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noisy audio input to system help

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andy257

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hi all,

i have been playing with a digital system for awhile now and have always had the problem of distortion due to no ac coupling at the input to my system.

today i got around to adding an ac coupling circuit to my circuit and the outcome sounds even worse than it did with no coupling. The noise is terrible.

I used the following circuit as an ac couple. I was aware the resistors may introduce noise but not this much. The noise overtakes the audio. However the small amount of audio that can be heard is no longer distorted.

I used a 1uF tant - was this a bad choice? i also used 10k resistors for the divider. Should they be higher?

If anyone has any suggestions how to remove the noise that would be great.

p.s the audio is fed into an A2D converter.

thanks

https://www.electro-tech-online.com/attachments/adcbias-jpg.17457/
 
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Use a much larger electrolytic, and put some small ceramic capacitors in parallel with the resistors. How noisy is VDD?
 
hi thanks for reply,

ive posted the circuit for the input to my system. I am wondering whether its anything to do with input impedances etc?

I dont think vdd is contributing to the noise. The power supply comes from the digital prototype board i am using and every thing else works fine.

thanks
 

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I'm assuming Vdd and Vcc are the same voltage.

If the power is coming from your digital board than the noise could be coming from that. Digital and analog do not readily mix. Digital circuits can create a great deal of power supply noise that doesn't affect the digital but will kill the analog.

Is the analog power well decoupled from the digital? An RC filter can be used. Add a resistor in series with the analog power that will give a voltage drop of about 0.1V at the current draw of the analog circuit. At the analog side of the power use a large electrolytic (>10uF) and a 0.1uF ceramic to analog ground with as short leads as possible.

Also, the ground for analog should be separated from the digital ground and connected at only one spot.

I don't think you want a capacitor in parallel with the top input divider resistor you added. It will just tend to couple noise from the power supply into the input. A small cap to analog ground is okay.
 
hi,

in my haste to upload the circuit i made an error. all components use the same supply.

i am puzzled where this noise is coming from. Without the ac coupling i hear distrortion but there is no noise. With ac coupling there is lots of noise even when i am not sending any audio out!!!

hopefully a bit more insight into the problem now.
 

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You're not listenng. Must be noise on the internet.

There's a direct path from the power supply digital noise into the input circuit through the input resistors you added. If you don't want to decouple the complete analog circuit power then at least decouple the input resistors.

Replace the top 10k ohm resistor to the power supply with two 5k ohm resistors. Connect a large capacitor (<5uF) and a 0.1uF from the junction of the two 5k resistors to common. That should reduce, if not eliminate, your noise.
 
hi crutschow,

i am sorry if i am not listening to your words. I will try and take on board what you are saying.

Do i replace the top 10k resistor with two 5k resistors in series? also do both capacitors go in series or are they in parallel.

here is what i think you mean. Having looked at it i think it should do the trick.
 

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andy257 said:
hi crutschow,

i am sorry if i am not listening to your words. I will try and take on board what you are saying.

Do i replace the top 10k resistor with two 5k resistors in series? also do both capacitors go in series or are they in parallel.

here is what i think you mean. Having looked at it i think it should do the trick.
Yes, the new schematic is correct. The resistors in series and the caps in parallel.

That should help. Let us know.
 
hi

just tried new modifications and the noise is still there. It does sound a little bit better but that could just be me thinking it sounds better when actually its the same.
 
Hey andy, long time no see...

This is exactly the problem I have with my setup. Although mine uses different ADC/DAC's I'm getting noise.

At first glance that input looks fine to me. Others have suggested that 'digital noise' could be the problem, and undoubtably there WILL be digital noise...but for it to be loud enough to drown out the actual signal..when the power supply is decoupled as in your schem? Not likely. At most there'll be an annoying hiss.

As for debugging, its a bit difficult to do it with a complete system. That is
analogue in -> buffer -> amp -> ADC -> digital stuff - > DAC -> analogue out.

So, if you assume that the DAC is getting the right information, in the right format, and the ADC is working fine. Try grounding the ADC input. Effectively that should output 0's for the sample, and the DAC should output 0v. Therefore you would only be left with noise from the analogue parts.

Also, I don't want to get into another issue here, but if the output of your DAC is say 0-5V...what are you plugging this into to 'hear' the output? One silly thing I did with my setup was forget to attenuate the signal before putting it into an amp...so I heard noise..but that was just low level noise, I just so happened to be amplifying it 20x so it clipped the amp. No audio was detected because of the same reason. So, I shall dig up the data sheet you provided for the DAC board, and have a butchers at the output circuit.

If you could remind me of the part numbers (or even link the datasheets again) in case I can't find them, I would be grateful.

Scott.

Ps. you got a provate message.
 
Hi Blueteeth, Thanks for the reply. Like i said in the PM i sent you i had a break through. The noise in my opinion was not coming from the power supply. It was far too loud to be noise. Just out of pure chance i was playing around with some capacitors and i managed to resolve the problem by doing the following modification.

I am baffled how it is actually working. The quality is not bad and there is no noise anymore, the distortion has gone also.
 

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Hmmm,

So, you're plugging in an audio source directly to teh RCA jack...then just adding a cap between the jack and input circuitry? Cool! Well if it works, it works...just one components though? Amazing..

Blueteeth
 
What was the modification? Did you reverse the polarity of the input capacitor? The + should be on the circuit side since the circuit is more postive than the input signal.

-----------------
Experience: The ability to recognize a mistake the second time.
 
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