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Old 3rd March 2014, 02:01 AM   #1
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Default Amplifier circuit with tone controls HELP!!!!

Hi everyone, I am new to electronics and for my first college project I am building an amplifier circuit with bass/mid/treble tone control and VU meter. I successfully combine the LM386 with the LM3915 and got a great sound out of my speaker. I decided to add tone controls but I'm not really sure what I'm doing is right. I would like to know if my circuit looks or if not is there any advice.

I have selected those chips as I have them already and do not want to change/buy any more so what you see is what I have to work with.

I have added a picture of my circuit diagram as an attachment.

Much appreciate any expertise beyond my own noobism.

Thanks
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Old 3rd March 2014, 05:03 AM   #2
Mihkus is offline Mihkus  Estonia
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Its never going to work like that.....
VU meter and speaker on the same amplifier? I bet its peak cannot be reached.
The SIG in on that 3915 is connected to speaker gnd? why?
Theres diode and capacitor missing on your vumeter input.
Theres some components missing on your LM386 circuit.
Id add a switch between mode and + supply.


College? Well, I built something similar for my thesis but with tiny 3" fullrange speaker in small ported enclosure with large plexy vu meter on top, the sound was really good
The best you could get from portable device.

You need another amplifier for that speaker.

Last edited by Mihkus; 3rd March 2014 at 05:12 AM.
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Old 3rd March 2014, 08:49 AM   #3
mjf is offline mjf  Austria
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there are a few flaws in the circuit.........
the 741 opamp needs a dc blocking cap at the output (usually an elcap with a few uF).
the zobel at the output of the lm386 is 10 ohms (and 47nF);
--------
PSU: you need only one 100uF elcap,the second one can be a plastic cap with 100nF or so.

what is the impedance of your speaker?
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Old 3rd March 2014, 03:55 PM   #4
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The speakers are 8 ohms
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Old 3rd March 2014, 05:07 PM   #5
mjf is offline mjf  Austria
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the 220uF elcap at the lm386 output is small and cuts off the bass at 90Hz.
1/(2*pi*R *C) = 1/(6,28*8*220^-6) ~ 90Hz cut off frequency.
you can use a bigger one, 470uF.......1000uF if you like.
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Old 3rd March 2014, 08:07 PM   #6
mjf is offline mjf  Austria
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The input of the lm3915 (pin5) is connected to ground in your schematic. I think this will not work. Did you have a look at the datasheet ?

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Old 3rd March 2014, 09:35 PM   #7
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Thank you so much for your input. That is an error in the drawing I did, pin 5 is connected to the output of the 386 after said 220uf capacitor. I have changed that for a 680uf. The 386 and 3915 circuit I have built performs well, clear sound until I turn the volume up to 7 then I get distortion, is this just the poor performance of the chip or is there something I can do?
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Old 4th March 2014, 05:04 AM   #8
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Yes, The distortion that you get is most likely of the poor performance of the LM386.

You can also increase the performance of the circuit by swapping out the LM741 with something a bit better as well such as a TL071 for a single opamp type as it has the same pinout.

Or use a dual opamp type (if you want to make a stereo version) like the TL072 or other similar types like the LF35x's (3, 5, 6, 7) series of opamp chips.

If you choose to try a better power amp chip then maybe a LM1875 or TDA2003 or TDA2005 design may be a good choice as well with minimal changes, as your choices are unlimited with what is available.

The LM386 isn't designed to do much more than about 1/4 to 1/2 watt of output power.

FWIW

jer

Last edited by geraldfryjr; 4th March 2014 at 05:22 AM.
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Old 4th March 2014, 06:14 AM   #9
mjf is offline mjf  Austria
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QUOTE The 386 and 3915 circuit I have built performs well, clear sound until I turn the volume up to 7 then I get distortion, is this just the poor performance of the chip ........

In this configuration you use the lm386 ,it has a gain of 20x.......that is a lot for a chipamp with only around 0,25W output.
I presume you simply overdrived the lm386........;


What is your PSU........a 9V battery or something else?
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Old 4th March 2014, 07:51 AM   #10
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My first impression is, "WHY are you using a 741 ?". Even at unity gain they are pretty awful compared to almost any other op-amp.
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