Reduce input noise

[YUTK]

How I can reduce input noise at input stage ,How many solution to fix it

 

[sagarverma]

signal capacitor(no electrolytic,m/f or poly cap recomended)
100k resistance to gnd(m/f recommended)

see image.

Sagar

 

[murphy]

Solution: Low Input Resistance

Reduce the input resistance of your circuit. So vou reduce the voltage noise ! If possible, it ist he best solution at lowest cost.

 

[lineup]

Hello

Put one 1nF capacitor
from input transistor base
to input ground

 

[K-amps]

Hello

Put one 1nF capacitor
from input transistor base
to input ground

That would reduce the HF extension wouldnt it? i.e. roll it off.

 

[lineup]

That would reduce the HF extension wouldnt it? i.e. roll it off.

Yes, it is a lowpass filter, for high freq. Say like above 500 kHz.
The size of this cap depends on how much resitance from source output.
It is a simple RC filter.
Most every serious amplifier design use this little cap
to input ground.

Sometimes, if not using this cap,
amplifier input can act as a radio receiver and we can hear radio programs!
The components before input, if not shielded,
act as an radio antenna.
Even if we can not hear anything, the HF-noise will disturb
the amplifier operation.
Increase distortion within circuit.

 

[K-amps]

SO what kind of noise can be reduced by the 1nf...Audible Hiss? radio programs?

thanks.

 

[lineup]

SO what kind of noise can be reduced by the 1nf...Audible Hiss? radio programs?

thanks.

Anything that enters a sensitive input stage will effect the circuit.
There is no musical information above 500 kHz.
There is nothing lost by blocking HF-noise.

We are surrounded by a constant HF-noise. A background noise.
It is a mixture of many different frequencies.
We can not hear it, by our mobile telephones and radio recievers
can filter out some information and transform it into data and sound
that a human can hear and understand.

For a sound signal amplifier this noise have no value.
So we filter away this BEFORE it enters amp.
It can only disturb, upset circuit opration.

I do not know if I can explain this more clearly.
I am sure there is more on the web on this subject.

 

[K-amps]

Spare a thought for the bats that enjoy a good track or two...

By the way, from what I hear, the effects of phase shifts caused by rolling off HF at inaudible frequencies can themselves be audible. Now having said that I do not know if 500kHz roll off would have any appreciable phase shift, hence my prodding you for info.

thanks again.

 

[pinkmouse]

Wouldn't it be a better option to treat the source component that's producing the noise?

 

[K-amps]

Sure, but Al, my guess is that the source is unidentifiable...

 

[forr]

Sagarverma
"signal capacitor(no electrolytic,m/f or poly cap recomended)"

An electrolytic does not introduce noise, even less than the recomended polycap, if of greater capacitance. A 10 µF bipolar electrolytic is very good and much smaller than any plastic cap.


LINEUP
"Put one 1nF capacitor from input transistor base to input ground"

A cure which is sometimes worse than no cap at all because it can introduce oscillation in the input device. A stopper resistance (150 Ohm) after the 1 nF, in series with the base, is safe in this case.


Checking the output of the amp with a scope could help to indentify the cause of the annoying noise.

~~~~~ Forr

§§§

 

[K-amps]

In some applications I have been able to hear an electrolytic cap... horrible sounding in signal path. Some are better than others but still...

 

[sagarverma]

Sagarverma
"signal capacitor(no electrolytic,m/f or poly cap recomended)"

An electrolytic does not introduce noise, even less than the recomended polycap, if of greater capacitance. A 10 µF bipolar electrolytic is very good and much smaller than any plastic cap.

use of non-electro(m/f etc) is for better speed,i.e faster operation with the audio i/p,not bec of noise.

Even if we can not hear anything, the HF-noise will disturb

the damage that hf does is introducing phase shifts in some parts of the audio band.

 

[thanh]

perhaps your amp isn't stable . Does it receive Radio program ? my amp's too . I increased value of miler cap and problem seemed to be decreasing .
I found that increasing miler cap will cause non-smooth turning on/off of amp .
If I don't use miler cap , amp will turn on/off smoothly and there is still sound of radio program in amp , quite small but audioble easily .

 

[thanh]

ha ha ! perhaps a low Iq cause ocssilation

 

[amplifierguru]

Hi Yutk,

First you need to be more specific about the type of noise - is it hum, is it hiss, is it erratic - what is it's character. This may help to home in on the source.

First put a shorting or up to 1K input plug and see if it still is there. With no connection to other equipment.

Cheers,
Greg

 

[AndrewT]

Hi,
thanks Amp Guru, nice and simple.
Trace the problem.
The shorting plug (or two) is a must for every DIYer.

 

[Eva]

Excess RF at the input of most op-amps and amplifiers is heard just as an increased noise floor. That's why it's desirable to introduce a HF roll-off for frequencies above 20Khz (but it should be done properly).

On the other hand, if you can hear AM radio it's because some stage of your circuit is suffering from strong instability and oscillation (some redesign or layout enhancement may be required to solve this, specially when long wires are involved).