Field Coils for Audio Systems

[dcathro]

As requested by @GroovySauce I will list the components that I bought to make the Coleman power supplies.

Chassis - Kesper bamboo box from amazon.
Transformer - 300VA 2 x 12Vac from ebay
Power Supply caps - Mundorf MLytic 22000uf @ 40V
Power Supply resistors - Mundorf Mresist 10W
I used speaker binding posts for the +/- outlets which were pure copper from ebay
The heatsinks were made from 2mm (150 x 100 mm) copper plate and a 10mm aluminium block (50 x 100 mm)
I made the power cord captive (no IEC socket) and connected everything up using ceramic terminal blocks bought locally. I also incorporated a slow blow 5 amp fuse before the transformer as recommended.
I replaced the electrolytic caps supplied for the regulator with AN Kasai 50uf @ 63v and most of the resistors with Takman non magnetic carbons. There are a few that Rod says need to be as supplied.
One important thing when building these is that you can hear everything, especially the orientation of the power transformer. There is one orientation or the AC inputs and AC outputs that sounds optimal.

 

[GroovySauce]

@dcathro Thank you much! That is very helpful.

 

[dcathro]

In Hi-End audio we all know that we have to spend a fortune to get improvements in sound!

Well I just got a huge improvement from 12 wooden clothes pegs.

Let me explain. When I built my power amp, 15 years ago, I did a lot of experiments with the chassis and the heatsink. I found that I could hear the fins of the heatsink ringing and ended up using a large thick (6mm) copper plate. Even then, the number of connections/supports of the heatsink could be heard.

Then a few months ago when visiting a friend, I noticed he had a new piece of kit - a turntable motor controller for his Garrard sourced from Hong Kong. What caught my eye was that he had all these clothes pegs attached to the heatsink of this device. We then listened with the pegs on and off, and the difference was profound.

So although my FC PS heatsinks don't have fins, I thought I should try wooden clothes pegs on the thin (2mm) copper plate. I was blown away at the difference, much better tone and texture without loosing anything else.

It just goes to show just how EVERYTHING is important in the Field Coil power supply.

 

[Superchunk]

There's a pair of the Lowther field coils, which never really came to fruition as a driver for sale, for sale in England for a long while that I keep considering to try. Has anyone here had experience with them?

 

[GroovySauce]

Is it bad to leave the field coils on 24/7? When I was running 11v+ they would get warm. Now that I'm running lower voltages in the 9-10v range the FC don't get nearly as warm even after long periods.

Is heat cycling worse than steady temps?

 

[Rensselaer]

There's a pair of the Lowther field coils, which never really came to fruition as a driver for sale, for sale in England for a long while that I keep considering to try. Has anyone here had experience with them?

Not with the Lowther field coils, but with several of their Alnico magnet drivers, one using silver for the voice coil and no "hi-ferric" metal filing coating on the coil (just turns to powder eventually blocking the voice coil's movement). Saying that, regardless of which, the paper cone with attached whizzer of Lowthers, IMHO, causes an unpleasant bump in mids that gives it a shouting character. Interestingly, the AER cone, though appearing identical, doesn't have that (but costs a lot more).

 

[Zeotrope]

You should turn them off when not in use. The heat will eventually cause metal fatigue and electrical conductivity will be lost. You’ll have to solder the break or rewire them at that point.

Low voltage doesn’t change the resistance in the coil, as they will use more/fewer windings to maintain the same magnetic strength as a high voltage FC.

 

[lok]

I have Supravox 380 field coil speakers. They require a 9-12V 2x5A power supply.
What do you think about this linear power supply?
Will it work with Supravox?
It is difficult to find adjustable power supplies with a current of 10A on the market.

https://www.rmitaly.com/pl/produkt/lps-120-s/

 

[dcathro]

I have Supravox 380 field coil speakers. They require a 9-12V 2x5A power supply.
What do you think about this linear power supply?
Will it work with Supravox?
It is difficult to find adjustable power supplies with a current of 10A on the market.

https://www.rmitaly.com/pl/produkt/lps-120-s/

You don't need 10 amps, your Supravoxs will likely draw between 1 and 1.5 amps. There are lots of commercially available power supplies from generic lab supplies to audiophile solid state and tube supplies. You can also build your own such as the ones mentioned earlier in this thread.

 

[Zeotrope]

My Supravox 285-2000 EXC run between 6-12V and draw 2.2-2.7A (cold-hot). Ask Supravox directly what current the 380 model draws.

While you can use any lab power supply, the quality of the power supply and whether it's current or voltage limited will make a big difference to sound quality.

 

[lok]

I have recommendation from Supravox

You can use any lab power supply from 8 to 12volts and at least capable of 2 x 5A, for example:

Buy VOLTCRAFT PPS-13610 Bench PSU (adjustable voltage) 1 - 18 V DC 0 - 20 A 360 W USB , Remote programmable No. of outputs 2 | Conrad Electronic

Top %brandid% Selection | Order easily now: VOLTCRAFT PPS-13610 Bench PSU (adjustable voltage) 1 - 18 V DC 0 - 20 A 360 W USB , Remote programmable No. of outputs 2 | Fast delivery | Reasonable prices

www.conrad.com

 

[Zeotrope]

That will work but it won’t sound nearly as good as a FC PSU made specifically for this purpose.

For example: this guy in Japan is probably the best field coil PSU guy alive. Unfortunately, he doesn’t sell completed units (at least when I asked him, he didn’t).

Field Coil PSU

theaudiofeast.com

I upgraded from a lab PSU to a Wolf von Langa PSU and the improvement was significant.

If you chose to upgrade, you’ll have to measure the resistance and get the actual current draw when cold and hot of your units. I now recall that Supravox oddly didn’t know this when I asked!

 

[dcathro]

My Supravox 285-2000 EXC run between 6-12V and draw 2.2-2.7A (cold-hot). Ask Supravox directly what current the 380 model draws.

It is easily calculated by measuring the impedance of the field coil. I = V/R, so if the impedance is 8 ohms and they are run at 12V, the current will be 1.5A. If the impedance is 4 ohms, I will be 3A.

My Supravox 215s have 4 ohm impedance and run at 9V and so draw about 2.3A. My WVL 15" bass have 8 ohm impedance and run at 12V and draw 1.5A

While you can use any lab power supply, the quality of the power supply and whether it's current or voltage limited will make a big difference to sound quality.

Supravox say that any supply will do but it is not true. I used lab supplies for years and it held the drivers back. The two favoured approaches are to use direct current regulated SS supplies or Tungar based supplies.

 

[dcathro]

That will work but it won’t sound nearly as good as a FC PSU made specifically for this purpose.

For example: this guy in Japan is probably the best field coil PSU guy alive. Unfortunately, he doesn’t sell completed units (at least when I asked him, he didn’t).

Field Coil PSU

theaudiofeast.com

I upgraded from a lab PSU to a Wolf von Langa PSU and the improvement was significant.

If you chose to upgrade, you’ll have to measure the resistance and get the actual current draw when cold and hot of your units. I now recall that Supravox oddly didn’t know this when I asked!

OK, we are saying the same thing

Your WVL supplies are basically the same as my DIY Coleman supplies, SS direct current regulated.

 

[Treehaus Audiolab]

I’d recommend a Coleman build for 12v range field coils.

 

[Superchunk]

What would be the audible effect, if any, of field coil overheating (psu and motors)?.

The PSU is too hot to touch for more than a few seconds. The field coil motor casing isn't that hot, but it's hot enough you wouldn't want to rest your hand on it. Class A amp sort of hot.

I've been chasing an issue of recessed, undefined, fuzzy vocals. Pack of detail. I thought it was elsewhere but last night I realized just how hot my field coils were getting....

 

[Superchunk]

Not with the Lowther field coils, but with several of their Alnico magnet drivers, one using silver for the voice coil and no "hi-ferric" metal filing coating on the coil (just turns to powder eventually blocking the voice coil's movement). Saying that, regardless of which, the paper cone with attached whizzer of Lowthers, IMHO, causes an unpleasant bump in mids that gives it a shouting character. Interestingly, the AER cone, though appearing identical, doesn't have that (but costs a lot more).

It was this shouting character that stopped me. The only Lowther I've really heard were $100,000+ and tuned to perfection, and playing music suited to them...

 

[LanschePlasmaGuy]

What would be the audible effect, if any, of field coil overheating (psu and motors)?.

The PSU is too hot to touch for more than a few seconds. The field coil motor casing isn't that hot, but it's hot enough you wouldn't want to rest your hand on it. Class A amp sort of hot.

I've been chasing an issue of recessed, undefined, fuzzy vocals. Pack of detail. I thought it was elsewhere but last night I realized just how hot my field coils were getting....

Buy an infared thermometer. I bought one at Costco for about $15 Aussie. So you can get an indication pretty quickly of how hot the unit is getting and if it's within its safe operating tempature or not (different on my Wolf Von Langa, Paladin and Rullit field coil motors).

LPG

 

[Treehaus Audiolab]

What would be the audible effect, if any, of field coil overheating (psu and motors)?.

The PSU is too hot to touch for more than a few seconds. The field coil motor casing isn't that hot, but it's hot enough you wouldn't want to rest your hand on it. Class A amp sort of hot.

I've been chasing an issue of recessed, undefined, fuzzy vocals. Pack of detail. I thought it was elsewhere but last night I realized just how hot my field coils were getting....

Which PSU are you using? Nothing should be getting that hot. The field coils should be dissipating 10-20 watts of heat (depending on your driver and how you are running them) so they should be warm but not burning.

Edit: I agree on the infrared thermometer. It removes a lot of guesswork and has uses all over the house and kitchen!

 

[Atmasphere]

What would be the audible effect, if any, of field coil overheating (psu and motors)?.

The PSU is too hot to touch for more than a few seconds. The field coil motor casing isn't that hot, but it's hot enough you wouldn't want to rest your hand on it. Class A amp sort of hot.

I've been chasing an issue of recessed, undefined, fuzzy vocals. Pack of detail. I thought it was elsewhere but last night I realized just how hot my field coils were getting....

That's scary. You really do want to find out how hot its getting. You don't want the coil getting so hot insulation melts!