Are Amplifiers Affected By Magnetic Flux?

[ttocs]

Mono amps are often placed directly adjacent to large speakers, so are these amplifiers influenced in any way by the magnetic energy of the woofers?

Are there design considerations for plate amps in subwoofers when placed so close to large magnets?




(I live in a State Of Flux.)

[KeithL]

The continuous magnetic flux from a magnet is NOT going to affect an amplifier.

(There's not that much flux leakage to begin with... and nothing in an amp it would bother. Have you ever seen a chunk of metal actually stuck to the side of a speaker cabinet?)

In theory the alternating magnetic field associated with the music that's playing could leak a tiny bit from the driver coils as well as any coils in the crossover.

And, in theory, this leakage could be "picked up" by certain things.... the most sensitive of those things being phono cartridges and phono preamps.... especially those running on vacuum tubes.

However, in practice, those are going to be bothered a loot more by the vibration than by the magnetic fields.)

In addition to that many powered subwoofers use switch mode power supplies or Class D amps...

And, since subs generally don't tend to have metal cabinets, there could be some noise leakage there...

But, again, the thing most likely to be bothered by that would be a tube phono preamp, and it's likely to be bothered a lot more by the vibration.)

Sept 1, 2021 8:00:24 GMT -5 ttocs said:

Mono amps are often placed directly adjacent to large speakers, so are these amplifiers influenced in any way by the magnetic energy of the woofers?

Are there design considerations for plate amps in subwoofers when placed so close to large magnets?




(I live a the State Of Flux.)

[ttocs]

Thanks Keith, that's a question I've wondered about for decades. I figure that the magnets affect CRT tv's, audio tapes placed inside a Fender Twin Reverb amp (stupid high school friend), etc, that maybe there was some influence on electronics devices in general.

[405x5]

Video display was the only instance where driver magnets could impact negatively, hence magnetic shielding for those magnets.
That was only during the days of CRT monitors.
Nothing in today’s flat screen displays to be impacted by such a thing.
Even at that, I believe it was rare for a center channel speaker (as the prime example) to have its magnets close enough to the “ guns” in the CRT to produce that negative effect

[KeithL]

CRTs are a special case... because they actually use a beam of electrons that is deflected (steered) by an electrostatic or magnetic field.

Tube oscilloscopes and a few ancient TV sets used electrostatic deflection... modern TVs all used electromagnetic deflection.

An outside magnetic field therefore deflects the beam very much like the internal mechanism does.

With a black and white TV or oscilloscope this is temporary and won't cause any damage.

You can actually move the dot around by holding a magnet near the screen.

(So, with a B&W TV, a nearby magnet could distort the picture, but the effect would be temporary.)

However COLOR CRT tubes include a sheet of perforated metal called a dot mask - right behind the front screen.

(Trinitron tubes actually use a row of wires instead - but it's the same idea).

This dot mask steers the electron beams for each color so that they only strike phosphor dots of the right color.

The problem is that this sheet of metal is almost always steel... so it can become magnetized.

And, if an area of the mask does become magnetized, the result is that "color stain" you sometimes see on old color TV screens that have had a magnet near them.

This effect is so sensitive that even the Earth's magnetic field can cause problems over time.

Because of this color TVs have what's called a "degaussing coil" that runs around the tube.

Each time you turn the TV on a brief high frequency signal runs through this - to "erase" any stray magnetism that the mask has picked up.

(But, since this isn't very powerful, if you get a strong magnet near a color TV screen, and it causes a color warp, it could take weeks to fully erase it, or may require an outside degausser.)

Modern LCD and OLED flat panel TVs are totally unaffected by magnets.

(In the old days many "A/V speakers" actually had "magnetic shielding" to limit their magnetic flux leakage to minimize the effect they would have on nearby TV sets.)

Since recording tape is magnetic it can obviously be erased or re-recorded by a magnetic field.

However, even then, it normally requires a strong magnet in very close contact, or an alternative field that is either very strong or very close, to do short term damage.

Putting a cassette next to a speaker magnet, or dragging it across a strong magnet, will USUALLY trash it...

Setting it on top of a speaker cabinet isn't a good idea since you don't know where all the magnets are inside.... but it would probably survive.

(And, if you still own any big old CRT color TVs, don't sit magnetic media on top of them... those degaussing coils I mentioned before can damage or erase them over time.)

Sept 1, 2021 8:44:06 GMT -5 ttocs said:

Thanks Keith, that's a question I've wondered about for decades. I figure that the magnets affect CRT tv's, audio tapes placed inside a Fender Twin Reverb amp (stupid high school friend), etc, that maybe there was some influence on electronics devices in general.

[AudioHTIT]

Sept 1, 2021 8:00:24 GMT -5 ttocs said:

…
(I live in a State Of Flux.)

The 51st State?

[ttocs]

Sept 1, 2021 10:08:50 GMT -5 AudioHTIT said:

Sept 1, 2021 8:00:24 GMT -5 ttocs said:

…
(I live in a State Of Flux.)

The 51st State?

Yes, it's one of the Constant States.

[cawgijoe]

I had a 34" Sony XBR 1080i TV (actually still have it) that developed a color "splotch" on one side while still under warranty. Sony actually sent a tech out to look at it. He tried degaussing the tube and that didn't work, so he called Sony. They said they had a possible solution they wanted to try. A week later the tech came back with magnets. He placed the magnets on the back of the tube and moved them around till the discoloration was gone. He then went into the service menu and calibrated the picture. The magnets worked and have been on the set ever since. No issues.

We just use the set now while riding the exercise bike in the basement.

[KeithL]

Interesting... and very "old school".

Apparently, rather than "something getting magnetized", he decided that there was a continuous magnetic field coming from somewhere.

(Most likely current flowing through an inductor somewhere in the set itself actually creating a "live" magnetic field.)

Therefore he used the magnets to "null it".

(And, if the field is "live", degaussing it won't help, because it will simply keep coming back.

Many older sets also had a whole selection of adjustments for use during setup...

Including the ability to actually adjust the location of the magnetic deflection yoke (wrapped around the back end of the tube).

Many also included a whole set of individual adjustments for things like "red position top left".

(And Sony had some interesting tricks to sort of minimize the need for this.)

You actually had to adjust the set so the three color images lined up - called "adjusting the convergence".

However, at least in theory, those shouldn't change once they've been set correctly, although sometimes occasional adjustments are required.

This is something still required on some projectors...

However, with modern flat panels, the individual pixels are printed onto a substrate, we don't need to worry about THAT any more.  

Sept 1, 2021 11:02:20 GMT -5 cawgijoe said:

I had a 34" Sony XBR 1080i TV (actually still have it) that developed a color "splotch" on one side while still under warranty. Sony actually sent a tech out to look at it. He tried degaussing the tube and that didn't work, so he called Sony. They said they had a possible solution they wanted to try. A week later the tech came back with magnets. He placed the magnets on the back of the tube and moved them around till the discoloration was gone. He then went into the service menu and calibrated the picture. The magnets worked and have been on the set ever since. No issues.

We just use the set now while riding the exercise bike in the basement.

[cawgijoe]

Sept 1, 2021 12:14:17 GMT -5 KeithL said:

Interesting... and very "old school".

Apparently, rather than "something getting magnetized", he decided that there was a continuous magnetic field coming from somewhere.

(Most likely current flowing through an inductor somewhere in the set itself actually creating a "live" magnetic field.)

Therefore he used the magnets to "null it".

(And, if the field is "live", degaussing it won't help, because it will simply keep coming back.

Many older sets also had a whole selection of adjustments for use during setup...

Including the ability to actually adjust the location of the magnetic deflection yoke (wrapped around the back end of the tube).

Many also included a whole set of individual adjustments for things like "red position top left".

(And Sony had some interesting tricks to sort of minimize the need for this.)

You actually had to adjust the set so the three color images lined up - called "adjusting the convergence".

However, at least in theory, those shouldn't change once they've been set correctly, although sometimes occasional adjustments are required.

This is something still required on some projectors...

However, with modern flat panels, the individual pixels are printed onto a substrate, we don't need to worry about THAT any more.  

Sept 1, 2021 11:02:20 GMT -5 cawgijoe said:

I had a 34" Sony XBR 1080i TV (actually still have it) that developed a color "splotch" on one side while still under warranty. Sony actually sent a tech out to look at it. He tried degaussing the tube and that didn't work, so he called Sony. They said they had a possible solution they wanted to try. A week later the tech came back with magnets. He placed the magnets on the back of the tube and moved them around till the discoloration was gone. He then went into the service menu and calibrated the picture. The magnets worked and have been on the set ever since. No issues.

We just use the set now while riding the exercise bike in the basement.

I remember him mentioning moving the set to another location in the room to see if that would "fix" the problem, but that simply wasn't possible. He also mentioned magnetic earth fields.
I was happy that this simple fix worked.

Also, as a side note, this was the time that XBR owners were well taken care of. They would actually send a tech out to your house....how often does that happen now?

[Ex_Vintage]

Any amplifier (or other sensitive electronics) that is encased in a metal (preferably steel)case should be shielded from reasonable external electromagnetic fields.