Electronics > Beginners

Pre-amp to connect microphone to speakers

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Zero999:

--- Quote from: SoundFan on December 03, 2018, 10:12:48 am ---
--- Quote from: Hero999 on December 03, 2018, 09:14:26 am ---No, the output is higher in to 8 Ohms than 32 Ohms.

--- End quote ---
Yes, since power (W) = U²/R
--- End quote ---
Yes, that's what I initially thought but I was wrong. Audioguru was right. The output is current limited, so the output power will be higher into a 32 Ohm load an highest into a 16 Ohm load. P = I2R.

In real life it will probably make little difference because a typical 8 Ohm speakers actually have a higher impedance than 8 Ohms at the frequencies they work best at. I don't know whether the same is true for headphones.


--- Quote ---
--- Quote from: Hero999 on December 03, 2018, 09:14:26 am ---There isn't that much which can be learned from looking at these modules because all the ICs are black boxes. If you want to learn you're better off looking at older discrete audio amplifier schematics.

--- End quote ---
I don't just want to look at them (you're right, that would be totally pointless), I would like to look at the gain range used and at the circuit in general, I just want to find the difference between the method described in this topic and the way the board is designed, so that I can try to make a better working circuit.

--- End quote ---
No doubt the modules will be similar, if not identical to the example schematics on the data sheets. I doubt you'll learn much by studying them.
https://datasheets.maximintegrated.com/en/ds/MAX9812-MAX9813L.pdf
https://www.nxp.com/docs/en/data-sheet/TDA1308.pdf

SoundFan:
Thank you very much ! The module works well.
I just have a few more questions please, I don't know whether I should open a new thread or not:

Which are the biggest electret mic capsules out there please ? I have seen 26mm ones but they are hard to get in my location. Ideally, I would like to find one with a diameter of around 40mm to experiment with but I don't think that there are any. It doesn't actually need to be an electret mic, any 40mm mic would be really great.

I had once read a post on another forum where someone said he's sometimes making his own electret capsules in his free time. I know it probably won't be easy, but do you know how it is possible to do that please ? Which materials would I need ? Is there any tutorial out there for that please ?

Richard Crowley:
There are many sources of large-diameter condenser ("LDC") microphone capsules.  When you start getting up into those sizes, they are no longer electret (permanently-charged).

You will have to provide a rather high-voltage to "charge-up" the condenser, and then provide an extremely high-impedance first-stage amplifier. This is quite esoteric circuit design and construction.  It requires extraordinary cleanliness and isolation to avoid leakage of the high voltage, the very miniscule audio signal and the very high impedance.

There are LDC microphone kits available to help with the very tricky bits.  A "scratch-built" condenser microphone (of any size) is not the kind of circuit most people find they can accomplish before they have a good amount of experience.

Primo is a popular source of mic capsules for people making their own mics. https://www.primomic.com/
Dynamic and electret capsules up to around 14mm

Larger capsules are available from places like https://microphone-parts.com/collections/capsules
Many of those run to rather expensive, so be prepared.
OTOH there are many offers on Ebay from vendors in China selling LDC capsules for < US$20
https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=large+diaphragm+condenser+microphone+capsule



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