Hello,

I recently picked up a couple GS26347 boards (https://www.petervis.com/Electronic...fier-module/lm386-audio-amplifier-module.html) and thought these might be perfect for a simple intercom project. The specific project is to integrate an intercom into one of these swim masks we have:

The problem is that when I (or one of my kids) has one of these masks on, you can't hear anything they're saying, so I thought it would be uber cool to build in a little voice intercom into the side of the mask, along the lines of:

- Rechargeable 9v battery for power
- LM386-powered GS26347 board (in a waterproof casing) for amplification
- Microphone inside the mask (I have a few broken headsets with little condenser microphones in them, would need to test for waterproof-ness)
- External speaker (also needs to be waterproof)
- Pushbutton switch that would activate power to the device

This is the first project I've looked at that required waterproofing of any kind, so I knew I'd have some things to learn there, but to start with, I ordered some cheap speakers and mics to start testing the circuit with and also check their ability to handle water:

- Microphone: https://www.amazon.com/gp/product/B01F5592EO/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
- Speaker: https://www.amazon.com/gp/product/B00OE6606Y/ref=oh_aui_detailpage_o06_s01?ie=UTF8&psc=1

This is also the first time I've messed much with audio components, and a lot of my questions are going to be super basic, but when testing I've found that:

- Using the microphones I ordered, as well as ones taking out of headsets, I can't get hardly any sound out of the system. Perhaps the GS26347, with its gain of 200, is not powerful enough to use this type of microphone?

- Using one of the speakers listed above as a microphone, I get a very low-noise mic that works pretty well.

- The speaker is loud enough to work well if you held it up to your hear, but not loud enough to work as what's needed in this project (was hoping for about a meter of range)

- The speakers do seem to be sealed and seem to be able to take some water, not sure if they'll survive a dive to say 12+ feet or so...

Some gentle guidance on this project would be greatly appreciated. I assume I need more power? Maybe that would solve my microphone issues too? Or maybe I'm missing something that's required for these condenser-type microphones? Or maybe this project is stupid and I should stop wasting my time on it? =)

 

Post your LM386 microphone circuit.

 

You need a transistor pre amp for that chip.

 

An electret microphone cartridge needs a small amount of DC to power it. A speaker acting as a dynamic microphone does not need any external DC to operate. However. both microphone types make only millivolts of audio signal. In very round numbers, you have millivolts of source and need volts at the destination (waterproof speaker), for a gain of 1000, and the 386 tops out at 200. So for either microphone type you'll need a preamp circuit of some kind.

Adafruit has two different microphone preamp modules, one with compression and one without. Both have a bias system for an electret mic. If you want to build from scratch, Dd's circuit is an old standard.

Beware of microphone preamp modules on ebay. Many use the LM386 inappropriately as a mic preamp. It works but does not sound well, and drains batteries faster than a proper preamp circuit.

ak

 

Don't need a preamp just a bias supply for the microphone. The LM386 module has the gain set at 200 and with a typical mic output of 10mv will get you 500mw at the speaker.
SG

 

Thanks for all the replies. To answer a few questions:

- I'm using the GS26347 board, so that's my circuit. I'm just connecting power, mic, and speakers.

- I wondered if these little microphones needed some kind of preamp power to them, I measured around 1mv signal coming from the mic, and I was wondering if a gain of 200 was simply not enough to do what I needed.

I didn't see any answers referencing if I'm wasting my time using the GS26347 board or not. If I add in a preamp for the mic, in conjunction with the GS26347 board, will that produce around the power I want (1m listening range)? The speakers I have, according to the link, are rated for 200mW max, is that going to be loud enough for this to work?

I see 200mW might give me around 23dBm, and that's just slightly higher than the "rustling of leaves" according to this scale: https://www.chem.purdue.edu/chemsafety/Training/PPETrain/dblevels.htm

 

How are you connecting the mic to the amp module?
EDIT: Suspect you would need to use larger size speakers in wattage and physical size to be effective under water. Maybe 3" and rated at least 1 watt.
SG

 

Oh, let me clarify in case it wasn't clear before: I was only going to use this speaker system on the surface, when above the water. I assumed it would take way too much power to make it audible under the water (though that would be really cool, so if it could still work with a 9v system, let me know, that would be awesome!)

 

Oh, let me clarify in case it wasn't clear before: I was only going to use this speaker system on the surface, when above the water. I assumed it would take way too much power to make it audible under the water (though that would be really cool, so if it could still work with a 9v system, let me know, that would be awesome!)

OK, but I'm still waiting for you to describe how you connected the mic to the amp board.
SG

 

I simply connected the leads from the microphone (or speaker, when I was trying the speaker as a microphone) to the GND & IN pins on the board: https://www.petervis.com/Electronic...fier-module/lm386-audio-amplifier-module.html

Now, with that in mind, if I needed a preamp on the microphone, as suggested before, how would I go about that?

 

I simply connected the leads from the microphone (or speaker, when I was trying the speaker as a microphone) to the GND & IN pins on the board: https://www.petervis.com/Electronic...fier-module/lm386-audio-amplifier-module.html

Now, with that in mind, if I needed a preamp on the microphone, as suggested before, how would I go about that?

That's not correct as I stated in post #5. The condenser mic requires a DC bias. Try connecting a 10K resistor from the + lead of the mic to the + side of the 9 volt battery. Then connect a 1uf capacitor from the + lead of the mic to the amp input for starters.
SG

 

You have GS26347 boards that have no detailed descriptions. Peter vis said that it has a capacitor for a gain of 200 but it also has an RC for bass boost. The tiny speaker will not be heard 1m away with only 200mW.
Guess what? The bass boost works by reducing the gain of mid and high frequencies so their gain might be only 50 which is not enough. You do not need bass boost and the tiny speakers will not produce bass sounds anyway.
Here is how the electret mic should be powered:

 

This layout maybe less confusing for connecting the mic as a start.
SG
View attachment 144686

 

You have GS26347 boards that have no detailed descriptions. Peter vis said that it has a capacitor for a gain of 200 but it also has an RC for bass boost.

I've eyeballed the PC board and see no bass boost. Peter vis said it "appears" the components are there but they are not actually.
SG

 

Audioguru: I did read that about the bass boost, and that did occur to me that if the bass is being boosted, it's at the expense of some other frequency and detrimental to my purpose. I'll check to see if the RC is connected at pins 5 and 1.

sghioto: thank you very much for the very simple explanation. I'll try that for powering the mic and see what kind of results I get.

If the LM386 is not powerful enough, what's my next bigger (and best, cheap) option for a project like this? One of the things I liked about the GS26347 board was that it puts all the components together in a very cost-effective package (versus building them from components).

 

The LM386 is powerful enough, just need better speakers. See post #14, no bass boost on board.
SG

 

Excellent, I will see about getting more powerful speakers and powering the mic and see how it goes.

 

Your simple circuit is missing the important filtering of the bias voltage for the microphone that I showed. As the battery voltage jumps up and down with the signal then it gets fed in to the input of the amplifier to cause oscillations. I assume that the GS26347 board has all the other important filters on it.

 

Your simple circuit is missing the important filtering of the bias voltage for the microphone that I showed. As the battery voltage jumps up and down with the signal then it gets fed in to the input of the amplifier to cause oscillations. I assume that the GS26347 board has all the other important filters on it.

I didn't want to overwhelm him with too much detail at first which is why I stated in post #11, "for starters" and again in #13. Yes the mic should have a decoupling circuit as you posted.
SG

 

Ok, I'm reading to see what the decoupling circuit looks like. I'll then try to wire up that preamp circuit for the mic before it's fed into the mic input pins on the GS26347 and see how she does.