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-   -   does some know how to build a 60hz hum Eliminator which stops the 60 hz hum?? (http://www.diyaudio.com/forums/solid-state/117097-does-some-know-how-build-60hz-hum-eliminator-stops-60-hz-hum.html)

prorms 9th February 2008 03:54 AM

does some know how to build a 60hz hum Eliminator which stops the 60 hz hum??
 
does some know how to build a 60hz hum Eliminator which stops the 60 hz hum?? i need some kind of filter using opamps
i need for guitar the pickup coils generate 60hz hum the guitar
will work the same as any other type of input

Nelson Pass 9th February 2008 05:16 AM

Assuming that it's the 50/60 Hz line, it's not difficult to inject some
of the line signal into a passive or active summing junction to
cancel it out. Such a system has a line sample (usually transformer
isolated) and a potentiometer which allows a +phase through
0 to -phase injection into the signal, preferrably with band-pass
filtering centered on the line frequency.

You have to adjust it at each gig, but it usually does the job.

taj 9th February 2008 07:33 AM

Interesting. My experience with guitar noise is mostly buzz picked up from stage lighting dimmers. Especially when using single coil strat type pickups. It's a sharp harmonic buzz picked up as a consequence of the tiny signal level and long unbalanced trip from the pickup to the amplifier, along with the massive amounts of AC dimmers laying nearby.

So the actual 60Hz content isn't as great as the multitude of its upper harmonics, which fall right smack in the middle of guitar sound bandwidth. A sharp filter centering on the line fundamental wouldn't help too much, but it wouldn't affect the guitar sound too much either, unless it was a bass guitar, so it's probably worth trying.

I think the best approach would be preventative measures first. Stay far away from the dimmer packs (or keep the dimmer packs far away from the stage); Reduce the number of AC cables, particularly lighting cables, laying around the stage. Make sure the guitar amp is well/properly grounded and shielded; use a VERY well shielded/maintained (ie: expensive) guitar cord; shield/ground the pot cavity inside the guitar body; use humbucker pickups if it suits 'your sound'; if you use a pedal board with multiple stomp boxes, make sure all the interconnecting cables and plugs are well shielded and grounded and their internal wiring is in top-notch condition (no missing barrels around the plugs).

Then, try some Nelson Pass magic. ;)

Or take up drums. :devilr:

..Todd

audiomachines 9th February 2008 08:38 AM

hi,

Here is what i do on live gigs, guitar amps and stomp boxes are plug on the same power strip that is plug to a line surge/conditioner or have a ground wrist strap (the one that they use when working on an esd sensitive device) and clip it to the ground portion of your Phone plug. A bit annoying but much better than a humming FOH speakers. Hope this helps.

Best Regards,
audiomachines

GRollins 10th February 2008 04:40 AM

The way I do it on the basses I build is to wind a pickup coil, but use it without a magnet. Since it has no magnetic field of its own, it does not generate a signal from the strings, however it remains sensitive to all EMF fields in the area. I then run that signal into the main output out of phase, thus canceling the hum.
Or you could just use humbucking pickups...

Grey

Gordy 10th February 2008 09:21 AM

Suggestion: 'direct injection' box consisting of the following, in this order...

- basic differential op-amp
- low pass filter (nothing to do with your 60Hz issue, but just because you can, and because it's a good place for it, with a -3dB point about two decades above your highest desired frequency)
- 60Hz series tuned filter (= inductor in parallel with resistor+capacitor)
- unity gain buffer
- a capacitor at the output (because you are likely to have a dc offset at the output. But then again, if the amp has a cap at the input then this output cap may not be required)

The basic differential amp, even with simple 1% resistors, should get you 25 - 30 dB of attenuation, and the tuned filter should do the rest.


I also agree with the suggestions from the other guys.

taj 11th February 2008 04:43 AM

Gordy,

Dumb question: What's a unity gain buffer and what does it do?

..Todd

nobody special 11th February 2008 04:37 PM

a unity gain buffer is an amp stage (usually opamp) that is configured for a gain of 1. It has a really high input impedance that doesn't load the signal, and has a low output impedance to drive a cable or input circuit. It converts impedance, basically, without affecting gain.

Gordy 11th February 2008 04:56 PM

Todd, it was not a dumb question.
The answer provided by 'nobody' was spot on.

: )

wakibaki 11th February 2008 06:04 PM

Up to 10~15 feet of cable you should not really be getting enough pickup to be heard in normal conditions, particularly as there is often some masking hum from other sources and other background noise.

You can of course use a DI box if distances are great, but if not, ensure that the cable and connectors are of good quality and that the joints are well made. Then make sure that the same applies to the guitar, and that it is well wired with spidered grounds without loops.

Failing this effecting a cure, you might want to ask another guitarist whether he gets an unusual amount of hum when he plugs into your amp. If he says yes, have the amp serviced or possibly replace it. Otherwise you might just want to ignore it. A guitar system that evidences no audible hum when listened to close up at high volume settings with the strings damped is unusual.

w


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