Simple electret mic --> headphone amplifier suggestions - Page 1
Forum section: electronics / projects

author: Delta
author_rank: Super Contributor
timestamp: July 06, 2016, 08:56:01 pm
content: What route would you recommend? Discrete transistors? (I've got a load of BC548s...) Opamps? (Some TL074s and LM358s on hand) A LM386? (would treat myself from eBay!)I'm erring towards a single transistor preamp, driving an LM386. The load will be a single earphone (not sure of impedance, whatever I've got lying around). Battery powered.The task is to build an old-fashioned hearing-aid. A box with a switch, mic, and volume control, with an earphone on a cable. Wide bandwidth and hifi is not required!The reason for this is that my Nan is a bit mutton, but refuses to wear her nice, small, expensive hearing aid. She'd would hopefully sit with a box on her lap and lift an earphone to her ear when people are talking to her. Maybe...

author: StillTrying
author_rank: Super Contributor
timestamp: July 06, 2016, 09:56:09 pm
content: Quote from: Delta on July 06, 2016, 08:56:01 pmI think the LM386 is OK as long as you leave it's gain at 20. You can higher it's volume control resistance a bit if it's being driven by just a single transistor preamp to reduce the loading.If this is powered from a 9V battery the transistor and the mics bias need plenty of decoupling from the +ve supply.

author: Audioguru
author_rank: Super Contributor
timestamp: July 07, 2016, 01:37:47 am
content: Hearing loss is normal with old age. High frequencies have the most loss so the hearing aid must have a custom equalizer with proper high frequency boost and a compressor/limiter to avoid overload from loud sounds that you will not have in the simple amplified mic you are making. Each ear might need different settings.Here are graphs of the typical amount of hearing loss for men and women at different ages and at different frequencies. But some people are worse and some people are better.Some people have profound hearing loss to the high frequency consonants sounds in speech and no hearing aid can be loud enough for those people to hear so the hearing aid takes the top octave and shifts it down one octave where the sounds can be heard. How will you do that in your contraption?

author: Delta
author_rank: Super Contributor
timestamp: July 07, 2016, 01:44:30 am
content: Quote from: Audioguru on July 07, 2016, 01:37:47 amI won't.I am going to make a battery powered mic preamp + headphone amp, just like my Dad has asked me to.I have taken on board what you have said though, so a bit of treble-boost might not go amiss.

author: Audioguru
author_rank: Super Contributor
timestamp: July 07, 2016, 02:57:48 am
content: Last year when I was 69 years old I ran out of excuses about people mumbling and radio and TV engineers having the sound set wrong. Then I received ads in the mail about free hearing tests and free 3 months demo for hearing aids. The audiologist was a professional and knew how to play random frequencies and timing so I did not guess wrongly that I heard the tones and speech during the hearing test.I was shown the graph of my normal (for my age and sex) high frequency hearing loss then I thought that I could make my own hearing aids. The demo hearing aids were tiny and were programmed wirelessly. There were about 5 settings that I wanted (music, speech, very sensitive, muted and automatic) which are selected by a button on one of the hearing aids and they wirelessly go to the same setting together. I can't Doo Dat! My design would be in a huge box with wires all over the place.What a big difference. When I put on my hearing aids in the morning then everything sounds better than when I was young and when I take them off at night I am suddenly deaf. Usually I forget about them until I hear something that makes me aware that they work really well. They were expensive but I got a discount and no interest monthly payments for one year. My government paid for a huge percentage of the cost.

author: Buriedcode
author_rank: Super Contributor
timestamp: July 07, 2016, 01:42:13 pm
content: My father is also losing his hearing - started off being directional sensitivity - I'll call him from a different floor and he'd be convinced I was on the ground floor. Now it's 'eh?' every sentence, so he got small hearing aids. Totally worth it and far better than anything I could make of reasonable size. Whilst its a nice project, like so many things these days, cheaper, easy, and better to buy one.With that said you should definitely have a limiter, adjustable treble boost, adjustable overall gain, and of course 'volume out'. You're looking at a few opamps and if you're powering it form a 9v battery, it wont' last long. Perhaps it would be better to use the ultra-miniature electret mic's, epoxied onto in-ear ear buds, the enclosure can have a belt clip. This would mean two separate circuits for stereo, and a phase shift to limit feedback. It's just so much better to buy hearing aids, but an interesting project nonetheless.

author: Delta
author_rank: Super Contributor
timestamp: July 07, 2016, 01:48:40 pm
content: Thanks for the replies, but I think some of you aren't listening (shit pun intended!)My Nan has just turned 90, and has an expensive, tailor made, in-ear hearing aid. It was designed specifically for her hearing. When she does use it, her hearing is much better. There's just one small problem with it...SHE WON'T USE IT!She doesn't like having things in her ear! She also gets quite grumpy when we try to persuade her to use it so we don't all have to shout!I am NOT trying to make something better than that, on the contrary; I am trying to make something very simple, big and bulky, that she can hold in her hand and useas and when she wants.I'm sure some of you also have lovely-but-stubborn old relatives!

author: Fungus
author_rank: Super Contributor
timestamp: July 07, 2016, 02:05:25 pm
content: You probably don't need any sort of pre-amp with an electret and an LM386 set to maximum gain.If you do, a simple transistor won't help much. Transistors are often called "amplifiers" but that doesn't make them audio amplifiers.If the LM386 by itself isn't loud enough then you need another op-amp. Another LM386 would do the job.LM386s cost about 20 cents each. Get half a dozen and have a play. All the circuits you need are in the datasheet.