Making a field coil speaker

[mike hadley]

After rebuilding both permanent magnet and field coil speakers the next step for me was to build a field coil speaker from the bottom up,

besides there is an awful lot of radios out there without the correct speaker, yes I could have repurposed a field coil speaker from another radio, but field coil speakers are beginning to get expensive, and in some cases the cost is prohibitive.

I have used resistors in the past but not anymore.

There is a good article called project Ryu, the author covers lots of information and goes into the theory of electromagnets magnetic flux voice coil types etc.,
this is all excellent information, however, I don't have access to professional machining equipment, or the software to check on the theory. and the designers in the late 1920’s didn’t have all of the modern alloys that are around today.

I read in another article about electromagnets that the purer the iron makes a better electromagnet, so always willing to experiment I thought it was time to try.
Armed with the above information, the materials were chosen
the pole piece is 1.5” En1a type steel,
the end plates are 8mm thick En3 grade,
the tube is 4” diameter BS1387 heavy wall type,
all these parts are readily available.

I used a modern speaker cone as the hard work is already done, the size can be found by measuring the basket opening, this is where the corrugated speaker cone edge fits,
cone height is measured by placing a straight edge across the cone opening and measuring down to where the voice could will sit, an allowance has to be made for the spider height, ideally the cone and spider should meet at the voice coil junction, but if the cone is a little short this is not a problem. for this job the cone is 282mm dia by 72mm high, there is a slight gap of around 1.5mm between the spider and cone, but this is ok.

there is also the decision wether to have the voice coil underung or overhung, both types have different effects on the final speaker performance, there is plenty of information about this on the internet, briefly underhung means the coil is shorter than the magnetic gap height, overhung means the voice coil is longer.

for this speaker I am using a commercially made coil, the winding length is 14mm that makes this speaker overhung, meaning there is voice coil showing either side of the magnetic plate

The tools used was a diy type drill press, the sort of thing that Harbor freight would sell,
I am lucky to own a metal turning lathe for those interested it’s a Myford ml1, and dates from around 1934.

here is the donor speaker, it’s a 1970 vintage Fane guitar speaker, I chose this as it has a cast basket,

you can see in the photo that the voice coil had seen better days

the metal discs were machined with a 1/4 “ wide 30 thou step that locates to the tube ends, the tube is 31/4” long, with a wall thickness just under a 1/4”, one end plate has a hole for a 10mm bolt, drilled with a little clearance for adjusting the pole piece position, the other disc has a hole machined to clear the pole piece plus 3mm, this will give a voice coil gap of 1.5mm,

Here is a drawing of what I’m trying to achieve

Because the original speaker was made for a ceramic magnet, an adapter plate was made, this is machined from 1/8” aluminium, this is strong enough to hold the field coil assembly in place.

The closer the gap the stronger the magnetic field will be, I have measured a few 12” speakers and the gap can be from 1mm to 1.6mm
here are the parts roughly assembled,

The bobbin for the field coil was made from 5/64" card with the ends epoxied on and given a couple coats of varnish to seal, it’s length is 21/4 long, taking into account the voice coil will need room to move, (excursion) if there is not enough room here the voice coil when driven with deep bass notes will hit the field coil former with a loud clack, don’t ask me how I know.

This former is then put on my cheap Chinese coil winder, with mdf discs that will stop the coil from spreading as it is wound, I am using 0.16mm wire or 34awg with high temperature enamel,
the photo shows a pause at 250 ohms of wire, one of the field coil taps required for this application, the speaker will have a further 1700 ohms wound on, giving 1950 ohms in total, that’s about a mile and a half of wire.

The completed bobbin mounted on the pole piece showing the resistances

Then the wound field coil can be fitted to the pole piece and tried, the coil will see somewhere around 140 volts when working, this means it will be dissipating roughly 10 watts, despite this the coil assembly stays fairly cool when running, the resulting magnet is incredibly strong, I thought to put a 3/4 steel nut across the gap, the coil has to be turned off to remove the nut.
the field coil assembly weighs in a 7lb 13oz

I usually wind my own voice coils, so I can alter dimensions to suit the individual speaker,
the resistance can be worked out by multiplying the circumference of the voice coil former by the number of turns required then looking up a suitable wire gauge to give the correct resistance, for 8 ohm speaker this will be around 6.5 ohms dc resistance,

here are a couple of examples made for previous jobs.

For this speaker I used a ready made coil as they are quite cheap to buy.

At final assembly the centre bolt that holds the field coil in position is slackened off and the voice coil gap is set. I’m using three 1.5mm drills here, the bolt is then tightened and not disturbed again.

The voice coil shims are fitted next, these need to be thick enough that the voice coil is a snug fit when slid over the top, they need to hold the coil firmly in position so the voice coil cannot move during the next couple of processes, I cannot stress enough that once fitted these shims should not be moved, as the alignment relies on these.

do not try to use one piece of shim material as three pieces are easier to remove when the cone is fitted. The shim I’m using here is 0.02mm, it’s a piece of plastic removed from an old lcd screen.

Fitting the voice coil in position the coil is shown in this position for the photograph, the coil sits with 3mm of the winding showing, placing the center of the coil in the center of the gap.

Next is the spider, these are made in standard sizes, but they will require some adjustment, I have found wrapping a pice of sandpaper so it makes a tube is inserted in the voice coil hole and twisted a few times, the spider will soon fit, be careful as it’s easy to over do it.

The spider is slid over the voice coil and glued to the speaker basket with contact glue,
when set the spider is glued to the voice coil with slow set epoxy,

The epoxy applied with a toothpick in small amounts until all of the joint is covered, the speaker is then placed cone side down until the glue is set, this will stop any epoxy running into the voice coil and ruining the speaker.
Don’t use quick set epoxy variety because this doesn’t set hard.

Once the glue has set the cone can be fitted, this will require the same type of treatment as the spider to make a perfect fit, leave the fuzzy edge the sandpaper makes as this will help the glue bond.

The voice coil wires are threaded up through the center hole in the cone and attached to some small eyelets for the connecting braid to the speaker terminals, these small eyelets are from ebay, and are used in circuit board repair.

the cone is then glued to the voice coil with slow set epoxy using the toothpick method as before taking care not to move the shims.
once this has set the cone can be glued to the basket with more contact glue, when this has set the outer gasket is fitted, and allowed to cure.

The shims can now be removed, if all has gone well the cone can be gently pressed by hand, there should be no rubbing and if tapped gently with a finger should sound like a drum.
the speaker can then have the braided leads attached, I use de fluxed solder braid,

I have read elsewhere that this is the wrong thing to use but over the years I have not had a failure yet.

Now the speaker can be tested, 140volts is applied across the total winding, this means that the coil is dissipating around 10 watts, this is right in the ball park for a field coil of this size
and the voice coil connected to a suitable amplifier, I have attached a a couple of screenshots taken from an iPad app while playing pink noise through the speaker, the same settings on the noise source and amplifier were used, the microphone (ipad was 1 foot away from the cone edge)
I know that this is not in the realm of professional test equipment but it does offer a comparison,

Here is the rebuilt speaker next to the speaker used for comparison

The first waveform is from the commercial speaker.
and here is the field coil version

Its a shame I cannot play sound here as the speaker does sound very sweet it does have more sensivity than the commercial magnet speaker.

In conclusion, yes there was a fair amount of work to get this far, but the speaker does sound smooth and easy on the ear, would I do this again.
Yes, in a heartbeat, I have a Mcmurdo V waiting for a speaker and amp, Supergiant anyone?

Mike

[Scott Seickel]

Looks good Mike.  I was wondering what you were up to, you been too quiet. 

[Brad Winder]

Wow! Amazing! 

For a larger speaker, will you try to find a cast basket? Seems they would be super difficult to find, since by the time the large 12" and 14" speakers came out in the later 30's everyone seems to have moved to the lighter (and cheaper) stamped steel baskets.

[mike hadley]

Hi Brad,

Cast baskets are fairly easy to find, I managed to pick up a dead cast 18" speaker yesterday from eBay for $10,

there are cheaper Chinese stamped baskets available I did use one on a Stromberg Carlson a while back I would have been happier if the had used the next gauge of steel up.

I intend to use the 18" speaker for my Mcmurdo V, as a super giant is way too expensive for me, just think of the radios you could buy for those bucks,

besides this method uses blown speaker, that makes the donor fairly cheap, some of the older odd sizes could be a problem though

Mike