LM741 and RF NOISE!!! - diyAudio
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Old 8th January 2004, 02:43 AM   #1
nfway is offline nfway  United States
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Default LM741 and RF NOISE!!!

Hello,

I'm fairly new to the world of electronics, and I'm doing the best I can here, but I'm stuck, and cannot find a way out.

I have added a very basic preamp to my homebuilt guitar amp (based on the Hitachi HA13118 BTL Amp Chip) to boost input signal, but now that I have the right amount of signal, I have tons of RF noise, and no amount of shielding eliminates the problem. I sometimes even pick up radio stations.

Is there any way of eliminating all this RF in my circuit so that I can still keep the gain/output the way it is? Maybe use a different OPAMP?? I'm so confused! The sound is perfect otherwise... if I could only remove the buzz.

My basic preamp curcuit:
Click the image to open in full size.

I have additionally tried to use a non-inverting setup with the LM741, and this just does not work. All I get is a pulsating. very upset speaker output, and no audio. I'm not really sure why this is happening, but it also happens when I try the same setup with a TL082.

Any help would be so very much appreciated.

thanks,
N
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Old 8th January 2004, 04:14 AM   #2
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Are you using a positive and negative supply with that? it looks like the negative supply is just going to ground.

741 is very old OpAmp design, I think there are a lot better ones now.

I made a wah wah for my brother with one once about 20 years ago and it was pretty noisy, he liked it though
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Old 8th January 2004, 04:42 AM   #3
nfway is offline nfway  United States
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Apologies. I should have said that earlier. I am using a single 12V supply.

I have confirmed that the noise is coming from the lm741 because I have connected my acoustic guitar (with built-in preamp), and the amp sounds great... no noise at all.
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Old 8th January 2004, 06:33 AM   #4
dhaen is offline dhaen  Europe
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If you are using single rail 12v: Pin 3 must be connected to a potential divider to supply 6v. Just 2 equal value resistors will do.

It's surprising that a 741 can "see' any RF. It's so slow!
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Old 8th January 2004, 06:45 AM   #5
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I believe pin 3 should be connected to a reference voltage "pseudo ground" made with a voltage divider. it should be connected through a resistor close to R1 + R2 to reduce errors from bias current.
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Old 8th January 2004, 06:52 AM   #6
nfway is offline nfway  United States
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I've tried that, but the second I do, I get a heavy pulsating in the speaker for about 3 seconds. Once the pulsating stops I tried the guitar, and I get a very reduced output.

The only way I get the output desired is through the circuit in my earlier post, albeit with the nasty noise.
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Old 8th January 2004, 07:02 AM   #7
nfway is offline nfway  United States
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btw, the resistor value I used for the divider circuit was 2 - 10K resistors.


N
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Old 8th January 2004, 07:11 AM   #8
nfway is offline nfway  United States
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Connecting pin 3 to "pseudo ground" created by the voltage divider circuit of 2 - 10K resistors gives me constant pulsating in the speaker with no audio.

Connecting pin 3 to +6V gives me the pulsating for about 3 seconds and reduced audio as replied to earlier.

thanks,
N
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Old 8th January 2004, 07:43 AM   #9
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Quote:
The only way I get the output desired is through the circuit in my earlier post, albeit with the nasty noise.
But your circuit is nevertheless defective - it only amplifies one half of the waveform - which may be a cool thing for a guitar. Follow the suggestions above and make sure the PS lines are decoupled close to the opamp. If your biasing is correct you should get around half the supply voltage at opamp output. Does the buzz remain with the input shorted to gnd?

Is 10nF not too low at the input?
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Old 8th January 2004, 08:08 AM   #10
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Besides adding the voltage divider, there are some other things you could change. The 10nF at the input should be increased to 10uF or so. If there is a long cable (anything above 30cm being considered long) connected to the output, a 220 ohm resistor between the op-amp and the cable may prevent the cable capacitance from making the op-amp unstable. If you pick up RF with the input wire, it may help if you split the 1kohm resistor into two 500 ohm (or 470 ohm) parts and add a 4.7nF capacitor from the point where the resistors are connected to each other to ground. Adding a decoupling capacitor directly between pin 7 and ground may also help, particularly if the wires to the supply are long.
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