Hi everyone,
I just started getting into electronics, and have been reading up, and primarily watching videos on YouTube, but am starting to get more questions than I know what to do with, heh.
I would like to start by building a basic 20dB gain LM386 amplifier:
Edit:
Let me have another stab at it:
Vin (3.5mm input from phone, computer, etc.) is fed through 10k ohm pull down resistor to ensure that the input is grounded, when nothing is connected.
9v connected to Vs (pin 6), and then fed through the IC.
The output of pin 5 is then sent through another pull down resistor, with a 0.05 microfarad capacitor (possibly to filter out certain frequencies? I'm not sure what the purpose of this is).
That current is then fed through a 250 microfarad capacitor, and into a speaker.
My main questions would be:
1) What I'm missing from my current understanding of the circuit's current flow?
2) What are the purpose of the capacitors on the output?
Thanks.
Korant
I just started getting into electronics, and have been reading up, and primarily watching videos on YouTube, but am starting to get more questions than I know what to do with, heh.
I would like to start by building a basic 20dB gain LM386 amplifier:
Edit:
Here is my best understanding:
Vin (let's say 9v battery) is sent through a 10k ohm pull-down resistor (to ensure that the input is 0v when nothing is connected?), and then input into the IC.
Pins 2 & 4 are grounded (for proper grounding of the circuit), and then pin 6 would be the input source (3.5mm input from a phone/computer, etc.)
Pins 1 & 8 are left open, as by design, to ensure a gain of 20dB.
The output of pin 5 is then sent through another pull down resistor, with a 0.05 microfarad capacitor (possibly to filter out certain frequencies? I'm not sure what the purpose of this is).
That current is then fed through a 250 microfarad capacitor, and into a speaker.
Just realized that my understanding was completely incorrect.Vin (let's say 9v battery) is sent through a 10k ohm pull-down resistor (to ensure that the input is 0v when nothing is connected?), and then input into the IC.
Pins 2 & 4 are grounded (for proper grounding of the circuit), and then pin 6 would be the input source (3.5mm input from a phone/computer, etc.)
Pins 1 & 8 are left open, as by design, to ensure a gain of 20dB.
The output of pin 5 is then sent through another pull down resistor, with a 0.05 microfarad capacitor (possibly to filter out certain frequencies? I'm not sure what the purpose of this is).
That current is then fed through a 250 microfarad capacitor, and into a speaker.
Let me have another stab at it:
Vin (3.5mm input from phone, computer, etc.) is fed through 10k ohm pull down resistor to ensure that the input is grounded, when nothing is connected.
9v connected to Vs (pin 6), and then fed through the IC.
The output of pin 5 is then sent through another pull down resistor, with a 0.05 microfarad capacitor (possibly to filter out certain frequencies? I'm not sure what the purpose of this is).
That current is then fed through a 250 microfarad capacitor, and into a speaker.
My main questions would be:
1) What I'm missing from my current understanding of the circuit's current flow?
2) What are the purpose of the capacitors on the output?
Thanks.
Korant
Last edited:
I'm not an expert in this stuff, so I can't answer all of your questions.
The LM3886 datasheet might help you with some of them: http://www.ti.com/lit/ds/symlink/lm3886.pdf
I'd also suggest reading up on op-amps in general.
Vin is not a voltage input. That's your signal input from your audio device (CD player, turntable, etc...).
That's not a 10k pulldown. Its a potentiometer, used to control volume by attenuating the input signal.
I'm not sure why pins 1 and 8 are left open. It has nothing to do with gain. In fact, there is no feedback in this circuit, so gain isn't really defined. This circuit will not act nicely, let alone do what you want.
Pins 1 and 5 are positive supply, so usually they're both supposed to be connected to V+.
Pin 8 is the mute pin - according to the datasheet, an open pin means the chip is muted, so it won't work at all. Maybe I'm missing something here, but I thought the mute pin needed to be tied to -ve supply with an appropriate pulldown resistor to draw >0.5mA.
Pin 7 is ground, so I don't know why its open. Its supposed to be, well, grounded.
The 0.05uF and 10 ohm resistor on the output form a low pass filter with a ~31kHz cutoff. Google RC lowpass filter for more information on that.
I'm not sure why there's a 250uF cap in front of the speaker. Usually you see stuff like that for removing DC offset from an AC signal.
The LM3886 datasheet might help you with some of them: http://www.ti.com/lit/ds/symlink/lm3886.pdf
I'd also suggest reading up on op-amps in general.
Vin is not a voltage input. That's your signal input from your audio device (CD player, turntable, etc...).
That's not a 10k pulldown. Its a potentiometer, used to control volume by attenuating the input signal.
I'm not sure why pins 1 and 8 are left open. It has nothing to do with gain. In fact, there is no feedback in this circuit, so gain isn't really defined. This circuit will not act nicely, let alone do what you want.
Pins 1 and 5 are positive supply, so usually they're both supposed to be connected to V+.
Pin 8 is the mute pin - according to the datasheet, an open pin means the chip is muted, so it won't work at all. Maybe I'm missing something here, but I thought the mute pin needed to be tied to -ve supply with an appropriate pulldown resistor to draw >0.5mA.
Pin 7 is ground, so I don't know why its open. Its supposed to be, well, grounded.
The 0.05uF and 10 ohm resistor on the output form a low pass filter with a ~31kHz cutoff. Google RC lowpass filter for more information on that.
I'm not sure why there's a 250uF cap in front of the speaker. Usually you see stuff like that for removing DC offset from an AC signal.
Thanks for the reply!
I realized right after I created the thread that my understanding was completely wrong.
I tried to edit it with another assumption as to what was happening in the circuit, but am no longer able to see my original message. 🙁
Also, for clarity, I'm referring to the LM386, not 3886. http://www.ti.com/lit/ds/symlink/lm386.pdf
You need to understand some of the basics methinks 🙂
The IC can be considered as having a ground connection (pin 4) and a positive supply connection (pin 6). In order to make the speaker cone move the amplifier must deliver a voltage to that speaker. So here is the first point... the output pin of the amplifier is biased (internally by the circuitry) to be at one half of the supply voltage, so 4.5 volts in the case of using a 9 volt battery. This means the output can go from 4.5 to 9 volts and from 4.5 to 0 volts. There is equal swing available each side. So that means we need the big capacitor (250uf) to block that permanent 4.5 volts DC from the speaker. The bigger the cap, the more extended the low frequency response will be. The cap lets only the AC signal through. The 10 ohm and 0.5uf form a "Zobel network" which is needed to terminate the amplifier and present a defined load at high frequencies. Without that, the amplifier could be unstable and oscillate.
The input (pin 3) must be ground referenced on this chip but we can also make that resistor variable to act as the volume control. The sliding wiper of the pot taps off a percentage of the input signal depending on its position. Pin 2 needs to be grounded for this chip to work in this circuit.
(There are two inputs, a - or inverting input and a + or non inverting input. It would be useful for you to look up simple "opamp" circuits to see how they work although the LM386 isn't a good example to learn about opamps because its purpose designed for a specific job)
The IC can be considered as having a ground connection (pin 4) and a positive supply connection (pin 6). In order to make the speaker cone move the amplifier must deliver a voltage to that speaker. So here is the first point... the output pin of the amplifier is biased (internally by the circuitry) to be at one half of the supply voltage, so 4.5 volts in the case of using a 9 volt battery. This means the output can go from 4.5 to 9 volts and from 4.5 to 0 volts. There is equal swing available each side. So that means we need the big capacitor (250uf) to block that permanent 4.5 volts DC from the speaker. The bigger the cap, the more extended the low frequency response will be. The cap lets only the AC signal through. The 10 ohm and 0.5uf form a "Zobel network" which is needed to terminate the amplifier and present a defined load at high frequencies. Without that, the amplifier could be unstable and oscillate.
The input (pin 3) must be ground referenced on this chip but we can also make that resistor variable to act as the volume control. The sliding wiper of the pot taps off a percentage of the input signal depending on its position. Pin 2 needs to be grounded for this chip to work in this circuit.
(There are two inputs, a - or inverting input and a + or non inverting input. It would be useful for you to look up simple "opamp" circuits to see how they work although the LM386 isn't a good example to learn about opamps because its purpose designed for a specific job)
Thanks for the reply!
I definitely need to understand the basics. I basically know nothing, but have been trying to soak up as much information as I can, heh.
I'm probably (read: definitely) jumping the gun, but I've always felt that targeting a build with a purpose, and then learning contextually around that sticks with me a lot better than reading through pages and pages of textbooks.
The sorcerer to the wizard, I suppose.
I'll keep reading more and more, and will look into the things you've mentioned in your reply.
Thanks, again.
Nothing like hands on to learn. And nothing like actually building something and getting it to work 🙂 Any questions, just ask.
A couple more questions, while I have you here. 😛
(Please forgive the level of ineptness)
1) From a video I was watching yesterday, it said that there are 2 rules to opamps:
a) No current flows through the circuit (making my initial post makes little sense)
b) Closed loop circuits will always try to keep the input & output voltage the same
You mentioned that the LM386 was designed with a specific purpose, and isn't a good opamp example.
Is that due to the LM386 intentionally ignoring #2, as the voltage is subject to biasing? (via a potentiometer?)
2) I was going to follow an instructables post to build the LM386, and the schematic differed from the pdf one:
Would you be able to explain the reasoning behind the differences in the schematics? :x
(Please forgive the level of ineptness)
1) From a video I was watching yesterday, it said that there are 2 rules to opamps:
a) No current flows through the circuit (making my initial post makes little sense)
b) Closed loop circuits will always try to keep the input & output voltage the same
You mentioned that the LM386 was designed with a specific purpose, and isn't a good opamp example.
Is that due to the LM386 intentionally ignoring #2, as the voltage is subject to biasing? (via a potentiometer?)
2) I was going to follow an instructables post to build the LM386, and the schematic differed from the pdf one:
An externally hosted image should be here but it was not working when we last tested it.
Would you be able to explain the reasoning behind the differences in the schematics? :x
I'll be back in a little while 🙂
Yes the LM386 isn't a good opamp example because despite being drawn similar to an opamp it functions a little differently. (that circuit doesn't look good on a number of fronts... 100 ohm pot to control a speaker feed... no way 😀)
Yes the LM386 isn't a good opamp example because despite being drawn similar to an opamp it functions a little differently. (that circuit doesn't look good on a number of fronts... 100 ohm pot to control a speaker feed... no way 😀)
This might help a bit.
Look at post #37 here and the draw the diagram in post #1 so that you can refer to it.
http://www.diyaudio.com/forums/analog-line-level/188560-mic-pre-amp-problem-2.html#post2565522
Closed loop (that means with feedback) circuits around opamps will always try and maintain the difference between the two inputs at zero.
For your LM386 follow the datasheet examples. That one you just posted... well I'm not 100% sure it would even work tbh.
The three big differences between the LM386 and a normal opamp are,
1) The LM386 is preconfigured internally for a set gain. An opamp gives you total freedom by choice of the feedback resistors.
2) The LM386 can deliver substantial power to a low impedance load like a speaker. An op can not and typically has a 600 ohm minimum loading.
3) The LM386 is designed for single rail operation, an opamp can use either single rail or dual rail (a plus and a minus supply) and over a much wider voltage range than the LM386.
Look at post #37 here and the draw the diagram in post #1 so that you can refer to it.
http://www.diyaudio.com/forums/analog-line-level/188560-mic-pre-amp-problem-2.html#post2565522
Closed loop (that means with feedback) circuits around opamps will always try and maintain the difference between the two inputs at zero.
For your LM386 follow the datasheet examples. That one you just posted... well I'm not 100% sure it would even work tbh.
The three big differences between the LM386 and a normal opamp are,
1) The LM386 is preconfigured internally for a set gain. An opamp gives you total freedom by choice of the feedback resistors.
2) The LM386 can deliver substantial power to a low impedance load like a speaker. An op can not and typically has a 600 ohm minimum loading.
3) The LM386 is designed for single rail operation, an opamp can use either single rail or dual rail (a plus and a minus supply) and over a much wider voltage range than the LM386.
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