LM741 and RF NOISE!!!

[GregGC]

Sorry for the delay in responding. I had to go out and buy some additional parts, and since Radio Shack is so well stocked, I had to take a major road trip.

Anyway, I have tried pretty much every suggestion listed, and let me say you guys are GREAT, but nothing seems to eliminate that noise. So, after deciding that I hate the LM741, I opted for using the TL082 Dual BiFET Opamp. I am only using 1/2 of the chip, but it seems to have eliminated the noise. In addition to swapping the chip, I also used some of gfiandy's suggestions (thanks so much btw).

Below is what I have ended up with. I'm still not sure that the TL082 is the right fit, but at least it doesn't produce that terrible buzz. If anyone had any suggestions for a better chip to use, I'll be glad to give it a shot. I will be trying the AD797 suggested by
jewilson (thanks).

An externally hosted image should be here but it was not working when we last tested it.

I know there must be some additions/modifications to the current curcuit, but this is my starting point (see attachment). I'm particularly concerned with the input. If a very high impedence load is plugged in, I'm worried that it might blow the opamp.

Also, (stupid question I bet), but you guys all refer to a 100R resistor. I'm guessing this is 100 Ohms. Is this correct?


Thanks again,
N

The input cap is connected in a wrong direction. The + lead should be connected to the 10k res. (I suspect your source doesn't have DC at the output and if it does it's less than 6V). Very important!!!!!

Greg

 

[nfway]

The input cap is connected in a wrong direction.

Sorry about that. My mistake in the schematic.

That looks a lot more reasonable.

I agree, but I still have some issues with this circuit. I have swapped the 56K resistors for 1K with no adverse affect, and I have also added the 100 ohm resistor at the output with no problems.

My problem comes when I turn the volume up past 3/4, I begin to get a pulsating in the speaker, and no matter what I do I cannot get rid of it unless I reduce volume back down to below 3/4.

Any ideas?

Thanks,
N

 

[GregGC]

Sorry about that. My mistake in the schematic.



I agree, but I still have some issues with this circuit. I have swapped the 56K resistors for 1K with no adverse affect, and I have also added the 100 ohm resistor at the output with no problems.

My problem comes when I turn the volume up past 3/4, I begin to get a pulsating in the speaker, and no matter what I do I cannot get rid of it unless I reduce volume back down to below 3/4.

Any ideas?

Thanks,
N

Is the 100uF cap connected corectly in your setup or it is upsidedown like on the sch. you posted above.

 

[nfway]

the 100uF cap connected corectly in your setup or it is upsidedown like on the sch. you posted above.

The 10uF cap is connected correctly... with + side connected to 10K resistor (schematic is wrong).

The 100uF is connected as shown in the schematic... with + side to ground. Is this incorrect?

Thanks,
N

 

[nfway]

I've tried reversing the 100uF position, and it did not help matters. I still get the pulsation in the speaker once I turn the vol. pot past 3/4.

 

[macboy]

The 100uF is connected as shown in the schematic... with + side to ground. Is this incorrect?

Yes. Change it.

I still get the pulsation in the speaker once I turn the vol. pot past 3/4.

Try adding a 10 k resistor between the wiper (middle) terminal of the pot and the amplifier's input.

 

[BobEllis]

I've tried reversing the 100uF position, and it did not help matters. I still get the pulsation in the speaker once I turn the vol. pot past 3/4.

pulsing like a motoroboat? Some form of oscillation - be sure that all chips (amp + pre) are bypassed as clost to the chip as possible with 100nf or so. rearranging the power leads may help, too.

 

[skyraider]

dude,
your booster has too much gain, a guitar pickup output is 30-100mV, and with gain of 100, you can expect clipping to happen in your amplifier. Im no expert in this, but I think the pulsating happens in your preamp. Try to lower your gain by lowering the 100k resistor.
another thing is that 1k is too low for your imput impedance. Try to use noninverting configuration and set at least 500k input inpedance. otherwise, you lost all the good 'highs'.
good luck

 

[skyraider]

I have built one of this. copied it off ESP website. Sounds great, and if you wanted a good preamp, I recommended this.
p/s- I eliminated the clipping diodes, though. I only used the clean sound from my amp.

 

[skyraider]

Ive built one of this preamp. Stole it off ESP webpage. Sounds great.