My wife and I are hearing impaired. We need an AVC amplifier for TV audio to keep the level constant going into WiFi to hearing aids. Commercial or build-it-yourself (no surface mount); Analog not digital.
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Here's a schematic from my archives that provides dual compression (signal and gain) using cadmium photo cells as variable resistors. Not normally seen in audio circuits photo cells actually have a fast enough response. This amp is designed for line level input and can use the USB jack on the TV for power (+5 volts).
The first half of U1 is the audio amp, second half is a DC amplifier to drive the LED. Limiter operation goes into effect when the LED begins to illuminate as the audio level increases. This causes both photo cells to decrease in resistance lowering the gain of the amp as well as the signal level at the input. The LED will need to be physically mounted between the photo cells in a light tight enclosure as seen in the schematic for best operation.
Devices I use that probably would do the job may not be directly applicable, but should give you some ideas.
Devices are called audio compressors or pedal amplifiers. They reduce the dynamic range of signals by amplifying the low signal levels and compressing the high signal levels.
I use a Sampson MXP124 microphone mixer. It has two channels with audio compression available. Simple, single knob per channel. Works well for what I use it for, which is a longer story not relevant here. Sampson may make much simpler equipment with similar audio compressors.
I also use a Dunlop MXR M-132 pedal amp or super compressor. Intended for professional guitar use, it is a single channel, and includes variable attack and gain. I use it with a 9-volt battery but the manual says an AC adapter is available.
Excellent device, small, but single channel (no stereo). (Pedal amplifier is a music term, do not worry, no feet required.) Again — these are sources of ideas, not necessarily solutions.
Right now, there are 304 "audio compressors" listed on ebay. Pick one within your price range and go for it.
What you want is to dynamically increase the volume of the soft passages, and diminish the volume of the loudest passages. The circuit to do this is called a compressor/limiter.
You don’t want a simple passive circuit because when you are hearing impaired, you need to retain the clarities and minimize the distortion of the higher frequencies to distinguish consonants and articulation of speech, and you don’t want to muddy the lower frequencies so you can catch inflection and vowels.
Radio stations (and to some extent TV stations) employ this to maximize modulation (so their signal comes in stronger) and yet not so loud as to over-modulate per their license.
As you experience in TV, marketing companies use separate circuits for each narrow band of frequencies to blast out commercials as loud as possible over the program material. But now you’ve got program material with softer speech passages and louder music passages and loud commercials. This is where the compressor/limiter comes in for hearing impaired use.
There are numerous examples available of all varying complexities, such as this one from Georgia Tech University: http://leachlegacy.ece.gatech.edu/ece4435/sp08/sp08dp06.pdf, or this one based on an Automatic Gain Control circuit by Jim Keith on Electro-Schematics: www.electroschematics.com/9400/audio-compressor-agc.
I myself am using a a circuit based on the TDA1054 IC as described here by P. Marion: www.electroschematics.com/232/audio-compressor.