Solid state amp to drive field coil woofers? Thoughts?

Atmasphere

[Industry Expert]
May 4, 2010
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I am curious, what are your field coils, and how do you find them compared to other approaches.

Thanks

David
They are made by Classic Audio Loudspeakers. IMO/IME field coils have speed that otherwise you can only get from ESLs and for very similar reasons- both have power supplies to run their motive forces. In a permanent magnet system, when you put current through the voice coil, the magnetic field sags a bit. Alnico sags the least, and is easiest to focus in the voice coil gap, which is why Alnico is the preferred magnet system. Neodynium is the most powerful magnet system, but its harder to focus and sags the most.

Field coil OTOH doesn't sag- so there is less compression, higher speed and lower distortion, and I really don't think its hard to hear that difference. Permanent magnets fall into that same arena of most of the big changes in audio history; they didn't go away because they were inferior, they went away because they are more expensive. This was the same motivator behind tubes to transistors and certainly analog to digital.
 

dcathro

Well-Known Member
Sep 16, 2016
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Melbourne, Australia
They are made by Classic Audio Loudspeakers. IMO/IME field coils have speed that otherwise you can only get from ESLs and for very similar reasons- both have power supplies to run their motive forces. In a permanent magnet system, when you put current through the voice coil, the magnetic field sags a bit. Alnico sags the least, and is easiest to focus in the voice coil gap, which is why Alnico is the preferred magnet system. Neodynium is the most powerful magnet system, but its harder to focus and sags the most.

Field coil OTOH doesn't sag- so there is less compression, higher speed and lower distortion, and I really don't think its hard to hear that difference. Permanent magnets fall into that same arena of most of the big changes in audio history; they didn't go away because they were inferior, they went away because they are more expensive. This was the same motivator behind tubes to transistors and certainly analog to digital.



lovely Ralph, are yours the Hartsfields or the T series?

What are you using to power the motors?

No argument from me on everything you have said!

I always felt that ferrite magnet speakers sounded somewhat grey and dead compared to alnico.

field coils seam to sound even more alive, and I am pretty sure the ultimate quality is dependent on the power supply.

Cheers David
 

Alrainbow

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Dec 12, 2013
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What’s the imp curve ? Can you post it ? do you have a field coil psu for each driver alone ?
if you can post some pics as asked and if possible vary voltage for drivers. My reason is field coil drivers from what I read and heard do vary greatly per amp used. BI amp is a way to lesson the effect of amp imp interaction. there was a very large horn setup all field coil horns. sadly they used one psu per side for 5 drivers.
 

dcathro

Well-Known Member
Sep 16, 2016
101
64
113
Melbourne, Australia
What’s the imp curve ? Can you post it ? do you have a field coil psu for each driver alone ?
if you can post some pics as asked and if possible vary voltage for drivers. My reason is field coil drivers from what I read and heard do vary greatly per amp used. BI amp is a way to lesson the effect of amp imp interaction. there was a very large horn setup all field coil horns. sadly they used one psu per side for 5 drivers.

Sorry, who are you referring to? The OP?
 

Atmasphere

[Industry Expert]
May 4, 2010
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lovely Ralph, are yours the Hartsfields or the T series?

What are you using to power the motors?

No argument from me on everything you have said!
I'm using the T-3.3.

I use an adjustable solid state regulated supply that can hold the voltage within 1-2mV. The voltage and current are displayed on the front panel.
 
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dcathro

Well-Known Member
Sep 16, 2016
101
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Melbourne, Australia
I'm using the T-3.3.

I use an adjustable solid state regulated supply that can hold the voltage within 1-2mV. The voltage and current are displayed on the front panel.

Gorgeous speakers!

Your bass drivers are essentially the same as mine, although you have 2 per side and running ported so you have way more efficiency!

I would humbly suggest that you experiment with the power supplies for the drivers. On my bass driver I started with a 20V 5A rated unit (the driver draws less than 1.5A), but found that a much higher rated 10A 30V (HP Agilent) unit made a profound difference. On my mids, I am using low ripple supplies - you can hear any PS noise, but want to try Tungars.

Cheers

David

P.s. love the look of your amps!
 
Last edited:

Atmasphere

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Thanks!

I built a power supply using Tungars because I'd heard so much about them, but I found them to mostly be a pain in the rear. The big deal is you have to regulate the power supply, since current in the voice coil affects the magnetic field, and that energy comes from somewhere- the more current in the voice coil the more sag in the magnetic field if you don't do something about it. Once there is a regulator in play, its all about the regulator(which is in essence a small power amplifier and so has to be very linear) rather than what rectified the power to it. Any 'quality' the rectifiers have upstream becomes irrelevent in the face of regulation, unless the regulator has a design flaw.

You really don't want the voltage to sag- otherwise it can introduce colorations on two counts- intermodulation and changes in the T-S parameters (including efficiency). While the latter is slight, it is measurable. The former is likely the lion's share of the difference. IME, there is a voltage drop across the wiring between the power supply and the drivers, so I keep the length of it as short as I can and run heavier gauge than it would appear to need.

I'm working on a new regulator using HEXFREDs (mostly for low noise) and a narrower adjustable range. I plan to build in a circuit to turn it on when audio is detected.
 
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dcathro

Well-Known Member
Sep 16, 2016
101
64
113
Melbourne, Australia
Thanks!

I built a power supply using Tungars because I'd heard so much about them, but I found them to mostly be a pain in the rear. The big deal is you have to regulate the power supply, since current in the voice coil affects the magnetic field, and that energy comes from somewhere- the more current in the voice coil the more sag in the magnetic field if you don't do something about it. Once there is a regulator in play, its all about the regulator(which is in essence a small power amplifier and so has to be very linear) rather than what rectified the power to it. Any 'quality' the rectifiers have upstream becomes irrelevent in the face of regulation, unless the regulator has a design flaw.

You really don't want the voltage to sag- otherwise it can introduce colorations on two counts- intermodulation and changes in the T-S parameters (including efficiency). While the latter is slight, it is measurable. The former is likely the lion's share of the difference. IME, there is a voltage drop across the wiring between the power supply and the drivers, so I keep the length of it as short as I can and run heavier gauge than it would appear to need.

I'm working on a new regulator using HEXFREDs (mostly for low noise) and a narrower adjustable range. I plan to build in a circuit to turn it on when audio is detected.

Am I understanding you correctly that the Tungar PS is unregulated, or that once you regulate it, the fact that it is Tungar makes no difference?

Your comments on the cables are interesting. What king of gauge do you think is necessary?

I am interested to know more about your power supply design.
 

Atmasphere

[Industry Expert]
May 4, 2010
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www.atma-sphere.com
Am I understanding you correctly that the Tungar PS is unregulated, or that once you regulate it, the fact that it is Tungar makes no difference?

Yes. Well- it'll be a lot bigger if you use Tungars (which was a GE trademark but became generic like Kleenex) and occasionally one will fail :) Honestly I don't see the point unless the glow of tubes is what's important.
Your comments on the cables are interesting. What king of gauge do you think is necessary?

I am interested to know more about your power supply design.

I've been running 10Ga wire. I can't be sure but it seems to make the system slightly faster, although it could be expectation bias.

The supply is pretty simple. I don't have a great degree of current needed since my woofers are Alnico still (I hope to install field coil woofers but I want them to have similar specs to my 1602s). The circuit is a simple pass regulator to filter out the sawtooth; teh resulting DC is applied that to the input side of an LT1083 and a potentiometer to allow me to set the voltage over a small range. I've been thinking about a vintage style meter (probably the same style as we use on our amps) to display the current so I can set that correctly.
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
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I'm working on a new regulator using HEXFREDs (mostly for low noise) and a narrower adjustable range. I plan to build in a circuit to turn it on when audio is detected.

If I may ask, which HEXFREDs are you using? I installed Vishay HEXFRED VS-HFA15TB60-M3 in my DAC, tuner and phono stage, and the results were clearly superior
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
6,351
811
610
Boston, MA

Alrainbow

Well-Known Member
Dec 12, 2013
1,902
538
370
NYC , USA
I'm using the T-3.3.

I use an adjustable solid state regulated supply that can hold the voltage within 1-2mV. The voltage and current are displayed on the front panel.
OMA says the original tube recti used on his field coil drivers of his flagship sounded best but measured bad lol. He is a really nice honest guy I enjoyed my time there. really cool looking Cust audio stuff that sounded As good as it looked.
 

dcathro

Well-Known Member
Sep 16, 2016
101
64
113
Melbourne, Australia
Yes. Well- it'll be a lot bigger if you use Tungars (which was a GE trademark but became generic like Kleenex) and occasionally one will fail :) Honestly I don't see the point unless the glow of tubes is what's important.

LOL, it's surprising to hear a tube guy say that there is no benefit!

I've been running 10Ga wire. I can't be sure but it seems to make the system slightly faster, although it could be expectation bias.

I will definitely try a bigger gauge - I have been running 18awg.

The supply is pretty simple. I don't have a great degree of current needed since my woofers are Alnico still (I hope to install field coil woofers but I want them to have similar specs to my 1602s). The circuit is a simple pass regulator to filter out the sawtooth; teh resulting DC is applied that to the input side of an LT1083 and a potentiometer to allow me to set the voltage over a small range. I've been thinking about a vintage style meter (probably the same style as we use on our amps) to display the current so I can set that correctly.

Sounds good. What voltage range will it have, and how much current?
 

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