60-HZ Notch Filter, Wien Bridge and Twin Tee

Keith Hilton · Post by **Keith Hilton** » 8 Mar 2000 8:43 pm

Here is something interesting to try. The next time you get a "TERRIBLE" hum, try building either a Wien Bridge or Twin Tee 60-HZ Notch Filter. These are pretty common filters and diagrams can easily be found. Make this filter the last thing before the cord goes to your amp. That way it will NOTCH out any AC power hum you have picked up from the various effects, pre-amps ,stuff you have in line, and ground loops. It will also NOTCH out any AC power hum your single coil pickups have picked up. That's why they call them "PICK-UPS"! They sometimes pick-up the wrong thing, noise.
I guarantee there won't be any 60-HZ power line hum. The only problem is you will hear this frequency that is missing. In other words you hear the NOTCH. So, it is really a trade off. I suppose a filter like this would have a place. I know when I played at the Roy Clark Theater, I could not get rid of the terrible hum. Away from the Theater, the same equipment hooked up the same worked fine. The hum was so bad in the theater, I did not even enjoy playing. I wish, now that I had built one of these filters. But--I don't know which would of been worse, living with the noise, or listening to the hole or notch in your sound.

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Earnest Bovine · Post by **Earnest Bovine** » 8 Mar 2000 9:11 pm

Trouble with 60 cycle hum is that a lot of it consists of harmonics at 120, 180, Hz etc. If you cut a deep narrow notch at 60 Hz that won't help get rid of the harmonics.

Now what about running your signal thru a 1/60 sec delay, reversing the polarity, and mixing that in with you original signal? It seems to me that would cancel out 60 Hz and all the harmonics too, without sounding like a notch filter. But then I haven't actually tried it so I don't jnow.

Keith Hilton · Post by **Keith Hilton** » 9 Mar 2000 8:10 am

Thanks for the reply Earnest. I never say something won't work until I try it. I might try a variation of what you said.
By the way Earnest, the 60HZ notch filter gets rid of the 60HZ ,and any harmonic multiples. That is why there is a pretty big hole left in your sound. The hum is gone, but quality in your sound suffers greatly.

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Joe E · Post by **Joe E** » 9 Mar 2000 12:27 pm

Kieth,

How much would that affect the PSG or 6 string Guitar? I don't thinks these instruments drop to 60hz. As apposed to a bass drum. I would think the only thing missing would be the harmonics being created out of the 60 hz. Isn't it the effect of the instrument and other factors like the strings ringing that create the harmonic. The pickup should still pick this stuff up. I would thinks that only the harmonics created from the freq's of 60hz would be missing. If a freq of 120 0r 180 are played it should be there.

In other words if I never hit a note at 60hz would I ever miss the freq. Alot of time you roll of the freq's below maybe 80 or 100.

That sounded like a statement, but its really a question.[This message was edited by Joe E on 09 March 2000 at 12:29 PM.][This message was edited by Joe E on 09 March 2000 at 12:32 PM.][This message was edited by Joe E on 09 March 2000 at 12:34 PM.][This message was edited by Joe E on 09 March 2000 at 12:36 PM.]

Keith Hilton · Post by **Keith Hilton** » 9 Mar 2000 3:49 pm

Joe, I have never actually put a notch filter on my oscilloscope. I simply took the diagrams that are pretty stock for these type filters. When I built these filters I did not use 1% or even 5% stuff. The tolerance was + - 10 to 20%. I created a loud hum, for test purposes, and was interested if the filters eliminated the hum.
They eliminated every bit of it. This is power line hum---call it what you want. Then I tried playing with the filter in place to see what it sounded like. It sounded like a big hole of frequency was missing. It was not a tone you would be proud of. It made you wonder which was better, the cancer or it's cure.

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BUDDY BARROWCLIFF · Post by **BUDDY BARROWCLIFF** » 12 Mar 2000 6:16 am

Any of you guys using a profexII?
I think you will find this feature is one of the programable effects in it.
But I will agree it does suck out a big junk of the spectrum in addition to the 60 cycles.
Check it out.

Sam Marshall · Post by **Sam Marshall** » 12 Mar 2000 10:50 am

If the 60 Hz notch is removing the harmonics, then it is a pretty low Q filter (ie broad bandwith)with a deep notch. There is nothing inherent in a narrowband (hi-Q active) 60Hz notch filter that will allow it to remove harmonics.

Regards,
Sam

Joe E · Post by **Joe E** » 13 Mar 2000 9:47 am

Hey Sam, That sounds good! I think thats what I meant above.

Maybe not, but it still sounds good!

Joe[This message was edited by Joe E on 13 March 2000 at 09:48 AM.]

Bobby Lee · Post by **Bobby Lee** » 13 Mar 2000 10:19 am

The 60 cycle filter in my TubeFex cuts out so much tone that it's useless, IMO.

Donny Hinson · Post by **Donny Hinson** » 13 Mar 2000 1:56 pm

The 60-cycle notch filter will do a decent job if it's an active filter with a high-Q. I tried an op-amp t-notch filter once, but it required about a volt P/P driving it to work good (it would have to be installed between the amp stages), so I ditched the idea. A well designed notch filter won't affect all the harmonics. The biggest problems are at 60, and 120 cycles. If you build filters for these, that should eliminate most of your problems. A good filter would put these two frequencies down 30-35 db, with little effect on frequencies over +/- 2.5 cycles of the notch. Most of these severe hum problems are be caused by cheap incandescent dimmers, or neon lights.

Why amp builders don't install switchable filters in every amp is beyond me![This message was edited by Donny Hinson on 13 March 2000 at 02:04 PM.]