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Is the LM386 Noisy ?

P

Phil Allison

Jan 1, 1970
0
Hi to all,

Natsemi do not supply a noise spec for their LM386 audio amp.

This has become a sore point with a poster called " Tom " .

Having a spare LM386N ( 8 pin DIL) in my bits box meant is was easy for me
to test one.

I used the most basic circuit with a gain of 20 dB and fitted a 47uF bypass
cap on pin7.

Input pins 2 & 3 were grounded to pin 4, DC supply came from a 9 volt radio
battery.

The output signal on pin 5 was fed first to a 60dB gain mic pre-amp and then
a 22Hz to 20 kHz audio band filter with 12 dB/ oct roll-off slopes.

The noise signal was observed on a scope and found to be hum free, then
measured with a True RMS voltmeter with over 100kHz bandwidth. The reading
was 144mV rms.

So, the LM386's output noise level was 144 uV rms.

Hence the EIN = 14.4 uV rms.

Input noise density = 100 nV/ rt Hz ( !!! )

Little wonder it is NOT speced.

Relative to a 3 volt rms output level, the s/n ratio is around 87dB.

Fine for the intended applications.




........ Phil
 
B

Bob

Jan 1, 1970
0
Phil said:
Hi to all,

Natsemi do not supply a noise spec for their LM386 audio amp.

This has become a sore point with a poster called " Tom " .

Having a spare LM386N ( 8 pin DIL) in my bits box meant is was easy for me
to test one.

I used the most basic circuit with a gain of 20 dB and fitted a 47uF bypass
cap on pin7.

Input pins 2 & 3 were grounded to pin 4, DC supply came from a 9 volt radio
battery.

The output signal on pin 5 was fed first to a 60dB gain mic pre-amp and then
a 22Hz to 20 kHz audio band filter with 12 dB/ oct roll-off slopes.

The noise signal was observed on a scope and found to be hum free, then
measured with a True RMS voltmeter with over 100kHz bandwidth. The reading
was 144mV rms.

So, the LM386's output noise level was 144 uV rms.

Hence the EIN = 14.4 uV rms.

Input noise density = 100 nV/ rt Hz ( !!! )

Little wonder it is NOT speced.

Relative to a 3 volt rms output level, the s/n ratio is around 87dB.

Fine for the intended applications.

My subjective opinion of the LM386 driving a 2.5" 8ohm loudspeaker
is "really noisy" although that was in a quiet workbench area,
in somewhere full of whiring IT equipment I might not have noticed
a thing.

A graph on the datasheet shows significant THD above 0.2w
(1.2v into 8ohm) and I recall a test sinewave starting to sound nasty
well below maximum volume.

At high output the THD reduces the s/n ratio, I'd say 80dB using your
measurement. If you want volume in reserve at normal listening
levels for a device that might be used somewhere noisy then you might
be
down to 60dB.

Anything i am likely to to build is small volume high margin or a hobby
project and I'l happily pay a couple of euro's more for something
better even if I could get away with mediocre performance.


Bob
 
P

Phil Allison

Jan 1, 1970
0
** Groper IDIOT Alert


My subjective opinion ...


** Is 100% complete shite.

A noise level of 144uV ( = 86 dB below 1 watt) is not audible via a normal
loudspeaker.





....... Phil
 
P

PeteS

Jan 1, 1970
0
Phil said:
Hi to all,

Natsemi do not supply a noise spec for their LM386 audio amp.

This has become a sore point with a poster called " Tom " .

Having a spare LM386N ( 8 pin DIL) in my bits box meant is was easy for me
to test one.

I used the most basic circuit with a gain of 20 dB and fitted a 47uF bypass
cap on pin7.

Input pins 2 & 3 were grounded to pin 4, DC supply came from a 9 volt radio
battery.

The output signal on pin 5 was fed first to a 60dB gain mic pre-amp and then
a 22Hz to 20 kHz audio band filter with 12 dB/ oct roll-off slopes.

The noise signal was observed on a scope and found to be hum free, then
measured with a True RMS voltmeter with over 100kHz bandwidth. The reading
was 144mV rms.

So, the LM386's output noise level was 144 uV rms.

Hence the EIN = 14.4 uV rms.

Input noise density = 100 nV/ rt Hz ( !!! )

Little wonder it is NOT speced.

Relative to a 3 volt rms output level, the s/n ratio is around 87dB.

Fine for the intended applications.




....... Phil

And this ignores any noise from the filter, which of course reduces the
actual noise of the LM386.

Nice job, Phil.

Cheers

PeteS
 
P

Phil Allison

Jan 1, 1970
0
"PeteS"
Phil Allison wrote:

And this ignores any noise from the filter,


** The audio band filter ??

Totally irrelevant since it come after 60dB of pre-amp gain & operates at
nearly line level.

which of course reduces the actual noise of the LM386.


** What drivel.

Care to rethink you words ??

Nice job, Phil.


** Shame you tried to spoil it.




......... Phil
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
you cannot check the noise that way ground all inputs and then measure the output. that is the best noise figure that you can get. the noise power rejection is -50db not that good so make sure that the power supply is good to begin with. otherwise that is the source of the noise . you might say that all noise regardless of gain or loop feedback is caused by the input first stage.
 
T

Tom

Jan 1, 1970
0
Phil,
So, the LM386's output noise level was 144 uV rms.

Thanks, that more or less confirms what I see. I remeasured the output
noise yesterday and it corresponds more or less with that (the figures
I gave before were from (incorrect) memory). I measure approx. 1 mV
peak-peak, and since for gaussian noise RMS ~ peak-peak/6 that implies
about 166 uV RMS.

In any case that is clearly audible as a hiss in the headphones.

So we can close that discussion...

greetings,
Tom
 
T

Tim Shoppa

Jan 1, 1970
0
Phil said:
Hi to all,

Natsemi do not supply a noise spec for their LM386 audio amp.

This has become a sore point with a poster called " Tom " .

Having a spare LM386N ( 8 pin DIL) in my bits box meant is was easy for me
to test one.

I used the most basic circuit with a gain of 20 dB and fitted a 47uF bypass
cap on pin7.

Input pins 2 & 3 were grounded to pin 4, DC supply came from a 9 volt radio
battery.

The output signal on pin 5 was fed first to a 60dB gain mic pre-amp and then
a 22Hz to 20 kHz audio band filter with 12 dB/ oct roll-off slopes.

The noise signal was observed on a scope and found to be hum free, then
measured with a True RMS voltmeter with over 100kHz bandwidth. The reading
was 144mV rms.

So, the LM386's output noise level was 144 uV rms.

Hence the EIN = 14.4 uV rms.

Input noise density = 100 nV/ rt Hz ( !!! )

Little wonder it is NOT speced.

Relative to a 3 volt rms output level, the s/n ratio is around 87dB.

Fine for the intended applications.

Very useful measurement. 87dB is damn good for a lot of applications,
and would've certainly qualified as "hi-fi" 50 years ago. (30 years
ago, probably too!)

Many hobbyist-type uses of LM386's allow a lot of power supply
feedthrough and have poor layout (large loops) that allow all sorts of
60Hz and RF hash to enter and get mixed/demodulated into what is nearly
just noise. This is part of its bad reputation. Lack of any rational
gain budget in a lot of hobbyist or cookbook type circuits don't help
either.

Even hobbyists who know how to and work hard to lay out a good PC board
to support a 600MHz $30 digi-RF IC will often neglect doing a good
layout for a part that costs pennies :).

Tim.
 
P

Phil Allison

Jan 1, 1970
0
"Tom"
Thanks, that more or less confirms what I see. I remeasured the output
noise yesterday and it corresponds more or less with that (the figures
I gave before were from (incorrect) memory). I measure approx. 1 mV
peak-peak, and since for gaussian noise RMS ~ peak-peak/6 that implies
about 166 uV RMS.

In any case that is clearly audible as a hiss in the headphones.

So we can close that discussion...



** YOU seem to be after a 1 watt ( low duty cycle) amp with about 120 dB s/n
ratio.

Dead simple.

Use a low noise op-amp like the NE5534 with about 5 times gain = circa 3 uV
output noise.

Add a pair of current boost transistors to the output - ie emitter
followers, one NPN the other PNP.

The op-amp can deliver 15 mA, the followers get you up to 500mA peak.



........ Phil
 
P

Phil Allison

Jan 1, 1970
0
"Tim Shoppa"
Very useful measurement. 87dB is damn good for a lot of applications,
and would've certainly qualified as "hi-fi" 50 years ago. (30 years
ago, probably too!)


** Consider that hi-fi amps vary from about 20 watts to 200 watts per
channel and that their s/n ratios are invariably expressed relative to FULL
power.

so 87 dB rel 1 watt

= 97 dB rel 10 watts

= 107 dB rel 100 watts


Or, looked at another way - hi-fi speakers have an electo-acoustic
conversion efficiency of about 92 dB for a 1 watt noise input at 1 metre.
Driving such a speaker with an LM386 creates a noise of 5 dB SPL at 1
etre - which is WAY below the threshold of audibility even in a quiet
room.

The OPs wacky problem is BECAUSE what he is doing with that earpiece is
SOOOO damn dangerous.

It (allegedly) delivers over 150 dB SPL for 1 watt of input.

Be like supplying a hi-fi speaker with 650,000 watts !!!!



....... Phil
 
R

RST Engineering \(jw\)

Jan 1, 1970
0
Care to post a url where this topology is discussed. I've always had a bit
of a problem with quiescent current in this circuit, especially in single
supply operation (which is what the op postulated as a requirement).

Jim
 
I

Ian Bell

Jan 1, 1970
0
Phil said:
So, the LM386's output noise level was 144 uV rms.

Hence the EIN = 14.4 uV rms.

Input noise density = 100 nV/ rt Hz ( !!! )

I think this is incorrect. The noise at the output is EIN x gain + output
noise. With only 20dB gain it is quite possible the output noise dominated.
I suggest you try again with a much higher gain. Note that 144uV is
about -80dBu which, depending on the value of feedback resistor, is
probably close to its normal output noise.

Ian
 
E

Eeyore

Jan 1, 1970
0
Ian said:
I think this is incorrect. The noise at the output is EIN x gain + output
noise. With only 20dB gain it is quite possible the output noise dominated.
I suggest you try again with a much higher gain. Note that 144uV is
about -80dBu which, depending on the value of feedback resistor, is
probably close to its normal output noise.

This should be fun !

Graham
 
P

Phil Allison

Jan 1, 1970
0
"RST Engineering (jw)"
Care to post a url where this topology is discussed. I've always had a
bit of a problem with quiescent current in this circuit, especially in
single supply operation (which is what the op postulated as a
requirement).


** The OP has no need for low distortion at high SPLs - may well be able
to use the followers in zero bias mode.
 
P

Phil Allison

Jan 1, 1970
0
"Ian Bell"
I think this is incorrect. The noise at the output is EIN x gain + output
noise. With only 20dB gain it is quite possible the output noise
dominated.


** The EIN of a circuit is DEFINED as being the measured output noise
level divided by gain.

The "E" stands for equivalent !!!

I suggest you try again with a much higher gain. Note that 144uV is
about -80dBu which, depending on the value of feedback resistor, is
probably close to its normal output noise.


** Gobbledegook.



........ Phil
 
P

Phil Allison

Jan 1, 1970
0
"RST Engineering (jw)"
That wasn't the question.


** Can you stop top posting ?

You will get no replies from me until you comply with normal etiquette.


......... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Phil Allison"


** Maths correction:


Natsemi do not supply a noise spec for their LM386 audio amp.

This has become a sore point with a poster called " Tom " .

Having a spare LM386N ( 8 pin DIL) in my bits box meant is was easy for me
to test one.

I used the most basic circuit with a gain of * 26 dB * and fitted a 47uF
bypass
cap on pin7.

Input pins 2 & 3 were grounded to pin 4, DC supply came from a 9 volt radio
battery.

The output signal on pin 5 was fed first to a 60dB gain mic pre-amp and then
a 22Hz to 20 kHz audio band filter with 12 dB/ oct roll-off slopes.

The noise signal was observed on a scope and found to be hum free, then
measured with a True RMS voltmeter with over 100kHz bandwidth. The reading
was 144mV rms.

So, the LM386's output noise level was 144 uV rms.

Hence the EIN = * 7.2 uV rms.*

Input noise density = * 50 nV/ rt Hz * ( !!! )

Little wonder it is NOT speced.

Relative to a 3 volt rms output level, the s/n ratio is around 93 dB.

Fine for the intended applications.




........ Phil
 
I

Ian Bell

Jan 1, 1970
0
Phil said:
"Ian Bell"


** The EIN of a circuit is DEFINED as being the measured output noise
level divided by gain.

The "E" stands for equivalent !!!

Fair enough. It does not invalidate my point.. It does therefore depend on
the gain of the amplifier and has two components viz: the ACTUAL noise at
the input x the gain plus the ACTUAL output noise of the op amp. Try making
EIN measurements at different gains and you will get difference answers.

Ian
 
J

Jan Panteltje

Jan 1, 1970
0
"RST Engineering (jw)"


** The OP has no need for low distortion at high SPLs - may well be able
to use the followers in zero bias mode.

That could be wrong.
If this is the thing I think it is, like they use to test your hearing,
a distorded wave at X amplituide may rattle the headphones mechanically
so although the person under test will not hear the original tone,
he / she will hear 'something'.
Having a pure undistorded wave is needed, I expect, that is why the 2 x EL34
(OK or the 50kW tubes).
LOL
 
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