Hum eliminator - crazy idea?

[Diablo]

Since tube amp hum is either 60 Hz or 120 Hz, I came up with a potential solution. Perhaps some of the experts on this forum could tell me if it's practical or possible or maybe it's an old idea, or that it can't work and why. The idea is to build a simple low voltage source with two outputs of AC voltage at 60 Hz and 120 Hz, and then have a polarity switch so you can change the phase and a volume control for the amplitude of the voltage. Connect to your speaker and adjust the voltage and phase to cancel all amp hum. Could this device knock the hum out of single coil pickups without changing tone?

[tubeswell]

Wouldn't it be easier to build your amp 'normally' but without that hum? (Seems like its adding another potential source of problems LoL). How would you make it work anyhow?

[Diablo]

You're right in that it's adding complexity, but I was thinking about when you have two or more amps hooked together or a standalone reverb unit + amp, and want to eliminate the ground loop hum without using a ground lift. Also, is there any other method to eliminate single coil pickup hum and still keep the tone of the single coil pickup?

[tubeswell]

Yes I have achieved 100% successful hum reduction for stand alone reverbs and am happy to tell you that:

R.G.'s grounding method really works with a stand alone reverb (and you have to isolate all but one of the input output and reverb send/ret sockets). (See article.)

Also you really need trannies with steel end caps in your main amp and in your reverb unit (to beat EMF-induced hum), and you need the unit's pan input transducer as far away as possible from any transformers.

As far as single coil hum (which has nothing to do with mains hum) goes, the only way I know of reducing that is to shield your geetar p'up cavity properly. There are various articles on the 'net about doing that, e.g. I have done this particular one before and it works well in that regard: http://www.guitarnuts.com/wiring/shielding/shield3.php

[jmaf]

Single coil hum is picked up from the environment. Playing in a studio where the lights are ohmic, not fluorescent, transformers are shielded in other rooms, etc, will lower the single coil hum down a lot. Also, what tubeswell recommened, shielding the guitar using foil or conductive paint.

About your idea, the problem is with the impedance mismatch between power and signal lines. You may cancel signals easily, due to the high impedance sources, basically just inject an inverted signal and you've got common mode rejection.

Now, with power lines it's different. If you inject a high impedance signal onto the power section, it'll be solemnly ignored. To cancel out power line hum, you'd have to inject the exact amount of opposite power, which means matching the low impedance and throwing away lots of energy(as heat).

As tubeswell said, when the subject is power lines, the best bet is to design the system well. If you really must, there are methods to get zero ripple! But that's not vintage tube amp design, you then get into 3 pin regulators and such sillicon solutions.

[diagrammatiks]

a well built amp and a well built single coil shouldn't really hum enough to go through the trouble of adding another system to the amp.

imo.

[Steve Conner]

It won't work for a whole bunch of reasons.

1. The signal has to be in the correct phase. Easy enough if you derive it from the same mains supply that's causing the unwanted hum, and provide a phase shift control. But you couldn't use it to null hum on a recording, for instance.

2. Hum is usually not just pure 60 or 120Hz. It includes a whole spray of harmonics: 180, 240, 360 etc. The harmonics are subjectively more of an annoyance than the fundamental. So, each harmonic must be cancelled in the correct magnitude and phase. Another, more intuitive way of stating this is that the unwanted hum waveform is rarely sinusoidal, and the cancellation waveform must be made the same shape only upside down.

3. I've seen DSP notch filters that do this, but I shudder to think what they'd do to your guitar tone. If you're interested there is an open source implementation by Paul Nicholson.

[Diablo]

Thanks Steve for the explanation. I was thinking that the hum may not be a pure sine wave and you verified that.

[jmaf]

If it were a matter of mimicking the hum wave form, just tap a capacitor out of +B and into a separate tube's grid, the output at the plate is phase inverted and containing all the harmonics of the original, plus some(distortion). Again that won't work because the high impedance signal will be ignored when reinjected on the power line.

[R.G.]

Actually, there is one hum eliminator I've seen that does exactly as proposed.

It takes a slightly distorted 60Hz signal from its power supply secondary, then uses filters to extract the 60Hz, 120Hz, 180Hz, and 240Hz parts of the waveform. These are then individually run into pot-controlled allpass phase shift filters, and added together to produce a hum cancellation signal. In operation, one turns the volume down on all four channels, brings up the 60Hz a bit, then diddles the phase until hum goes down, then messes with the volume, then phase, etc. until hum is as low as possible. After that, you do the 120, 180, and 240. By the time this is done, the hum is significantly reduced.

It's a super deluxe version of what the old hum-balance pot on the heater windings tried to do, and works great... if you have the patience to retune it every time something changes, like amp location/orientation, pickup orientation/location, etc. It's pretty good for tuning out hum generated inside the amp though.

However, stopping hum is much, much better than trying to cancel it out once it's already in the amp.

[teemuk]

3. I've seen DSP notch filters that do this, but I shudder to think what they'd do to your guitar tone.

I think steep notch filters @ 60Hz and @120 Hz would have very little effect since the frequency of lowest fundamental is about 80 Hz and most guitar amps really cut down lows quite a lot and rely more on amplifying the harmonics of those lower notes. This is even more prominent with high gain circuits, which are likely the ones that are also excessively troubled by hum. Notch filters to cancel hum are definitely more a problem of full-bandwidth audio amps that need to have a flat response down to 20 Hz.

Personally, I still feel the phase-inverted hum cancellation seems far more logical idea. As far as I know, it works pretty well in all the amps that use the idea.

[loudthud]

Didn't Fender make a Strat that had an extra pickup coil with no pole pieces that was selectively connected with the pickups in the proper phase to cancel the hum picked up by the pickups? The hum-canceling coil was under the pickguard where you couldn't see it.

[tubeswell]

Merlin also has a number of power supply ripple stabilisers in Chapter 6 of his 2nd book, including interesting and simple design using triodes to amplify an inverted signal to cancel out the ripple. But these would obviously use up some heater current.(However, you really should get the book to look at that). But I still think it is better to build the amp (normally) without hum.

[Diablo]

Didn't Fender make a Strat that had an extra pickup coil with no pole pieces that was selectively connected with the pickups in the proper phase to cancel the hum picked up by the pickups? The hum-canceling coil was under the pickguard where you couldn't see it.

That's an interesting idea. How was the tone with the ghost pickup in series? Did it still sound like a traditional chimey single coil PU?

[jmaf]

Didn't Fender make a Strat that had an extra pickup coil with no pole pieces that was selectively connected with the pickups in the proper phase to cancel the hum picked up by the pickups? The hum-canceling coil was under the pickguard where you couldn't see it.

Well, I always learn something new on this forum. TIL about the Marauder because of loudthud's post. Fender Marauder - Wikipedia, the free encyclopedia