Hi everyone,
I am a super novice at this stuff, but I have made a "sound system". A speaker unit with 3 channels, which can listen to the 3 channels simoutanously or by demand. On the microphone unit you can setup the channels.
I want to reduce the "background" noise / noise from the amplifier. There are way too much noise, when all 3 channels are active at same time. How can I possible do this the easiest or the best way?
I hope you understand the problem and you're able to help somehow. I am very thankful for any suggestions.
Best Regards,
Michael Trolle
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Hi Vbase,
Well it is noisy when 1 channel is active, but it gets 3x worse when all 3 are active at same time. So I believe changing from LM741 to NE5534 will change a lot? But I take any advice, that can improve the sound.
The grounding / earthing is done correctly, but the hum/noisy is still there.
You did not describe the "background noise". Is it high frequency hiss, low frequency hum, low frequency rumbling or acoustical feedback howling?
The design of the 741 opamp is 47 years old. It cannot produce high audio frequencies and was never intended for audio (maybe for a telephone?) so it is noisy with hiss and rumbling.
Your circuit also shows a cheap old little LM380 power amplifier that also produces plenty of hiss and rumbling noise but a better replacement will have different pins and will not fit on the circuit board.
The circuit has a high frequency filter directly following the volume control that cuts frequencies above 457Hz when the volume control is halfway. It cuts most hiss and high frequency squealing feedback.
Acousical feedback howling occurs when the microphone can hear the speaker and causes sounds to go around and around (out the speaker, through the air, into the microphone, amplified and out the speaker again, over and over). It is stopped when the mic and speaker are far apart and when the volume control is turned down.
Low frequency hum can be caused when your wires from the microphones are not shielded audio cables and act like antennas to pickup electricity hum that is all around you.
Hi,
It seems you use serially connected electrolytic capacitors in the signal path. I personally try avoid this with high quality audio circuits.
They may cause noise.
They are polarised. Do you expect DC? If yes, are they connected the correct way? Try to use audio style capacitors. Best are film capacitors, but they are big and expensive. Maybe try a single one if it really brings a benefit.
Are the shields (ground) of the input signals connected elsewhere? Maybe causing ground loops via earth signal?
Can you post a true schematic... It may be easier to read than the pcb layout.
Klaus
Can you recommend a change for the LM380? Then I will just change the circuit board - I believe that is possible?
Do you require the same power delivery?
If you can settle with a higher power delivery, then you can use the TDA2030, which is very common. It's packaging is different, and may require heatsinking if you decide to increase the power to be dumped in the load.
In any system, the SNR (Signal to Noise Ratio) is highest at the input. It keeps on degrading as we go through the stages of the amplifier, and hence some amount of noise is added in every stage. Replacing the 741s with NE5534 will onviously yeild a higher SNR.
So, let's see how the replacement of 741s works out, and then you will be able to figure out the amount of noise generated in the power amp.
Whether it is acceptable or not? Whether you need to re design the power stage again? I think things should be more clear after the replacements.
This sounds like some HF related problem.and the speaker is catching some radio channel signals / music
Hi,
I should clarify some things:
I personally don't like (standard) electrolytic capacitors in an audio signal path.
(--> I'm not talking about power supply)
With "serially connected" I mean the capacitors act like an LPF, or for DC blocking.
If they should act as an LPF, then they are meant to block low frequency AC and DC. But they are not made for AC...so if there is AC it will cause distortions.
If they should act for DC blocking, then you should check correct polarity.
There are special - low noise - audio type electrolytic capacitors. And there are special non polarised electrolytic audio caoacitors.
This sounds like some HF related problem.
Try to install HF RC filters at each input. It's hard to recommend values, because I don't know your source impedance.
You could play with 10p to 1000p at the input and check if the sound is still OK.especially the higher audio frequencies.
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