Removing hum

[jbord39]

Hey guys.

A recent pedal is plagued by humming when it is near other electronics.  This is all 60 Hz hum (as viewed on oscilloscope), which looks like

_______|_______|_______|_______|________
              '             '             '             '               
I am trying to build a 58-62 Hz notch filter.  The problem is I would like to supply this from a +9V source (not a dual supply).  I was wondering if anyone knew which (if any) of these op-amps could operate from +9V. (Or split +/-5V)

LM386
TL082
LM741
RC4558

Thanks,

John

[01370022]

Searching for the datasheets on google I found that all will run on +9 Volts (look under max voltage) and all 'cept the LM386 will take dual supply (+/-5V). Again, on the datasheets  .

Just as a guess and you may be ahead of me here, but have you checked what the power rail looks like on the scope? If you're getting mains hum induced on that, it'll show on the output. Chuck a whopping great big cap on the rail (>=100uF) and that may help the prob.

[CynicalMan]

The LM386 is not an op amp. It is an audio power amp. As for the others, you could use any of them, but the TL072 and 4558 are dual op amps, and you probably only need one. I'd suggest TL071 or TL081, but using the TL072 with one op amp grounded would be fine. LM741 is not ideal for audio. All of the op amps you mentioned would work from a 9V source, but you'd need to add a virtual ground to simulate a +/-4.5V supply.

[R.G.]

A recent pedal is plagued by humming when it is near other electronics.  This is all 60 Hz hum (as viewed on oscilloscope), which looks like

_______|_______|_______|_______|________
              '             '             '             '               

Actually, that's not all 60Hz. Those little pips do have a 60Hz component, but the fact that they are spikes mean that they have a big high frequency component at harmonics of 60Hz. A single frequency is a sine wave. Spaced out blips are composed of a whole array of frequencies. So...

I am trying to build a 58-62 Hz notch filter. 

This is a good first idea, but it is unlikely to do what you want, removing the hum. Even if you were able to do it perfectly, and it magically removed only the 60Hz, the buzzing would still be there, as all of the high frequency stuff remains. Worse yet, your ears will *insert* a low frequency which is the basis for a hrmonic string even if it's not actually there.

Then there is the problem of doing it perfectly. Removing only 60Hz requires a very sharp filter. The filter necessarily has some bandwidth. The narrower the frequency range it's active over, the harder it is to accurately position it. Getting this thing to exactly the middle of 58-62Hz is a daunting task for analog. There are digital filters which would do it, but they come with their own set of problems.

I encourage you to do the experiment, but I think you may find out some of these things as you do.

The problem is I would like to supply this from a +9V source (not a dual supply).  I was wondering if anyone knew which (if any) of these op-amps could operate from +9V. (Or split +/-5V)

As CyanicalMan noted.

And almost all modern opamps will work from a single supply voltage when properly biased.

Moving to the bigger issue, it would be far more fruitful to figure out why this one pedal picks up this noise, but none of the others.

[CynicalMan]

[Moving to the bigger issue, it would be far more fruitful to figure out why this one pedal picks up this noise, but none of the others.

I think the OP has given up on that:
www.diystompboxes.com/smfforum/index.php?topic=85158

[R.G.]

I think the OP has given up on that:

Ah. So it seems.

That's too bad. He's going to be one frustrated puppy over this one.

[jbord39]

Haha I have not given up, just exploring fixes.

It's strange because there is no humming at my apartment when using the pedal.  However, when I try to integrate it into my friends pedal-chain, there is a hum.  It's not horrible or anything, but it is definitely there.  I think this could be from the fact that he has a better amp (the hum is very quiet, but persistent).  Or possibly that more electronics near the pedal is causing more induced parasitic inductance/capacitance.

I built a regulated PS, which really helped the problem but did not remove the hum.

Thanks,

John

[R.G.]

However, when I try to integrate it into my friends pedal-chain, there is a hum.  It's not horrible or anything, but it is definitely there.  I think this could be from the fact that he has a better amp (the hum is very quiet, but persistent).  Or possibly that more electronics near the pedal is causing more induced parasitic inductance/capacitance.

How does your friend power his pedal chain?

WHERE do you try it in your friend's pedal chain? At his place?

A ticking buzz at line frequency is often associated with things like poorly grounded fluorescent lights.

[wavley]

I found this very interesting article from acoustics expert Ethan Winter a while back http://www.ethanwiner.com/filters.html it's a hum injector that has the offending frequencies 180 degrees out of phase and you can adjust to get rid of 60, 120, and all the harmonics.  It's a pretty interesting concept.

Finding the source of the hum and getting rid of it is really the answer.  It can be frustrating, I have a ground loop that I have been trying to bust all weekend, I have parallel effects chains going to two space echoes, when you put a meter on the two sleeves going in the space echoes I have almost 6mV of difference causing a noticeable hum, after rearranging and a few other measures I'm at like 2.4mV, we're getting there.

[R.G.]

I found this very interesting article from acoustics expert Ethan Winter a while back http://www.ethanwiner.com/filters.html it's a hum injector that has the offending frequencies 180 degrees out of phase and you can adjust to get rid of 60, 120, and all the harmonics.  It's a pretty interesting concept.

I built one of those. Works. As long as nothing changes.  It cancels any hum that's constant. Can't cancel out single coil hum though, unless you stand very still while playing.

Finding the source of the hum and getting rid of it is really the answer.  It can be frustrating, I have a ground loop that I have been trying to bust all weekend, I have parallel effects chains going to two space echoes, when you put a meter on the two sleeves going in the space echoes I have almost 6mV of difference causing a noticeable hum, after rearranging and a few other measures I'm at like 2.4mV, we're getting there.

You're dead right!!