LM386 - max gain and "bass boost"

Thread Starter

djbondango

Joined Jan 17, 2020
17
Hi guys.
I have a circuit for an LM386 to be used as a preamp with max gain off 200 with bass boost, which looking at most online
discussions is actually more of a mid pass boost.
Regarding the attached Schematic its a little different from what I have normally seen online, so have a few questions on it.

1. R4 and C3 im guessing RF filter?
2. C10 and C6, not seen these on any circuits but guessing just extra filtering IC power?
3. R2 and C13 (boost circuit) does it have an effect on the gain circuit Pin1,8 when added this way?

Cheers
M
 

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MrChips

Joined Oct 2, 2009
21,161
LM386 is an output amplifier, not a preamp. You should be using an opamp for the preamp stage.
LM386 is a low power audio amplifier that can only output about ¼W. You are not going to get much bass power out of such a low power amplifier.
 

Thread Starter

djbondango

Joined Jan 17, 2020
17
LM386 is an output amplifier, not a preamp. You should be using an opamp for the preamp stage.
LM386 is a low power audio amplifier that can only output about ¼W. You are not going to get much bass power out of such a low power amplifier.
[/QUOTE
Hi
The circuit is to amplify a mic signal before sending to a headphone IC. I have used a $2 LM386 x200 gain module from ebay for testing which works perfectly fine, however a little noisey as no real power bypassing on it. So, as it works for my purpose i just want to improve on the design.
 

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AnalogKid

Joined Aug 1, 2013
8,468
While there are lotsa low-cost ebay modules and websites that use the 386 as an audio preamp, it really is a poor choice. Granted, its gain-bandwidth product is higher than most "normal" opamps, but it has relatively high noise and distortion. OK for intercom audio, but I'd recommend against it for any application needing more fidelity.

ak
 

Thread Starter

djbondango

Joined Jan 17, 2020
17
Thanks for the replys. Yes, this is not in anyway a pro audio setup - its just for monitoring purposes. I have tried many many circuits designs but this one actually works, hence the reason i have tried to improve the design somewhat. So, i am just looking suggestions or changes to the circuit i have provided. As the el cheap ebay module works for my requirement, then it just improvements on it i would like to implement.

M
 

Wolframore

Joined Jan 21, 2019
1,748
Why not just design a proper preamp stage to the 386 output? You would get more control using an active filter... again you’re limited by the 386 but it would be better.
 

Thread Starter

djbondango

Joined Jan 17, 2020
17
Why not just design a proper preamp stage to the 386 output? You would get more control using an active filter... again you’re limited by the 386 but it would be better.
Well, this works for my purposes, has a very small footprint and as mainly smd fits my needs. That why I just want to improve if possible on the ebay modules short comings (your attached schematic) . The original schematic i posted is an attempt at doing so, and would have liked some input on tht design or changes. All compontnets are in SMD form including electrolytes.

M
 

Thread Starter

djbondango

Joined Jan 17, 2020
17
To be clear, are you modifying a pre-built module, or cloning that design with changes?

C6 is connected from GND to GND.

Yes cloning that design, and yes, had spotted just spotted that error - so left side ground connection is now removed, thanks :)
 
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Audioguru again

Joined Oct 21, 2019
1,509
Where did you get the schematic for the Chinese module? Its C3 will kill and muffle all audio frequencies if the volume control is turned down a little.
The value of C21 is high so that it can drive an 8 ohm speaker. it will take "all day" to charge when feeding the 100k input of a headphones amplifier. The value of C7 will also take time to charge and its high value is for earthquake frequencies, not audio.
The schematic in post #4 also has errors.

Power bypassing has nothing to do with the noisy transistors in the LM386 power amplifier.
What is your signal source that needs such a high gain?


The "bass boost" fools you because it does not boost anything, Instead it actually reduces the gain of the highs. The datasheet shows it on a graph:
 

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MrChips

Joined Oct 2, 2009
21,161
If you want lower frequency response you need to increase the value of the output capacitor.

Edit: AG types faster than me.
 

Papabravo

Joined Feb 24, 2006
13,759
Well, this works for my purposes, ...
This is probably the worst justification for going down a blind alley that I've ever heard. It's like trying to dig a ditch with a dinner fork. Ya, you can do it, but so what. Why would you want to clone a piece of crap. It is still a piece of crap. Why not look for a good design and clone that?
 

Thread Starter

djbondango

Joined Jan 17, 2020
17
Where did you get the schematic for the Chinese module? Its C3 will kill and muffle all audio frequencies if the volume control is turned down a little.
The value of C21 is high so that it can drive an 8 ohm speaker. it will take "all day" to charge when feeding the 100k input of a headphones amplifier. The value of C7 will also take time to charge and its high value is for earthquake frequencies, not audio.
The schematic in post #4 also has errors.

Power bypassing has nothing to do with the noisy transistors in the LM386 power amplifier.
What is your signal source that needs such a high gain?


The "bass boost" fools you because it does not boost anything, Instead it actually reduces the gain of the highs. The datasheet shows it on a graph:
The second schematic is of the ebay module, posted bu wolframore, the module i am currently using for testing.

I see now with the bass boost circuit it actually just a filter, I am actually looking at the reverse of that and concentrating on the 3K -12K range so not sure how to implement that.
On the first schematic would you recommend around a 1000uf on the output (C21) and omitting the input (C7) completely??
 
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Thread Starter

djbondango

Joined Jan 17, 2020
17
This is probably the worst justification for going down a blind alley that I've ever heard. It's like trying to dig a ditch with a dinner fork. Ya, you can do it, but so what. Why would you want to clone a piece of crap. It is still a piece of crap. Why not look for a good design and clone that?
I am limited to a 5V single supply, I have tried many opamp configurations (opa's ne's, jfets) - the device i am amplifing was
never intented to be used as an audio Mic - but through trial and error i have found that the 386 works for the purpose thats
why i am set in this route. I am not cloning "a piece of crap" merely trying to improve on an LM386 circuit which happens to work for my needs.
 

Papabravo

Joined Feb 24, 2006
13,759
The best advice I can give you is to set out your requirements and get a design that you understand and can live with. You're just wasting your time with a part and a board that cannot do what you want it to do.
 

Wolframore

Joined Jan 21, 2019
1,748
Well, this works for my purposes, has a very small footprint and as mainly smd fits my needs. That why I just want to improve if possible on the ebay modules short comings (your attached schematic) . The original schematic i posted is an attempt at doing so, and would have liked some input on tht design or changes. All compontnets are in SMD form including electrolytes.

M
You are getting inputs on the design. It’s not the approach that makes sense, you can add an SMD preamp and since you’re insisting on your present course you will get the results you get and that's all. You can’t boost just the bass with your approach... best you can hope for is a band pass or low pass... which will actually cut, not boost, a portion of your frequency as I thought that's your goal.
 

Papabravo

Joined Feb 24, 2006
13,759
I am limited to a 5V single supply, I have tried many opamp configurations (opa's ne's, jfets) - the device i am amplifying was
never intended to be used as an audio Mic - but through trial and error i have found that the 386 works for the purpose that's
why I am set on this route. I am not cloning "a piece of crap" merely trying to improve on an LM386 circuit which happens to work for my needs.
The LM386 itself is the piece of crap. Any circuit that includes it, by definition, is a piece of crap. Just because it happens to be one thing that "works for your needs", doesn't mean it is the only thing, or even the best thing. Just because the device you are using for an input was not intended to be used as an audio microphone doesn't mean everything you use has to be intended for something else. Of course you are free to pursue any path you desire, and you have time and energy invested in your present course of action -- so go for it.
 

Audioguru again

Joined Oct 21, 2019
1,509
The second schematic is of the ebay module, posted bu wolframore, the module i am currently using for testing.
Its values for R2 and C4 are completely wrong causing high frequency oscillation. Look at the datasheet.

I see now with the bass boost circuit it actually just a filter, I am actually looking at the reverse of that and concentrating on the 3K -12K range so not sure how to implement that.
Then remove your "bass boost" R2 and C13 and calculate the value of your C21 to pass 3kHz to the input impedance of your headphones amplifier. I agree that you do not need your C7 since the input of the LM386 is at 0VDC anyway.

You might need an additional input capacitor to prevent DC from the signal source from crackling your volume control. Then both capacitor values must be increased to avoid reducing 3kHz.
Don't bother adding an RC to reduce frequencies above 12kHz since the LM386 with a gain of 200 reduces frequencies above 12kHz anyway.

Again, what is your low level signal source?
Why do you want sounds from only 3kHz to 12kHz?? To hear LM386 hiss and birds?
 
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