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URGENT! Question about in-line audio control between amp and speakers?

I have a unique project that I am almost finished with, but need to make one last adjustment to. I am running a 3.7W Class D Audio Amplifier. The amp has two outputs. I need one to run as designed (controlled by my iphone before the signal gets to the amp which will be set to almost max volume). I need to then control the other output channel to reduce the volume. Any suggestions? It seems like I might be able to just get an inline control out of an old set of earphones, but I am not entirely sure. I always assumed (and pardon my ignorance if this was a foolish assumption) that the buttons in my iphone earbuds actually signal the phone to change the volume, and isn't really "in-line". So, I don't think those would work. Would an old set from Goodwill work for this?

I have seen posts where others are asking similar questions and the answer seems to be a 1K Ohm Pot (which are apparently really tough to find). Since I have a 3.7W amp, perhaps a 10K Ohm Pot will work (people have said this doesn't work well for their headphones). 

To summarize:

1) Will adjusting one output channel on a two-channel amp work? (or will the increased resistance simply close off the channel and pump all the signal to the other channel?)

2) If this will work, can I use a 10K Pot?

3) If not, will an old-school in-line volume control from some headphones work?

Sorry for all the info...trying to be thorough!

Thanks a ton for any advice!



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You NEVER control the volume on the OUTPUT, all you need to do is put a LOG or AUDIO track pot on the inputs, so you can independently adjust the volume of both channels.

WhiteTyrone (author)  steveastrouk2 years ago
Thanks for your response Steve. A couple of quick questions for you.

1) isn't an inline volume adjustment on a set of headphones basically doing the same thing as controlling the output, not the input? Maybe I don't have a grasp on how those function.

2) the amp I have only has one input, but I need to control the two outputs separately. So controlling the input won't work for what I am trying to do. Is there another option?

Thanks again!

Earphones are handling milliwatts. You're talking about several WATTS.

According to the link you posted you ARE using a stereo amp. You might have tied both input channels together, but you still have two amps.

WhiteTyrone (author)  steveastrouk2 years ago

Thanks again Steve, I think I am getting a better understanding. So, my phone puts out a stereo output, meaning two independent signals (L and R), and my amp treats each individually and then outputs them to bothe L and R channel outputs respectively. Is this correct? My "amp" is really two amps on one circuit board, right?

So my best bet is to intercept both signals before they get to my adafruit amp, and manipulate one channel's amplitude before it gets to the amp, and leave the other alone. is this right?

If so, what would I need to get a hold of that would allow me to make these changes to one signal without affecting the other signal! Is it still a 1k Ohm pot that I would put on the left output and just run the right out out directly to the amp?

Yes, that's right. The usual method of adjusting it would be to use a pot with two tracks mounted together, wired so one goes louder when the other goes quieter. You COULD just do this by adjusting the balance out of your iphone.

WhiteTyrone (author)  steveastrouk2 years ago

DUUUUDE! Best solution ever! So simple and it will work perfectly for my prototype! Thanks for all of your help on this. Much appreciated.

Don't forget to mark this as best answer !

WhiteTyrone (author)  WhiteTyrone2 years ago

sorry for the multiple typos, writing this from my phone.