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Old 9th March 2016, 07:50 PM   #1
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Default LM380N Headpone amp.

Hi Guys,

Not sure if I am putting this in the correct section so apologies if not.

The sound from my PC is pretty dire (amp before so never a problem), just built up a amp based on the LM380N with a 15V 500mA regulated supply. All is working fine as I expect it would, It is basically the sound monitor from a matamp supernova.

Problem is it really starts clipping from 80% onwards, if I reduce the line input it is still the same. Any ideas or tips of things to check?

Wondering if the impedance of my headphones could be too high. Sensitivity 100dB/mW and 40 Ohm @ 1khz.
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Old 9th March 2016, 07:52 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by mark31385 View Post
it really starts clipping from 80% onwards, if I reduce the line input it is still the same.
Post the schematic.
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Old 9th March 2016, 07:55 PM   #3
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Quote:
Originally Posted by rayma View Post
Post the schematic.
Don't have the schematic at home or the software to draw one, would a picture suffice lol?
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Old 9th March 2016, 08:01 PM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by mark31385 View Post
Don't have the schematic at home or the software to draw one, would a picture suffice lol?
Can you post a link to a schematic, if you got it from somewhere?
Are there DC blocking capacitors at the inputs?
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Old 9th March 2016, 08:08 PM   #5
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Unfortunately not this has been copied from a proven working designed product and no designs are available online etc. Basically it uses 2xLM380N's yep got some 10uF caps on the inputs.
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Old 9th March 2016, 08:20 PM   #6
rayma is online now rayma  United States
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Quote:
Originally Posted by mark31385 View Post
Unfortunately not this has been copied from a proven working designed product and no designs
are available online etc. Basically it uses 2xLM380N's yep got some 10uF caps on the inputs.
Ok, then do you have a scope, generator, etc?
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Old 9th March 2016, 08:22 PM   #7
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All at work, have just got rid of all my personal test equipment due to space after a recent move.
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Old 9th March 2016, 08:25 PM   #8
rayma is online now rayma  United States
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Quote:
Originally Posted by mark31385 View Post
All at work, have just got rid of all my personal test equipment due to space after a recent move.
Ok, then run a variable amplitude 1kHz triangle wave into it. See where it clips, on both polarities.
No need for a load. Is it close to symmetric? If the amp running on a single supply, is it still biased
to half the supply voltage at the outputs?

Last edited by rayma; 9th March 2016 at 08:27 PM.
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Old 9th March 2016, 08:26 PM   #9
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Are you wanting me to inject something like 1khz and follow the input to the output and see where it starts clipping?

It is certainly clipping on the output, I can control the input from min to max (connected to PC) even on 5% volume from the PC it is still clipping. I could try changing resistor values on +Ve
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Old 9th March 2016, 08:29 PM   #10
rayma is online now rayma  United States
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Quote:
Originally Posted by mark31385 View Post
I can control the input from min to max (connected to PC) even on 5% volume from the PC it is still clipping.
It does sound like there is not a DC biasing of the circuit to half the supply.
Without that, it would be like a half wave rectifier.
https://www.ieee.li/pdf/essay/single...amp_design.pdf
http://www.ti.com/lit/an/sloa030a/sloa030a.pdf

Last edited by rayma; 9th March 2016 at 08:33 PM.
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