Field Coils for Audio Systems

[MarcelNL]

that final capacitor is important in EVERY linear PSU, it even matters the world on the 12V rail going into the CPU in my audio server....

from what I heard in discussion with folks I trust on the matter is the hysteresis difference between various permanent magnets and FC an important factor for sound quality differences, a side effect of having a PSU is you can tweak a driver somewhat yet I am not aware it's a dial a T-S parameter smorgasbord ;-)

 

[Atmasphere]

In supplies where you see Tungars in use you will likely also see a rather prodigious capacitance at the output of the supply. If you don't you'll likely hear a bit of hum. At any rate, whatever capacitor is at the output of the supply will obviate whatever is used as a rectifier for the supply. Even more so if the output of the supply is regulated- then the rectifier is of no consequence whatsoever.

 

[MarcelNL]

(I was referring to very small capacitance, in the nF range)

 

[Zeotrope]

Hi,

A field coil doesn’t give more efficiency per se. A modern production neodymium magnet can easily provide enough flux density to saturate any metal poles including permendur without issue. Same goes for other magnets such as alnico. The theoretical maximum flux density achievable is 2.4T and numerous drivers have done this for many, many years with fixed magnets - Lowther PM4a would be an example.

The performance benefits largely reside in the incredible ability to tune the driver to the speaker by altering the magnet flux and consequent t/s parameters. Simplifying it a bit with an example of a field coil 8” full ranger - as you increase the voltage/current the magnetic force increases which leads to increased sensitivity and lower q. This results in a driver suitable mainly for horn loading applications but will also result in lower bass output. The same driver being fed a lower voltage/current will be less sensitive but its q will be higher meaning it will lend itself much more readily to open baffle application since the driver cone will be more readily able to move. This will give more bass output but at expense of sensitivity.

There are other arguments related to the field itself - that is the field coil (fed with appropriate power supply) magnetic field will be incredibly stiff and not fluctuating with increased heat like fixed ferrite magnets.

Then there is the area which is very hard to quantify relating to the sound itself. I have spent most of my time recently together with my Viking partner in crime (Leif) trying to extract the maximum from our field coil carts. People would tell us we lost the plot when we tell you that the type of capacitor used on the last filtration drastically alters the sound. Even though it is just a cap filtering dc for the magnetic field of the cartridge. If you read widely, there are all manners of opinion on which power supply types to use for best sound. Purists seem to opt for Tungar bulb type supplies, others on battery, other using various medical supplies and then there is the constant voltage and current crowd. I recently spoke to someone who said never to use constant voltage or current as it fights with the voicecoil.

Best regards.

Bill, how would you expect the frequency response to change for a bass horn when the field coil current is reduced? On one hand, sensitivity decreases which would reduce the SPL at the frequencies where the horn operates; but on the other hand bass output is increased… is that correct?
I’ve been running my Supravox 12” woofers at their max 12V / most sensitive setting, so that they match the sensitivity of the midrange and tweeters; but now I’m experimenting with 10V. It does seem like the (upper bass- they run from 60-600Hz) is more natural and a bit more pronounced. But how is the frequency response affected?
Thank you

 

[Audiophile Bill]

Bill, how would you expect the frequency response to change for a bass horn when the field coil current is reduced? On one hand, sensitivity decreases which would reduce the SPL at the frequencies where the horn operates; but on the other hand bass output is increased… is that correct?
I’ve been running my Supravox 12” woofers at their max 12V / most sensitive setting, so that they match the sensitivity of the midrange and tweeters; but now I’m experimenting with 10V. It does seem like the (upper bass- they run from 60-600Hz) is more natural and a bit more pronounced. But how is the frequency response affected?
Thank you

Hi,

Yes it will move in the direction you expect. As you lower the voltage/current the Supravox 285 will produce more mid bass output at the expense of sensitivity. The difference in sensitivity between the 285 mkII at 6v is 93 and at 12v it is 102dB! I assume you have the t/s data - here is the mkII driver data I found just now:


You can see that the driver is still good for horn loading at 10v. I wouldn’t go much lower though as the Qts is starting to climb quite a bit by 8v.

In terms of the frequency response in your horn in your system, I can run a simulation for you in Hornresp if you send me details of your horn geometry/mouth/throat/chamber volumes and length.

 

[Zeotrope]

Thank you, Bill!
You posted the data for Mk1, note the sensitivity of 97dB @ 6V, which is the driver I have.
Thanks for the offer to run the simulation, but I don't have the horn geometry details, unfortunately. I think the upper bass horns will be difficult to measure...

 

[Audiophile Bill]

Thank you, Bill!
You posted the data for Mk1, note the sensitivity of 97dB @ 6V, which is the driver I have.
Thanks for the offer to run the simulation, but I don't have the horn geometry details, unfortunately. I think the upper bass horns will be difficult to measure...

Who made the upper bass horn for you? We can always do some digging if you want. Whatever works but offer is open to run simulations with your chosen amp (power) on all the different voltage settings. I will be able to send you frequency response curves of all the different settings (and a whole lot more if you wanted haha).

 

[Zeotrope]

EletronLuv in Utah built the horns, but not sure if he still has the CAD files...

I had a crossover engineer voice the horns and plot the responses when we redesigned the crossover. But we only used the 12V setting on the woofers. For the crossover we were trying to maximize efficiency, as I was using a 7W amp at the time. Now I'm using a 150W amp, so sensitivity is not a concern

 

[Audiophile Bill]

EletronLuv in Utah built the horns, but not sure if he still has the CAD files...

I had a crossover engineer voice the horns and plot the responses when we redesigned the crossover. But we only used the 12V setting on the woofers. For the crossover we were trying to maximize efficiency, as I was using a 7W amp at the time. Now I'm using a 150W amp, so sensitivity is not a concern

All I need from him is what type of horn (tractrix, exponential etc), what diameter (or area) is the throat, diameter (or area) of mouth and total length of the horn.

 

[Zeotrope]

It's the LaScala style folded(?) horn... I can ask but doubt I will get an answer

 

[Audiophile Bill]

It's the LaScala style folded(?) horn... I can ask but doubt I will get an answer

No probs!

 

[Zeotrope]

No probs!

So in theory the reduction in sensitivity should be offset by the increase in low end response, say from 12V to 11V? Does it instinctively make sense?

 

[Audiophile Bill]

So in theory the reduction in sensitivity should be offset by the increase in low end response, say from 12V to 11V? Does it instinctively make sense?

Yes it makes sense theoretically but hard to quantify by how much unless we simulate.

Think of it in this crude way. The power of the motor is lower therefore the cone can potentially move more easily displacing the air required for lower frequencies.

 

[Audiophile Bill]

FYI. If you study bass drivers en masse, you’ll quickly discover there is no free lunch. Look at fs and sensitivity data. You quickly discover that if you want very low fs, you can’t concomitantly have high sensitivity (except for rare instances). The huge diameter pro woofers of 21” have decentish fs around 30hz and high sensitivity but if you are looking at high sensitivity (98-100dB) 15” as a breed, most will have fs around 40-45hz.

 

[Zeotrope]

I think I got it, thanks. So the frequency response should remain flat, because the increase in output will offset the reduction in sensitivity.

 

[Audiophile Bill]

I think I got it, thanks. So the frequency response should remain flat, because the increase in output will offset the reduction in sensitivity.

No the frequency response will more than likely see a small increase in your mid bass relative to your mids/treble.

 

[Zeotrope]

No the frequency response will more than likely see a small increase in your mid bass relative to your mids/treble.

Ah ok, so a net increase, despite the reduction in sensitivity. I know a simulation or measurement is the only way to know exactly; but absent that the theory is good to know, and of course how it sounds.

 

[Audiophile Bill]

Ah ok, so a net increase, despite the reduction in sensitivity. I know a simulation or measurement is the only way to know exactly; but absent that the theory is good to know, and of course how it sounds.

Yep. That is the point. No free lunch. You want more mid bass, you can’t get it at same time as more sensitivity.

This is same reason that the open baffle field coil crowd don’t usually end up running their motors too stiff (high voltage/current and therefore flux) cos the net sound can be too bright.

 

[Zeotrope]

No the frequency response will more than likely see a small increase in your mid bass relative to your mids/treble.

What’s interesting is that there’s a perceived improvement in clarity and ’realism’ in the mids and highs as well, even with just the bass horn voltage decreased.
I read this elsewhere: that adding more low end will improve the upper end sound quality, or at least our perception of it.

 

[Atmasphere]

FYI. If you study bass drivers en masse, you’ll quickly discover there is no free lunch. Look at fs and sensitivity data. You quickly discover that if you want very low fs, you can’t concomitantly have high sensitivity (except for rare instances). The huge diameter pro woofers of 21” have decentish fs around 30hz and high sensitivity but if you are looking at high sensitivity (98-100dB) 15” as a breed, most will have fs around 40-45hz.

The TAD 1602 was one of the few that had an fs in the 20s while still being 97dB. Too bad they don't make it any more.