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| Pre-amp to connect microphone to speakers |
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| Zero999:
--- Quote from: spec on November 27, 2018, 05:47:18 am --- --- 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? --- End quote --- Data sheets typically show 2k when specifying the output drive. In reality there is no set minimum load impedance requirement. It's determined by the op-amp's maximum output current and the desired voltage swing. If the op-amp can drive 10mA peak, then it'll be able drive a 600Ohm load with a peak voltage of 6V. --- Quote from: SoundFan on November 27, 2018, 08:41:41 am ---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 ? --- End quote --- How much gain do you want? The LM368 configuration you linked to has a gain of 200, not 20, because C1 bypasses an emitter resistor inside the IC. The data sheet provides lots of information about configuring the LM386 for different gains. http://www.ti.com/lit/ds/sbos869/sbos869.pdf The circuit you've posted with the op-amp and LM386 stages cascaded has a total gain of 200 000, which is far too high. The single stage LM386 circuit, with a gain of 200, will probably have enough gain to drive a small speaker, form an electrect mic. |
| Audioguru:
I think either the electret mic is connected backwards or it is not an electret mic, maybe it is a dynamic mic and the bias voltage is causing it to work poorly. EDIT: I think the tiny speaker (squeaker) is fried. |
| SoundFan:
--- Quote from: Hero999 on November 27, 2018, 09:41:07 am ---How much gain do you want? The LM368 configuration you linked to has a gain of 200, not 20, because C1 bypasses an emitter resistor inside the IC. The data sheet provides lots of information about configuring the LM386 for different gains. http://www.ti.com/lit/ds/sbos869/sbos869.pdf The circuit you've posted with the op-amp and LM386 stages cascaded has a total gain of 200 000, which is far too high. The single stage LM386 circuit, with a gain of 200, will probably have enough gain to drive a small speaker, form an electrect mic. --- End quote --- I'm sorry, I thought I've read somewhere that the LM386 has a fixed gain of 20. I'll try that when the LM386s arrive. |
| SoundFan:
--- Quote from: Audioguru on November 27, 2018, 04:19:19 pm ---I think either the electret mic is connected backwards or it is not an electret mic, maybe it is a dynamic mic and the bias voltage is causing it to work poorly. EDIT: I think the tiny speaker (squeaker) is fried. --- End quote --- I've tried different mics and they work fine if connected directly to the PC. They are unpolarized but I have tried them in both directions already. The "squeaker" (I like that name) might be fried but the headphones work well. |
| Zero999:
--- Quote from: SoundFan on November 28, 2018, 09:29:07 am --- --- Quote from: Hero999 on November 27, 2018, 09:41:07 am ---How much gain do you want? The LM368 configuration you linked to has a gain of 200, not 20, because C1 bypasses an emitter resistor inside the IC. The data sheet provides lots of information about configuring the LM386 for different gains. http://www.ti.com/lit/ds/sbos869/sbos869.pdf The circuit you've posted with the op-amp and LM386 stages cascaded has a total gain of 200 000, which is far too high. The single stage LM386 circuit, with a gain of 200, will probably have enough gain to drive a small speaker, form an electrect mic. --- End quote --- I'm sorry, I thought I've read somewhere that the LM386 has a fixed gain of 20. I'll try that when the LM386s arrive. --- End quote --- Whoops, I posted a link to the OPA196, when I meant to post a link to the LM386. http://www.ti.com/lit/ds/symlink/lm386.pdf The LM386 can be configured for any gain between 20 and 200. Adding a 10µF capacitor between pins 1 & 8 increases the gain from 20 to 200. Including an additional resistor in series with the capacitor, enables the gain to be set to other values between 20 and 200. Page 8 of the data sheet shows a 1k2 resistor in series with the 10µF capacitor, gives a gain of 50. |
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