What's the highest corner frequency....?

Started by Mark Hammer, May 04, 2018, 05:28:23 PM

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Mark Hammer

...that one could make a highpass filter for guitar, to filter out hum, without affecting "normal" tone?  Let's say one had a form of "brickwall" filter - steep slope, maybe 6-pole, with no resonant bump - to remove most hum from a guitar signal that carries too much of it.  I'm trying to think of something that would not engage in any level-based electronic "decisions", like a gate, but something that could simply work all the time, and eliminate that part of what accompanies the guitar signal that you don't need or want.

I'm guessing that this would NOT work for bass, since near-complete eliination of hum would interfere with the lowest content of a bass.

Something tells me that if it could be done, someone would have probably done it by now.  On the other hand, most noise-control devices produced to date have usually attempted to address both hum and hiss in the same monolithic unit, so maybe it just slipped under the radar.

phaeton

When you say hum, do you mean the ubiquitous 60hz hum?
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Rob Strand

#2
Probably a narrow notch filter is the tool for the job.  If you have a pot to tweak the notch frequency you can use an narrow bandwidth.  For a fixed filter the bandwidth you need to trade bandwidth and the amount of hum removed when tolerances are considered.  A very common hum filter for "HiFi" was a narrow twin-T.

The advantage of a notch over a high-pass is the notch can be deep and you don't have to trade against removing the wanted signal as much.

QuoteSomething tells me that if it could be done, someone would have probably done it by now. 
As for the high-pass there's degrees of audible.  If you do palm muting and chunking it's surprising what you can hear.  Same goes for slapping on the bass.  Whether you want to keep those low frequencies is another matter.  In practice filters have a finite slope.   I suspect you could hear the effect of a 1dB drop at the fundamental with careful A/B testing.  So 11Hz filter for a first order .  For second order order maybe 60Hz.    Obviously not much good for hum removal.  You might get away with a 70Hz cutoff for a second order and above and get a small amount of attenuation.  I don't know the answer for a brick wall filter.
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Mr.Kite

The low E on a guitar tuned at 440Hz concert pitch is around 81Hz, so, theoretically, you could cut anything below 80Hz and be fine...however, when I was modding my amps and when I make some recording, I noticed that "sub-harmonics" are useful sometimes, and the "full picture" of a guitar tone includes them: an HPF at 80Hz sounds tinny sometimes, or very focused (depending on the mix) but you can definitely hear a difference...but you can still hear some hum if it was present in the beginning, so it's not an ideal solution IMO...
As a reference, when mixing, I generally cut anything below 20-150Hz and above 8-15kHz on electric guitars, depending on the mix and the tone I'm looking for, but if you don't want to alter the tone stay below 40Hz and above 10-12kHz I'd say...

amptramp

If you are using fullwave rectifiers, the 60 Hz component cancels out and what you have left is 120 Hz and all other harmonics of 60 Hz.  The lead guitar output starts at 82 Hz, so there is no way you could filter without cutting some of the signal in-band.  Unless you have something like tube heater wiring interfering with the signal, 60 Hz is not worth cutting and if you do have 60 Hz from a heater, twisted pair heater wire is a better solution than a filter.  Many preamps use DC heater supplies just to avoid this entirely.

Rob Strand

QuoteIf you are using fullwave rectifiers, the 60 Hz component cancels out and what you have left is 120 Hz and all other harmonics of 60 Hz.
Yes, there's many different sources of "hum".  It's almost always best to get rid of the mechanism causing it than to fix it up stream.    Like filtering DC supplies.   Audio filtering should be a last resort.

Hum filters would probably work best for ground loops and magnetic coupling.   It would be least effective capacitively couple noise like you get from a guitar pickup.  The higher frequencies are boosted and fundamental isn't the main problem - that makes it go from hum to buzz.
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Transmogrifox

Quote from: amptramp on May 04, 2018, 07:00:54 PM
If you are using fullwave rectifiers...
I was wondering if somebody would bring up the harmonic content thing.  It's not only full-wave rectifiers, but any kind of nonlinear power conditioning.

For example, a 1/2-wave rectifier usually produces short-narrow high-current pulses at the peak of the waveform.  It's the commutation of the diode and rapid current pulses that are likely to couple into the audio path at periodic intervals of 60 Hz and related harmonics.

A thought experiment:  Imagine 1us, high-energy pulses coupling into the audio path on a periodic interval of 60 Hz.  Do you think you could remove that with a high pass filter?  Do you think you could better attenuate it with a low pass filter?  What about bursts of 900 MHz energy from a cell phone? Is it effective to filter at the burst rate?

Maybe a flanger tuned perfectly to 16.67ms delay would notch all the 60 Hz harmonics and remove hum.  I think we all have a good idea what that would sound like, not to mention the 16.67ms delay is discernable for some people.  Whatever the case, it sounds pretty weird when you do the filter matrix set to more than 10ms.

Ron & Rob pretty well sum it up that the best solution is to prevent it from getting into the audio path.
trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.

Danich_ivanov

I don't know the frequency, but i rebuild a wah sometime ago, and it had a ton of noise, and the lowest cap from power to ground that sorted things out was 4700uf, and i bet i could go up to 10000uf with even better results, since a tiny bit of noise is still there, but it is manageble. Not really the answer, but might be useful.

Rob Strand

QuoteI don't know the frequency, but i rebuild a wah sometime ago, and it had a ton of noise, and the lowest cap from power to ground that sorted things out was 4700uf, and i bet i could go up to 10000uf with even better results, since a tiny bit of noise is still there, but it is manageble. Not really the answer, but might be useful.
If you put a small resistance in series with the +V power rail, like 100 ohms, then put the cap after the resistor, you will find the hum is dramatically reduced and you don't need such large caps.

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samhay

I have too tried filtering out the LF stuff with very steep filters and it isn't a good option.
Gave up and spent more time shielding guitars, etc. Only so far this will work with single coils though.

Perhaps instead we should be thinking noise cancelling - i.e. the way Bose et al. do it.
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GGBB

Quote from: Transmogrifox on May 04, 2018, 10:12:29 PM
Quote from: amptramp on May 04, 2018, 07:00:54 PM
If you are using fullwave rectifiers...

...

Ron & Rob pretty well sum it up that the best solution is to prevent it from getting into the audio path.

Which is what I got from Mark's original question. Isn't he asking about single-coil mains hum, not power supply hum? That would always be 50 or 60 Hz wouldn't it?
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Digital Larry

This isn't an attempt to answer the question.  It's my version of "How I Learned to Stop Worrying and Love the Hum".

When I hear recordings which have hum or some cable noise it almost never bothers me.  Rather it imbues a sense of honesty, "you were there" or however you would describe it.  I'm adjusting my electric guitar collection so that I have more single coil guitars (hum cancelling in middle position) than humbuckers as I'm exploring surf and jazz.  This doesn't mean I never kick on the high gain channel, but I only hear the hum and buzz when I stop playing.  Lesson #2.  Never stop playing!

Glad I could help!

:icon_rolleyes:
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GGBB

Quote from: Digital Larry on May 05, 2018, 09:33:14 AM
This isn't an attempt to answer the question.  It's my version of "How I Learned to Stop Worrying and Love the Hum".

When I hear recordings which have hum or some cable noise it almost never bothers me.  Rather it imbues a sense of honesty, "you were there" or however you would describe it.  I'm adjusting my electric guitar collection so that I have more single coil guitars (hum cancelling in middle position) than humbuckers as I'm exploring surf and jazz.  This doesn't mean I never kick on the high gain channel, but I only hear the hum and buzz when I stop playing.  Lesson #2.  Never stop playing!

Glad I could help!

:icon_rolleyes:

It doesn't bother me either, but there are some situations where it needs to be avoided if possible. I play in a worship band at my church, and coil hum is actually distracting in that setting.
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Transmogrifox

Quote
Which is what I got from Mark's original question. Isn't he asking about single-coil mains hum, not power supply hum? That would always be 50 or 60 Hz wouldn't it?
That word "always" again.  There's nothing to guarantee the E&M fields correlated with mains power frequency come from a nearly pure 60 Hz source.  The wiring surrounding your stage is likely carrying harmonic currents, garbage from different equipment rectifiers, even stuff from variable frequency motors, pulsed current loads, light-load burst-mode switching power supplies, etc..

What you pick up in your single coils isn't *always* dominantly the fundamental 60Hz such that notching 60 Hz would always take out the majority of it.  Your pickups are getting the magnetic fields, which are generated by the currents, so even if the power in the outlet looks mostly pure doesn't *always* mean the magnetic fields are nearly pure 50/60Hz.

Whether "always" can be substituted with "usually" is something I don't know. Somebody who develops hum-canceling technology and studies the stuff that couples into guitars would probably know what is *usually* encountered.

I think my comment applies regardless of the source of the power frequency hum -- no matter where it's coming from it's not guaranteed to be a pure 60Hz sine wave.

The difficulty with narrow-band or steep filters is they have a long transient response.  At a cut-off in the low end the effect is likely to be audible even if (especially if) it has negligible attenuation at the lowest guitar fundamental.

That said, my intuition tells me a twin-T passive notch filter might be the best for getting the 50 or 60Hz fundamental and minimal effect on the pass-band..  It might be some of the attenuation on the low end can be corrected with an EQ without significantly pulling hum back up, since the T notch filter when matched theoretically has perfect attenuation at the notch.  A real filter is probably better because it's a little bit wider to capture the inevitable variation from true 60.0000Hz, but still very good attenuation near the notch frequency.

Make yourself a guitar booth lined with foil and ferrite plates.  I guess you have to keep eye contact with your band mates through low latency video...
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GGBB

^ TL;DR

I'm reasonably sure Mark is asking about 60Hz hum picked up by single coils. My main point was that he's not asking how to filter out rectifier or other power conditioning hum, which seems to have dominated the discussion so far. That info is useful, but most of the time what we predominantly hear is 60Hz (or 50) hum, so tackling that if possible - "at the source" is going to probably help the most.

Mark - are you out there? Is that what you are asking about?
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Rob Strand

QuoteThat info is useful, but most of the time what we predominantly hear is 60Hz (or 50) hum, so tackling that if possible - "at the source" is going to probably help the most.
I mentioned above about the pickup noise case.  It's not really a hum problem but a buzz problem.  (The harmonics get boosted at 20dB/decade so it doesn't take long before they become bigger than the fundamental even though they are smaller than the fundamental on the mains signal.)
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GGBB

Quote from: Rob Strand on May 05, 2018, 08:42:13 PM
I mentioned above about the pickup noise case.  It's not really a hum problem but a buzz problem.  (The harmonics get boosted at 20dB/decade so it doesn't take long before they become bigger than the fundamental even though they are smaller than the fundamental on the mains signal.)

So how does the notch filter suggestion help with that?
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Rob Strand

#17
QuoteSo how does the notch filter suggestion help with that?
It doesn't!   My first post was a general answer.   Amptramp posted about the PSU.   Then I posted about the pickup.   Then you guys worked out Mark might have been talking about pickups.

BTW: the DSP/noise cancelling stuff samhay mentioned effectively works out what to filter.  If you have a number if harmonic noise frequencies it will actually generate a filter which has a number of narrow notches.   For biomedical stuff you might come up with tougher algorithms that focus on the removing harmonic interference not noise in general.    If you have ever played around with noise removal software its pretty clear it can leave some unwanted artefacts - sometimes more objectionable than the original problem!  This is stuff largely impractical for analog circuits.
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According to the water analogy of electricity, transistor leakage is caused by holes.

amptramp

One way to avoid hum pickup is to use a buffer at the guitar.  It can be simple - a Tillman will do and you can put the gate resistor, FET and source resistor in the guitar or in the cable plug at the guitar end and have the drain resistor, coupling capacitor and resistor to ground in a stompbox at the entry to your pedalboard.  Your cable impedance goes from a variable (with the guitar level controls) number that may go as high as 250 Kohms to a constant 6800 ohms, meaning it will be far more immune to electrostatic fields.  If you have a magnetic field problem in the guitar, electrostatic shielding will help overcome it because the shielding acts like a mirror for magnetic fields.

In the modern world of fluorescent and LED lighting with harmonics from their internal inverters, shielding of the guitar is essential.  You have no hope of overcoming buzz in any other way.  Even old fluorescent fixtures have a hitch in the sine wave when the mercury starts conducting and they can interfere without needing an inverter to do so.

Rob Strand

QuoteOne way to avoid hum pickup is to use a buffer at the guitar.  It can be simple - a Tillman will do and you can put the gate resistor, FET and source resistor in the guitar or in the cable plug at the guitar end and have the
That only really solves coupling around to the cable.

If the pickup shielding is poor it's virtually impossible to remove electronically.  Some basses use a diff-amp to the pickup.   While that might seem to solve all the problem it doesn't since the pickup isn't physically symmetrical.   Noise couples differently to the winding start and the winding finish so the diff-amp doesn't cancel perfectly.  If you shield the windings it evens things out and the diff-amp takes up the remainder.

Some noisy pickups can be quietened by making sure the winding finish is the ground but then you might find it can get noisy touching the pole pieces.
Send:     . .- .-. - .... / - --- / --. --- .-. -
According to the water analogy of electricity, transistor leakage is caused by holes.