I'm trying to built a circuit that amplifies music from a phone to a headset, based om ambient noise level. So if you're surrounded by noise, your music volume will turn up and in silent surroundings it will turn down.

My problem is that I want to implement a maximum where it stops turning up the volume no matter how much ambient noise there is, to avoid damaging the ears of the user. I think the max volume output (where the graph flattens out) should be in the range of 1-2V. I have included a figure to illustrate what I'm trying to do. I have looked into voltage regulators and I can see that it is quite simple for DC circuits, but not so much for AC circuits. Also, it would be easier to come up with a digital solution, but the learning goals for this semester limits me to work only with analog circuits... Any suggestions?

tl;dr I need an AC voltage regulator for an analog circuit


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    \$\begingroup\$ Get yourself a THAT2180 VCA, a microphone and and OP-amp and you should be good to go. \$\endgroup\$ – winny Nov 8 '16 at 9:41
  • \$\begingroup\$ Thanks Winny! Unfortunately I don't have the expertise to fully understand that solution but I'll read up on it. Is the VCA somewhat the same as an Automatic Gain Control? \$\endgroup\$ – ebonne Nov 9 '16 at 12:46
  • \$\begingroup\$ Yes! If you find a good AGC and replace the auto part with your microphone + minor tweeks, you have your desired function. \$\endgroup\$ – winny Nov 9 '16 at 15:13
  • \$\begingroup\$ Also, look for "audio compressor". \$\endgroup\$ – Peter Bennett Nov 17 '17 at 5:46
  • \$\begingroup\$ "My problem is" No, Sorry, but that's not your problem, Your first problem is distinguishing (and measuring) noise from music. If you can't do that, you can't increase the music level appropriately. And this is not a simple problem. \$\endgroup\$ – WhatRoughBeast Apr 26 at 23:15

You have a very good idea there. A way to to what you want is to build an amplifier that has the maximum gain you want. Then feed it with this jfet attenuator; JFET antenuator

The maximum volume is then limited by the maximum gain of the amplifier.

You can find more about this antennuator here; App Note for JFET as a Resistor

  • \$\begingroup\$ Thanks for the quick reply, it's really helpful! I think it's a good and simple solution, but I might have a problem because of the volume-button on the phone. If the user changes the volume on the phone, the input of the amplifier will change, which will also change the output, regardless of the maximum gain. It will allow the max output volume to be higher than I want if the user chooses to turn up the volume on the phone. What I really want is a max output volume, that can never be exceeded no matter how much ambient noise there is and how much the user has turned up the volume on the phone \$\endgroup\$ – ebonne Nov 9 '16 at 12:38

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