Humm stops as soon as the coil is disconnected. Again there is no signal to the speaker at all. No Amps in circuit. Just an open baffle setup with three drivers one 16mh coil and a bunch of caps. Just loose cables to the LF xover and a second xover between the mid and tweeter. No devices. It's hard to wrap ones head around it.
Fascinating stuff, Mr. Rockaway. 
Just for interest I looked at the coil/capacitor values that might resonate at 60Hz with the low damping of a bass unit, which often has an Fs around 50-60Hz.
Alan Yates' Laboratory - VK2ZAY's Engineering Calculators - LCR Resonance
It comes out at 16mH and 500uF for about 60Hz. I don't know anything much about your speaker, of course. I think we are assuming there is some sort of return path for the current which might be induced by a stray magnetic field. So you would be interested if shorting the speaker input affects this.
A 16mH toroidal coil ought to be immune to stray magnetic fields. As would two 8mH coils wired in series, but oppositely aligned. Some magnetic shielding is affected by using a thick metal box to enclose a coil, but as you observe, it might affect the inductance value slightly by inducing currents in the box.
I could digress and talk about how SMPS power supplies in computers seriously degrade the quality of the mains coming into your house and put a DC offset on the AC supply which affects the amplifier's power rails. How amplifiers can pick up noise on the Earth and Neutral rails of your mains. How what seems to be an Earth is often just a noisy reference point. And Radio Interference, of course, which is often a problem near radio transmitters.
There is also a phenomena known as poor common mode rejection ratio, which makes amps pick up Earth rail noise caused by careless design in mismatching resistors on the two arms of a differential amplifier. Good engineers give these matters some thought.
Just for interest I looked at the coil/capacitor values that might resonate at 60Hz with the low damping of a bass unit, which often has an Fs around 50-60Hz.
Alan Yates' Laboratory - VK2ZAY's Engineering Calculators - LCR Resonance
It comes out at 16mH and 500uF for about 60Hz. I don't know anything much about your speaker, of course. I think we are assuming there is some sort of return path for the current which might be induced by a stray magnetic field. So you would be interested if shorting the speaker input affects this.
A 16mH toroidal coil ought to be immune to stray magnetic fields. As would two 8mH coils wired in series, but oppositely aligned. Some magnetic shielding is affected by using a thick metal box to enclose a coil, but as you observe, it might affect the inductance value slightly by inducing currents in the box.
I could digress and talk about how SMPS power supplies in computers seriously degrade the quality of the mains coming into your house and put a DC offset on the AC supply which affects the amplifier's power rails. How amplifiers can pick up noise on the Earth and Neutral rails of your mains. How what seems to be an Earth is often just a noisy reference point. And Radio Interference, of course, which is often a problem near radio transmitters.
There is also a phenomena known as poor common mode rejection ratio, which makes amps pick up Earth rail noise caused by careless design in mismatching resistors on the two arms of a differential amplifier. Good engineers give these matters some thought.
Attachments
TBH, my friend, as an experienced old lag in the engineering game, and TBH, I have been around. I would never venture a conclusion without sleeping on it!
We old Engineering lags trust the "Law of the Sod" which says that anything that can go wrong, probably will.
We also trust "The Law of Unexpected Consequences" which says that a small change to a system which seems to make sense at the time, might cause utter failure due to something the original designer of a system wasn't expecting us to do.
This is why the Chernobyl Nuclear Reactor blew up. Causing untold misery to the people of the Ukraine and maybe us nearby Europeans.
So I'm gonna sleep on it. But I think we might solve this one. It's what us old lags call an INTERESTING PROBLEM. I do have a vast knowledge of valve amps too. This might have a bearing. They have high output impedance.
I hate to steel anyone's thunder, but an amplifier with a much higher damping factor (lower output impedance) will likely solve your hum problem. But this will not reduce the likely HUGE magnetic fields that seem to permeate your living environment. Resonance or not, the fields required to generate the power required for you to hear the noise in a passive speaker is not inconsequential. This level of EMF may effect your health.
Any chance of sketching out the circuit that exists in there?
I would have thought that if the 16mH indictor was for a low pass filter, then it would NEED the speaker wired shorted to be able to generate noise from magnetic coupling into the coil.
A quick question / test: If the speakers are connected to the power amp, and the power am is switched on, how much noise is generated?
- I have never seen a speaker coupling noise of an audible level from an external source...
- I HAVE seen coupling between coils in a crossover when the coils are very poorly placed.
Things you might like to do:
- Get a CRO (or even AC voltmeter) and put a coil of wire across it (say 10 turns at about 30mm diameter). Sweep the area and see what voltages it picks up. If there is astrong field, you will see it on the CRO, or measure an AC voltage of a number of mV.
- Call in an exorcist, and get him to sort out your possessed listening room and gear.
I would have thought that if the 16mH indictor was for a low pass filter, then it would NEED the speaker wired shorted to be able to generate noise from magnetic coupling into the coil.
A quick question / test: If the speakers are connected to the power amp, and the power am is switched on, how much noise is generated?
- I have never seen a speaker coupling noise of an audible level from an external source...
- I HAVE seen coupling between coils in a crossover when the coils are very poorly placed.
Things you might like to do:
- Get a CRO (or even AC voltmeter) and put a coil of wire across it (say 10 turns at about 30mm diameter). Sweep the area and see what voltages it picks up. If there is astrong field, you will see it on the CRO, or measure an AC voltage of a number of mV.
- Call in an exorcist, and get him to sort out your possessed listening room and gear.
No need for an exorcist but this (or something like this) could help:
YSHIELD® shielding paint NSF34 | LF | 1 liter | NSF34 | For electric fields | Paints | YSHIELD European Union (Inside EU)
YSHIELD® shielding paint NSF34 | LF | 1 liter | NSF34 | For electric fields | Paints | YSHIELD European Union (Inside EU)
The driver's coil can pick up magnetic stray fields as well, notably with high efficiency drivers.
Try short the driver terminals directly, if there still is hum then this is what's happening. I've had this problem once and it took me quite some time to find the root cause, in an all-active system, turned out the amplifier's mains transformer was located too close to the driver (rotating the transformer helped though not perfectly removed the issue).
Try short the driver terminals directly, if there still is hum then this is what's happening. I've had this problem once and it took me quite some time to find the root cause, in an all-active system, turned out the amplifier's mains transformer was located too close to the driver (rotating the transformer helped though not perfectly removed the issue).
Googlyone: please read the datasheet of linked product. It need to be grounded this is not just a paint, it is a shield!
Very specific and effective product to protect (shield) for emi and or rfi (both kind exist, i linked the one for low frequency protection).
This is not snake oil but serious product initially developped for health care of very sensitive persons to emi.
One coat in the inside of the enclosure and a link to ground should cure the symptom (not different from what Tannoy or other serious brands sometimes offer).
Very specific and effective product to protect (shield) for emi and or rfi (both kind exist, i linked the one for low frequency protection).
This is not snake oil but serious product initially developped for health care of very sensitive persons to emi.
One coat in the inside of the enclosure and a link to ground should cure the symptom (not different from what Tannoy or other serious brands sometimes offer).
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Krivium,
This material is effective for electric fields only. This product will not be effective at attenuating a magnetic field.
To attenuate "stop" an electric field requires a conductive film, which is what the paint on material is. (more or less the first line of the datasheet in bold... take a look).
To attenuate a magnetic field requires either a superconductor or material with a high magnetic permeability, i.e. iron / etc. Note copper is not effective at stopping magnetic fields for this exact reason.
for what it is worth, I am somewhat dubious - or possibly instead - amazed that there might be a magnetic field strong enough to couple into a speaker inductor. I still want to know what the crossover looks like, as there is still something about the observations that leaves me wondering if the issue is actually magnetic coupling.
In particular, I am wondering how the crossover is arranged such that a voltage induced in a coil creates a current in the speaker. Sometimes we look for evidence to support a theory rather than collect all the data...
This material is effective for electric fields only. This product will not be effective at attenuating a magnetic field.
To attenuate "stop" an electric field requires a conductive film, which is what the paint on material is. (more or less the first line of the datasheet in bold... take a look).
To attenuate a magnetic field requires either a superconductor or material with a high magnetic permeability, i.e. iron / etc. Note copper is not effective at stopping magnetic fields for this exact reason.
for what it is worth, I am somewhat dubious - or possibly instead - amazed that there might be a magnetic field strong enough to couple into a speaker inductor. I still want to know what the crossover looks like, as there is still something about the observations that leaves me wondering if the issue is actually magnetic coupling.
In particular, I am wondering how the crossover is arranged such that a voltage induced in a coil creates a current in the speaker. Sometimes we look for evidence to support a theory rather than collect all the data...
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Just woke up to this flurry of debate.
If memory serves me the coil is in parallel and the bank of capacitors maybe 100uf in total,
are shorted across the terminals of my LF driver which is 15ohms. Xover should be in the range of 150hz. 12" driver,15ohm, runs fullrange. LF is in parallel to the 12". Tweeter has a .47uf cap in parallel and that driver is in parallel to the 12". Drivers are as follows: wharfedale w15 (LF). Knight kn850 (basically a goodmans triaxiom) tweeter, wharfedale super3 This information will likely cause further debate. I honestly don't know what the summed impedance is for the system since the impedance info on these drivers is limited.
If memory serves me the coil is in parallel and the bank of capacitors maybe 100uf in total,
are shorted across the terminals of my LF driver which is 15ohms. Xover should be in the range of 150hz. 12" driver,15ohm, runs fullrange. LF is in parallel to the 12". Tweeter has a .47uf cap in parallel and that driver is in parallel to the 12". Drivers are as follows: wharfedale w15 (LF). Knight kn850 (basically a goodmans triaxiom) tweeter, wharfedale super3 This information will likely cause further debate. I honestly don't know what the summed impedance is for the system since the impedance info on these drivers is limited.
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this might be way off base but the more i think about your description of what you have, i think you may have succeded in creating a crude self sustaining oscillator.
loudspeakers can be microphones and with enough enviromental stimulus (background noise) to start things going your circuit becomes a self sustaining oscillator.
does mechanically blocking or touching the cone of the 12" or tweeter change anything?
6 year old's are demanding and great sources of background noise!
loudspeakers can be microphones and with enough enviromental stimulus (background noise) to start things going your circuit becomes a self sustaining oscillator.
does mechanically blocking or touching the cone of the 12" or tweeter change anything?
6 year old's are demanding and great sources of background noise!
Turk, they (6yo's) are indeed sources of interference on so many levels! I tried to restrain the cone which revealed another possible Issue. (Ugh they groaned). The 12" driver has a concentric horn tweeter that I disabled by cutting the leads to the "built in" xover. What's odd is that my cursory observation shows the sound coming from that part of the driver. There is however a whizzer that theoretically
Emits up to 6k. Very hard at the moment to differentiate the two.
Emits up to 6k. Very hard at the moment to differentiate the two.
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