cord to hook my headset to any typical headphone jack but with the noise of the aircraft it is often hard to hear the audio when plugged into a laptop or iPhone/iPad. This is where my project comes into play. The project is an amplifier to enable me to hear the audio from the above mentioned when I am flying. The current version of this project is V3. The first one was just a good o' radio shack pre printed project board with a LM386 (I think that is the chip I used sorry I failed at taking good notes when I first got into the hobbyist electronics field) in a project box. This actually worked very well but who wants to carry that around right. The second version I came up with was a home made PCB with a LM386 (once again I think that is the chip I used) that was installed inside one of the earpieces of the headset. Once again this was good but it was difficult to install a power switch so the battery had to be changed all the time and it was also inside the earpiece. V3 is an inline adapter installed in a U94 casing (which is just a connecter with a push to talk button that is used on the communication cords on the aircraft). My problem is that I get a ridiculous noise, to the point you cant hear the audio, unless I find the sweet spot with the volume on the device it is plugged into, on the headset, and on the project. The goal is also to power the chip with 6V (using two 2032 batteries). This caused there to be more noise than when using a 9V battery. After doing some research here on the forum there some things I over looked or did not take into consideration. Once again I am using the LM386N-1. I am using the exact circuitry from the datasheet for the 200 gain setup. The first thing I misunderstood was the ohms of the speakers. There are two 8 ohm speakers wired in parallel in the headset, which from what I read comes out to 4 ohms. I started doing some research on chips set for 4 ohm speakers. The only chip that I was able to find that was through hole and setup for my project was the NJM2070 (http://semicon.njr.co.jp/eng/PDF/NJM2070_E.pdf). I was just looking for some input on anything else I may be missing and if anyone has used the NJM2070 chip. One other item in all the datasheets that I don't understand is the THD spec. I have tried to figure out that one out but have been unsuccessful. If you have made it this far through the novel I appreciate the interest and am thankful for ANY input. I am sure I left some details out that would be important, but please let me know if more info would be helpful.

Thanks!!

[grenert]

I don't think you'll get enough current out of coin cells to drive an audio amplifier.  The horrible noise you hear might be oscillation.  Have you tried adding 10uF directly across the power rails?

[tszaboo]

200 Gain is too much for a headphone amplifier, which takes an already headphone-ready volume. That is 43 dB.

[auto_218]

I don't think you'll get enough current out of coin cells to drive an audio amplifier.  The horrible noise you hear might be oscillation.  Have you tried adding 10uF directly across the power rails?

Yea that is what I figured when I switched to the 9V supply (as for the current). I think I might have to abandon the coin cell batt idea and stick with the 9V batt. I have not tried putting a cap on the power rail. I will give that a try and put another post up about how that works out. Thanks again for the input!

[auto_218]

200 Gain is too much for a headphone amplifier, which takes an already headphone-ready volume. That is 43 dB.

That is something that I did not take into consideration! I did try the 20 Gain setup from the datasheet and that setup did not produce enough volume difference from the inline patch cable that I spoke about. There is a 50 Gain circuit in the datasheet that I gave a try in the early stages of development. That one didn't really make much of a difference though. I will take the 50 Gain circuit and play with it to try a few different gains and see how that turns out. Thanks again for the input!