Schematic is incomplete

[ziggyone]

The included schematic is incomplete. Can you include the schematic for the MAX9814 IC module and the MAX98306 IC?

Thank you.

[Tarricko]

Hi! he is using the modules instead of the IC so can anyone can solder it.

[ziggyone]

I did not know that modules existed.
Thank you!

[Tarricko]

they are pretty usefull cause today ic are very complicated to soldering. i will use the issue to ask now jajaja
i would like a photograf of a prototype if its possible!
thanks

[ziggyone]

What is a recommended headphone (ear piece)?

[Tarricko]

i asume that with all of this,that u create a box with the microphone and a plug in for your headphone

[akshitgaur815]

As I am new to this, can anyone tell me if I am correctly guessing SW1 to be a switch and if yes, please tell me what kind of switch?

[bobmounger]

There are some things about the schematic that puzzle me.

If the component called MAX98306 is, as the photos seem to indicate, the breakout board of that Maxim chip, C7 is in series with a 1uF cap on the breakout board already & does nothing. In fact it creates a floating node. The minus amplifier inputs are also floating. Shouldn't they be grounded? & then having the 10k potentiometer on the output of your power amplifier is counter intuitive to me.

To my mind it would be better to remove C7 entirely & replace R1 with the pot & have the tap drive the power amplifier which would hook directly to the audio jack.

Did you try that & have a problem? I would be interested to know what it is. Or does your MAX98306 breakout board not have the input caps? But the minus inputs are still floating?

[ziggyone]

I ordered all the parts to try this project. There are many design problems with this project.

Yes, you are correct, the Adafruit MAX98306 module already has the input caps, so C7 is unnecessary.
Your are also correct that the minus amplifier inputs should be grounded.

Second, you can not cascade 2 identical passive RC high pass filters like that. The second high pass filter is presenting too great a load to the first passive high pass filter. In order to get the the desired filter slope, the impedance of the second RC filter must be an order of magnitude greater than the first RC filter, or else the loading effects will ruin you curve.

Passive RC filters must be driven by a low impedance on it's input and have a negligible load on it's output to perform properly. The second one is not driven by a low impedance, it is driven with the same impedance as itself has.

Bread boarding the circuit as shown in the schematic does not work. The losses in the filters are so great that there is not enough gain to hear anything on the amplifier output. However, if you remove the grounds from R1 and R5 (effectively removing the high pass filtering), you do get adequate volume amplification from the device, which may benefit someone with mild hearing loss, but there is no more high frequency emphasis other than that provided by the electret microphone.

As far as I can tell, R1 and R5 are not grounded in the board files that are provided, so you might get adequate volume if you use his board.

Also, the MAX98306 has considerable background noise at high gain, so it's not an ideal amplifier if you need a lot of gain.

[bobmounger]

Thanks for the reply. Good point about the filter loading. When I said "you" I meant the people who uploaded all this stuff, not you, ziggyone.

I think something like this:

https://circuitchemas.blogspot.com/2019/08/lm386-electret-microphone-amplifier.html

might work better, & be a lot cheaper. Probably need a 9V battery, but some of the things I have read say the electrets sound better if you run them around 4V anyway. Haven't tried it, but I think it would be more productive than this approach. I've got tinnitus, & I think a gain of 20 would be more than enough for my ears.

[ziggyone]

The adafruit Max9814 module has selectable gains of 40, 50, and 60 db.
The adafruit Max98306 module has 5 selectable gains of 6 db up to 18 db.
So if you leave out the passive RC filters there can be a total of 78 db from mic to earphone.

The LM386 has a selectable voltage gain of 20 to 200 (which is a gain of 26 db to 46 db)
If you aren't going to use the Adafruit Max9814, then I think you'll need some other electret mic preamp before the LM386 in those schematics you referenced. Perhaps the ones with a transistor, but I don't know the gain of the transistor circuit.

Here is a mic preamp circuit:
https://www.electronics-lab.com/project/low-noise-mini-electret-microphone-preamplifier/

But the adafuit max9814 isn't a bad preamp. It's the adafruit max98306 that isn't a very good amp bacause of the background noise (hiss) at high volume. I connected the Max9814 module output directly to my stereo hifi aux input with out the max 98306 in the circuit and then plugged headphones into the stereo. The result was much less background noise, convincing me that the max98306 and it's class C amplifier wasn't up to snuff.

[bobmounger]

Yes, but I also need to limit the low end. I think even 26dB is enough for my ears, but I need to cut off the low end so as to not blast my ears at those frequencies. I think the raw electret with a 4.7nF cap into the 10k potentiometer with 26db in the LM386 would be adequate for me. At least if my audiology report is correct. I have hearing aids, but they irritate my ears. Thanks a lot for your excellent input.

[ziggyone]

Be sure and report back. I'm interested in the same.

[bobmounger]

Well we got through the ice storm & I finally found the parts. I don't think this sounds too bad. I might want to put a cap on the gain pins of the lm386 to boost it a little more, but I kind of like the way it sounds. The click when it comes on is a little loud, but I think with a LC power supply filter it would be OK. The datasheet for the electret said to put 3V & 2.2kohms in series with the microphone. I did that & measured .17mA. With that I chose 27kohms for a bias resistor for the 9V case. I put 3nF between that & a 1

0kohm pot to ground & the wiper went to the input of the lm386. the output of the amp into my headset. I think a 400uF-ish cap in series with the output would be good to isolate the headset, but it worked.

[bobmounger]

[lightsaber2003]

Thanks for the help ziggyone. I was disappointed with my result following the diagram on my own breadboard. But i grounded the L- and R- on the amplifier input and disconnected the r1 and r5 resisters from ground but still connected together and its working much better now. less static and useful amplification. before all it was getting was high pitch sounds and lots of static when the pot was at max volume 0 ohms. but the sound obstruction from the head phones was so great that it effectively made your normal range hearing blocked off. With those changes it seem to still pick up high pitch sounds better than normal and does a good job of general sound amplification.

I also set the amp at 9db gain. my only other complaint would be if the 16 uF capacitor is useless its a hard part to source. i made mine with a 100,33,and 47 uF in series since that's supposed to be a about 16 uF and my ordered set isn't coming for a month. Also the 10k pot seems to be way too much a 5k or maybe even 1k seems like it would be enough. Also i had to unhook the ground from that pot as when the resistors went to ground i was getting sound when it was turned down at 10k on the input but was 0k from ground to the headphones.