Hello People
I have the attatced circuit that I require to be as quiet as possible.
Here are my questions:
1-Should I make a double sided board, one side being completely a ground only, to shield it?
2-There is a microphone input that I wish to turn completely off when not in use, to kill any noise generated by it when its not in use. How is the best way?
3-I have a external audio input that I wish to turn completely off when not in use, to kill any noise generated by it when its not in use. How is the best way?

Thank you very much, in advance for any help.
Yours truly,
Kim

 

"As quiet as possible" is NOT a meaningful or useful performance specification. If you spend a million dollars, you can get it pretty quiet. If you spend a billion dollars, you can get it quieter. But even that is probably not "as quiet as possible."

So... how quiet do you NEED it to be in order to be acceptable for your application? You need to quantify this is a way that can be measured.

 

"As quiet as possible" is NOT a meaningful or useful performance specification. If you spend a million dollars, you can get it pretty quiet. If you spend a billion dollars, you can get it quieter. But even that is probably not "as quiet as possible."

So... how quiet do you NEED it to be in order to be acceptable for your application? You need to quantify this is a way that can be measured.

As is always the case with projects that I have posted here, this is a request for quite a simple project, not NASA-class . I am asking for assistance in making this as quiet as possible within in the hobbiest-class. If simply turning down the input Potentiometers when not in use will produce the circuit to be relatively quiet, in people opinion, then so be it!

 

The traditional approach is to ground the inputs not being used.
Now, there will always be thermal noise coming from the resistors themselves. Use the lowest practical values.
Use a battery to power the circuit. Shield everything. Use decoupling caps at each stage. And yes, use a ground plane.
Now, the LM386 is definitely not a low noise amp. Improve it with a capacitor on the bypass pin, as indicated on the datasheet.

If all that fails, you can always cryogenically cool the circuit. Just kidding.

 

As is always the case with projects that I have posted here, this is a request for quite a simple project, not NASA-class . I am asking for assistance in making this as quiet as possible within in the hobbiest-class. If simply turning down the input Potentiometers when not in use will produce the circuit to be relatively quiet, in people opinion, then so be it!

You can achieve as quite as possible by grounding the signals at J2 and J3.

 

Hello People
I have the attatced circuit that I require to be as quiet as possible

When you say quiet is that for the headphone output and record out?

 

This problem may have a ridiculously simple solution.

Please state the overall problem You are trying to solve.

Why do You need absolute silence ?

Do You already have noise problems that You are trying to eliminate ?

Is your "Audio-Input" device the source of the "Noise" ?

Do You have problems related to Audio-Output-Levels ?
Where does your Microphone get it's needed Gain to bring it up to the needed "Line-Level" Output ?
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Define "Noise". Since Noise is the opposite of quiet, what kind of noise are you trying to mitigate? An automobile electrical system is "Noisy". But that's not a noise you can hear. Electrical noise can interfere with other electronics. Is this the noise you want to control? Other electronics can be putting out noise. Is this the kind of noise you're trying to shield against?

It seems like you're working with an audio system. I can imagine you want to eliminate the noises, pops and squeaks that other things emit in the form of electrical noise. AC hum is a noise that when amplified you can hear. So please define what kind of noise is it you want to eliminate?

 

I think tht I have the answers to my questions. Thank you to eveyone who has taken the time to respond

 

My response at post #5 was meant to be tongue-in-cheek.
You still have other options.

1) At J1, don't plug in a source.

2) J1 has three terminals. It is customary to use one of the terminals to ground out the input when the plug is not inserted. You might have to replace J1 with a proper shorting jack.

3) As for the microphone, some microphones have an ON/OFF switch which shorts the signal when in the OFF position.

 

I think tht I have the answers to my questions. Thank you to eveyone who has taken the time to respond

.
Ok fine, now please let everyone know what the solution turned-out to be
so that others can benefit from your experience.
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.
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All answers to my questions hare encorperated in all the responses given

 

It looks like the circuit has an optical coupler between the Q2 summing transistor and the U1 output driver, made of discrete components D2 and SC1.

Why?

Both sides of the optical isolation barrier have common power and grounds, so there is no true isolation. If SC1 really is a solar cell (as opposed to a photodiode), its frequency response will be very non-audio.

What is the intent of the circuit?

ak

 

What is the point of having a "light bridge" if the input and the output use the same power rails?

 

Where does your Microphone get it's needed Gain to bring it up to the needed "Line-Level" Output ?

Q1.

ak

 

A.K. is quite correct in post #13! In addition to poor frequency response, a solar cell may be able to detect other noise sources because of being able to respond to IR as well as visible light.
If you had requested "noise in some frequency span" to be 60dB or 80 dB below "Zero VU" then it would have been a perfectly defined requirement. Probably obtainable as well, within the class of average hobbyist capabilities. and budget.