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Pre-amp to connect microphone to speakers

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Zero999:

--- Quote from: SoundFan on November 23, 2018, 04:53:59 pm ---@Hero999 First of all, thanks for your quick reply!
I'm driving a very small 0.5W and 8 ohm speaker.
You are right, the LM393 is totally inappropriate for the job but unfortunately the max9814 also doesn't work well in my project somehow (it is almost as bad as the transistor preamp with LM393).
I won't be needing much energy at all. This 0.5W speaker and headphones are the only things I want to connect to the output. And even those won't be connected simultaneously.

--- End quote ---
The MAX9814 won't drive a speaker directly, if you attempt to, it will current limit and give lots of distortion.


--- Quote from: SoundFan on November 23, 2018, 09:09:39 pm ---@Richard Crowley
Thanks for your fast reply

Yes I found out about that power consumption a few days ago. Can I use the first op-amp diagram you sent me and simply replace the LM741 by an OPA134 please ?

--- End quote ---
The OPA134 will not drive a speaker directly either.

You need to connect the output of the MAX9814 or OPA134 to a power amplifier such as the LM386 to drive a speaker.

Audioguru:
Most opamps have a minimum allowed load of 2 thousand ohms. It has a peak output current of only about 20mA which will produce a power output of about 0.0016W into an 8 ohm speaker.

A cheap clock radio produces only 0.5W. Without any clipping distortion and an 8 ohm speaker, an LM386 little power amplifier produces 0.2W with a 6V supply or 0.45W with a 9V supply.
If the microphone can hear the speaker then you will have acoustical feedback howling.

spec:

--- Quote from: Audioguru on November 26, 2018, 04:27:07 pm ---Most opamps have a minimum allowed load of 2 thousand ohms.

--- End quote ---
I am not arguing- just interested. Where does that requirement come from?

Richard Crowley:

--- Quote from: spec on November 27, 2018, 05:47:18 am ---I am not arguing- just interested. Where does that requirement come from?
--- End quote ---
Most op-amps are used for internal circuit configurations where impedances are designed around optimal power consumption and/or signal-to-noise ratio.  So there is no need to drive particularly low-impedance loads.

However, there are some kinds of op-amps which ARE designed with lower output impedances because SOME circuits require driving lower impedances.  For example op-amps designed to drive audio outputs where you might want impedances in the low 100s of ohms.  And, of course some circuits which are designed to drive speakers where impedances are around 10 ohms.

SoundFan:
Hello,
I have tried out the circuit from AudioGuru and unfortunately it still doesn't work.


--- Quote from: Hero999 on November 26, 2018, 10:01:30 am ---The OPA134 will not drive a speaker directly either.

You need to connect the output of the MAX9814 or OPA134 to a power amplifier such as the LM386 to drive a speaker.

--- End quote ---

OK, I will be ordering some LM386s. I think using the LM386 directly won't be possible because of its low gain (x20), right ? I have found a post of someone using the LM386 to connect an electret mic to a speaker : https://circuitdigest.com/electronic-circuits/lm386-audio-amplifier-circuit


It seems like this works but I think the gain is really low here. If I connect the LM386 to the OPA134, which changes should I make to the circuits please ? Would the one I've attached work pleas ?

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