Field Coils for Audio Systems

[Atmasphere]

Have I heard the field coil 215-2000 exc? No I haven’t but cone geometry and design is identical other than magnet so, while exc version might sound more pure in some way, the high frequency extension shouldn’t change. I can tell you that the bass quality and punch from this driver is very good in a TQWT cabinet (the guy in the video used Supravox designed cabinet just like mine).
It is not issues per se, it’s just physics. An 8 inch driver will beam like hell at high frequencies. It’s fine for some and at least it isn’t nasty in the highs like most Fulleange drivers with a Whizzer cone.

Look at the Cessaro Opus-1 speaker. They have their own version of the Supravox 215-2000 Alnico (they still use the original in the Wagner) and they also choose to match it with a horn tweeter (same with Wagner). A good compression driver to my ears just sounds more realistic up top in a good horn (probably where your ALTECs are letdown).

I recently built a TQWT cabinet for the 6" PHY driver and it worked surprisingly well. That kind of cabinet seems to get the most bass extension out of a driver, if its an appropriate driver.

Every 'full range' driver is beamy! Any successful design is going to incorporate a tweeter, even if the tweeter is firing up or to the rear. You need something to maintain off-axis tonality and room energy.

 

[Zeotrope]

If you are using field coil drivers, don’t make the mistake that I did for many years and assume that any lab-grade DC power supply will work! It will work, but you are significantly impacting sound quality. The upgrade to the Wolf von Langa constant current supply made a big difference, and then another massive leap was achieved by supplying it with filtered AC (Shunyata Denali) and reducing noise from the AC ground (Shunyata Altaira/Denali).
Improvements were:
- more high end detail
- lower background noise/noise floor
- better localization, especially in front/behind the speakers
- more authority and presence

Results may vary of course, but it does prove that the quality of the incoming AC into the DC power supply, and the supply itself is absolutely crucial to the performance of the field coil driver!

 

[arivel]

Is a field coil loudspeaker suitable to be current driven?

 

[Zeotrope]

Is a field coil loudspeaker suitable to be current driven?

Yes and this is actually preferred.
Note that constant current on a lab supply is not really the constant current source that we need. Every lab supply I have seen doesn’t really maintain constant current as resistance changes.

 

[Atmasphere]

Is a field coil loudspeaker suitable to be current driven?

Are you talking about the field coil or the voice coil. If the latter, no.
If the former, if the voltage is tightly controlled the current will be as well.

 

[Zeotrope]

Especially for low voltage field coils you want to maintain a constant current, not voltage. As the coils heat up, resistance decreases, so voltage should also decrease to maintain constant current.
There is also interactivity with the amplifier signal, which will affect the magnetic field. A good field coil supply will maintain a constant current in these situations.

 

[Atmasphere]

Good point about the heat.

 

[Zeotrope]

Field coil power supplies are quite misunderstood, insofar as what contribution they make to the sound. I now consider the field coil to be in the signal path, just like a permanent magnet is technically part of the sound generation path.

 

[morricab]

Especially for low voltage field coils you want to maintain a constant current, not voltage. As the coils heat up, resistance decreases, so voltage should also decrease to maintain constant current.
There is also interactivity with the amplifier signal, which will affect the magnetic field. A good field coil supply will maintain a constant current in these situations.

Usually as things heat up the resistance increases…

 

[Zeotrope]

Usually as things heat up the resistance increases…

Right! Sorry, wrote it quickly and got it mixed up.

More on the relationship between the voice coil and field coil [this is a quote from another person, whose name I unfortunately didn’t note down]:
When music is playing there will be a magnetic field from the voicecoil 'reflected' as back EMF into the field coil and thus to the PSU. I suspect that the ability of the PSU to drain that is responsible for the difference in SQ between different supplies.

 

[arivel]

no I am not referring to the power supply which must supply energy to produce the magnetic field but rather the mobile coil connected to the cone which produces the sound waves.
there are amplifiers called transconductance which work in the opposite way to traditional amplifiers

 

[Zeotrope]

Just learned about hysteresis in audio and the fact that electromagnets exhibit far less hysteresis than permanent magnets. Since hysteresis is essentially distortion, this could validate why field coil drivers sound better than permanent magnet drivers (all things equal).

Hysteresis distortion - This kind of distortion occurs in audio products based on magnetic principles: loudspeaker drivers, crossover inductors, or all those magnetic components in a Class D amplifier. Once the passage of a varying audio signal magnetizes a piece of magnetic material in one direction, it retains that state. When our constantly changing audio signal then reverses direction, the magnet's historic memory adds or subtracts from the audio signal causing distortion.

Hysteresis

Hysteresis describes how something appears or responds based on its history. What we see in the present is that way only because of what happened in the past. Stretch a rubber band and upon its release, the band does not return to its original shape. That's hysteresis. Understanding hysteresis...

www.psaudio.com

Which type of magnet produce less hysteresis loss?
Soft magnets (low coercivity) are used as cores in transformers and electromagnets. The response of the magnetic moment to a magnetic field boosts the response of the coil wrapped around it. Low coercivity reduces that energy loss associated with hysteresis.

 

[MarcelNL]

that ties in with what I read a good while ago, it also could be an explanation for the fact that AlNiCo magnets make a speaker sound better than an equally strong (or stronger) rare earth magnet.

 

[tima]

that ties in with what I read a good while ago, it also could be an explanation for the fact that AlNiCo magnets make a speaker sound better than an equally strong (or stronger) rare earth magnet.

I would not go quite that far in claiming it a fact that AlNiCo magnets sound better; it may be more of a preference for a particular type of sound. At least there is debate about permanent magnet types and which "is best". Among other things, there are different magnetic properties, implementations, different durabilities and different costs. Pros and cons abound.

Alnico was most widely used in the 1950s and some say it yields a warm, classic tone -- a bit more 'vintage' than ceramic or neodymium. It tends to lose its magnetism more quickly than other types although it stands up to heat over a wide temperature range. Neodymium has a higher resistence to losing its magnetism (coercivity) but can be impacted more by high heat. Speaker builders sometimes incorporate different ways of venting or maintaining air flow to reduce heat build-up. Neodymium can yield a greater flux field density with a smaller size and weight, which makes them suited for use in tweeters and headphones. I suspect sonic quality is as much a result of implementation as material type.

 

[Zeotrope]

Given this, I wonder why field coils have not (yet) enjoyed a renaissance, especially for higher-end speakers?

 

[MarcelNL]

they are having one, not as much as tube amps recently did...

Part of the issue I guess is that you need a PSU per speaker, and the 'newer is better' virus..


@tima I was not implying AlNiCo always sounds better, but it does have lower hysteresis than other permanent magnets. I disagree that AlNiCo has a 'vintage sound'. Speaker magnets are a huge untrodden area IMO, we spoke with Wolf about this and it's mind boggling how much aspects there are that are not 'out in the field'.

 

[Zeotrope]

@tima Speaker magnets are a huge untrodden area IMO, we spoke with Wolf about this and it's mind boggling how much aspects there are that are not 'out in the field'.

To this I would add the importance of vibration control and clean AC to the field coil PSU. I improved the sound from my WVL PSU significantly by grounding and AC filtering (Shunyata) and adding active vibration control (Seismion). Cannot overstate how big of a difference it made.

 

[MarcelNL]

I can imagine, the AC to the speaker is very important!

I'm toying with the thought to have 4 Tesla's Field Coiled and only 4 of them in the new house (rather than 3 per side as before)

 

[Solypsa]

I can imagine, the AC to the speaker is very important!

I'm toying with the thought to have 4 Tesla's Field Coiled and only 4 of them in the new house (rather than 3 per side as before)

Not four per side

 

[Atmasphere]

Just learned about hysteresis in audio and the fact that electromagnets exhibit far less hysteresis than permanent magnets. Since hysteresis is essentially distortion, this could validate why field coil drivers sound better than permanent magnet drivers (all things equal).

Hysteresis distortion - This kind of distortion occurs in audio products based on magnetic principles: loudspeaker drivers, crossover inductors, or all those magnetic components in a Class D amplifier. Once the passage of a varying audio signal magnetizes a piece of magnetic material in one direction, it retains that state. When our constantly changing audio signal then reverses direction, the magnet's historic memory adds or subtracts from the audio signal causing distortion.

Hysteresis

Hysteresis describes how something appears or responds based on its history. What we see in the present is that way only because of what happened in the past. Stretch a rubber band and upon its release, the band does not return to its original shape. That's hysteresis. Understanding hysteresis...

www.psaudio.com

Which type of magnet produce less hysteresis loss?
Soft magnets (low coercivity) are used as cores in transformers and electromagnets. The response of the magnetic moment to a magnetic field boosts the response of the coil wrapped around it. Low coercivity reduces that energy loss associated with hysteresis.

Hysterisis is an issue with AC. Magnets are essentially DC so hysteriesis does not explain why field coils sound better. more:

I would not go quite that far in claiming it a fact that AlNiCo magnets sound better; it may be more of a preference for a particular type of sound. At least there is debate about permanent magnet types and which "is best". Among other things, there are different magnetic properties, implementations, different durabilities and different costs. Pros and cons abound.

Alnico was most widely used in the 1950s and some say it yields a warm, classic tone -- a bit more 'vintage' than ceramic or neodymium. It tends to lose its magnetism more quickly than other types although it stands up to heat over a wide temperature range. Neodymium has a higher resistence to losing its magnetism (coercivity) but can be impacted more by high heat. Speaker builders sometimes incorporate different ways of venting or maintaining air flow to reduce heat build-up. Neodymium can yield a greater flux field density with a smaller size and weight, which makes them suited for use in tweeters and headphones. I suspect sonic quality is as much a result of implementation as material type.

My understanding of why Alnico magnets sound better is that they are more easily focused into the voice coil gap and also do not sag as much when current is introduced to the voice coil as ceramic magnets; to my understanding neodymium sags the most. I might be mistaken about the latter bit about neodynium but I know it took a while before speaker designers figured out how to use it without making a bad sounding speaker (neodynium had a very bad reputation in the guitar amplifier world on this account...). Field coils OTOH sag far less since they can be powered by a regulated power supply.