Design & Production of original-designed full range field coil speaker unit. [AUDIO-2]

I want to upload the article about the Origianal designed speaker unit.

Designing concepts are
1) Original design without using "Speaker parts" for Voice Coil, Magnet, Cone, Frame
2) Full range unit with diameter around 9'
3) Edgeless, Dual spider damper
4) Very light cone paper (Straight cone)
5) With Field Coil magnet
6) Short voice coil & Deep Gap
7) Around 8 ohms impedance
8) Simple and Don't waste money.
9) Make and use Original wire winding equipment

Although I had no experience in disigning of speakers, I picked up some characters based on my past listening experience.
There are a lot of information when I searched through WEB, I can leverage those information.

speaker view.jpg

***Over all Assembly Drawing***
____1) Magnet wire (Polyurethane Enamel wire)
____2) Yoke Front Main Plate
____3) Pole Piece
____4) Spider Damper made by Bakelite
____5) Yoke Side Plate & Rear Plate
____6) Yoke Rear Plate
____7) Voice Coil
____8) Voice Coil Bobbin
____9) Yoke Front Sub Plate
____10) Cone Paper

Unit overall diameter is around 9 inches, Voice coil size is around 1.2 inches.
But detail design is not fixed yet. But To design detail, need to consider cost, easy production.

Items related to fixing voice coil diameter are
____1) Diameter of pole piece
____2) Gap width
____3) Jig for voice coil bobbin
____4) Thickness of voice coil bobbin material

1)About pole piece, considering cost and Relative permeability, I decided to use SS400 round bar. Close to 1.2 inch, I chose the size called diameter-28.0mm.
2) To get high Magnetic flux density, have to make gap inbetween pole piece and front yoke plate as narrow as possible. Narrow gap create difficulty to install voice coil bobbin without touching to pole piece and front yoke. I'd checked several spec exmple thuru internet, narrowest case is 0.1mm, ordinary case minimum gap is 0.2mm and maximum gap is 0.7mm. I tried to find jig with cylinder shape to make voice coil bobbin for dia-28.0mm to 29.0mm. Around dia 30mm, there are a few candidate like aluminum foil center core, small glass bottle for red chilli pepper, etc. But nothing suit. Finaly, I've found dia-28.6 stainless pipe for automobile muffler sold with cutting. I decided to use it. So, bobbin's inner diameter is 28.6. It means gap inbetween pole piece to bobbn is (28.6-28.0)/2=0.3mm
3) Choose bobbin material with certain strength to endure for many years usage.
Thickness of bobbin should be around 200um(0.2mm). Suppose I use Polyurethane Enamel wire with diameter-0.2mm 2layers, then total thickness of voice coil is around 0.6mm, And consider 0.5mm of outer gap, the hole diameter of front yoke will be 28.6+0.6*2+0.5*2=30.8mm. Decided front yoke hole diameter as 32.0mm. Balance between inner and outer gap is not even, I will attach paper with certain thickness to stainless pipe jig for adjustment.

***Design of Field Coil Bobbin***
I already decided the diameter of pole piece.
I tried to find bobbin material for field coil. Core inner diameter needs more than 28mm, and found polyvinyl chloride pipe with 31mm inner diameter. Outer diameter was 38mm. Wind coil magnet wire to this bobbin.
I arranged the bakelite endplate for this bobbin.

Choice of field coil wire (Polyurethane Enamel magnet wire) was depend on the seller's line-up.
Checked seller's web site, 1kg unit wire has many diameter range.

Resistance ratio and copper weight /kg is depend on wire's diameter. Previously, I repaired Magnavox P232, field coil 's resistance was 5,200 ohms. This time, coil wire is dia 0.18mm. current value for this Magnavox P232 is around 70mA. This is just good for Pentode current output. This is first time for me to make origial designed Speaker unit, sharing this power supply for newly designed fullrange unit, because I cannot tell whether original unit can work properly or not. As a result, I chose 1kg of diameter 0.18mm UEW wire (Polyurethane Enamel Type) to use.
Specification of this wire is 757 ohms/km, and 0.24kg/km. So, 4,160m is the length for 1kg of wire. And total resistance value is 3,155 ohms. compare to Magnavox P232 15 inch unit, total resistance value is about 60% only. Original unit is 9 inch diameter, so, I decided to use 1kg of this UEW 0.18 wire as field coil. Field current 70mA case, Power consumption is 0.07(A)*0.07(A)*3,155(ohms)=15.5W.

Consider about field bobbin thickness, width of bobbin is 64mm due to other factors.
Outer diameter of Vinyl chloride pipe is 38mm, this is starting point of winding.
Calculate the thickness of wound wire, weight ratio of copper is 8.96, volume of 1kg copper is 1,000/8.96=111.6 cubic cm without consideration of volume of viod.
Volume of pipe core is (38/2)^2*Pai*64/1000=72.6 cubic cm
Total volume is 184.2 cubic cm. SQRT(184.2/(6.4*Pai))=3.03 cm, Thickness of copper part is 3.03-3.8/2=1.13cm.
Confirmed 64mm width has no problem, continue designing with 64mm width.

Attach field coil bobbin drawing below.

Handmade Bobbin for Field Coil

***Original hand made wire winding equipment***
Polyurethane Enamel wire's length is 4,160m, if wind such a long wire to the bobbin, it's necesarry to use wire winding equipment. I'd check auction and found some items around US$50 to 60 price range.
It is not so expensive to buy, but this time I decided to make my original wire winding equipment.
Main parts are
base board, 4 pillars, shaft for bobbins, 4 ball bearings, rotating handle. It cost about US$20.

No drawing exist, all Gauging alignment.

***Design of remaining of magnetic circuit and voice coil***
Pole piece diameter = 28.0mm
Voice coil diameter (inner) = 28.6mm
Front yoke hole diameter = 32.0mm
This time design remaining part of magnetic circuit and voice coil.
Concept is "short voice coil" and Deep gap.
This means voice coil will not travel until outer of gap.
Thickness of yoke front plate is 9mm, but I need deeper gap, then add 2 pcs of sub-plate to front plate.
Total thickness of front plate is 15mm.

Tentatively calculate following specs,
Divide this 15mm into 3 parts, front travel stroke=5mm, rear travel stroke=5mm, voice coil width=5mm.
Start design with this tentative value.

1 turn of voice coil wire length is 28.6*Pai= 8.985mm
if use diameter 0.2mm Polyurethane Enamel wire, I can wind 25 turns in 5mm width.
Wind wire 2layers then number of turn will be 50.
Total length of wire will be 4.49m. Resistance of this 0.2mm wire is 607.6 ohms/km
then this voice coil DC resistance is 2.73 ohms, it's too low for my design.
I need to recalculate voice coil width and wire specs.

I finalized design with 7.9mm width, use 0.18mm wire.
Wind 44 turns 2layers, length=7.91m, DC resistance= 5.99 ohms. It may be used as 8 ohms impedance.
Front and rear travel stroke =3.55mm

I want to make this unit without edge, then I needed double damper.
So voice coil bobbin need to have 2 places for 2 spider damper.
Total voice coil bobbin's width is 42mm.
voice coil bobbin design.jpg

***Production of Voice coil bobbin and refine of design***
Pole Piece is dia-28mm SS400 round rod (bar), Hole of front yoke is dia-31mm. So, Gap is 1.5mm. In 1.5mm gap, need to place 2layers voice coil and bobbin without touching.
As a Jig for bobbin and Voice coil winding, I prepared dia-28.6 stainless pipe. I chose "Shibukami" for the bobbin material.
Shibukami is Japanese Traditional Stencil Design washi-paper, originaly use for making Kimono pattern . Very thin persimmon juice painted paper, combined into 3 layers.

I chose the 110um thickness shibukami paper for bobbin. Put 1 layer of copy peper on the stainless pipe to make bobbin's diameter slightly big, and Roll shibukami-paper for bobbin on it. I cut bobbin paper exactly follow the length, no extra length to wrap, to fix the bobbin shape, use very thin 5mm width heat resistance capton tape from inner side.

To make 45 turns voice coil with dia-0.18mm Polyurethane Enamel wire, it takes about 8mm by calculation. But actually need almost 10mm width to wind 45 turns.

Marking the line for voice coil alignment, wind Polyurethane Enamel wire slowly and carefully for 1st layer 45 turns, then spray electro isolation varnish to the voice coil to firm the coil. Wait about 1 hour, then can start 2nd layer.
To wind 2nd layer nicely, 1st layer's alignment is very important. No gap, wind tight is required.

As a result, bobbin width needs 42mm, because spider damper fixing point is 20mm from center of voice coil for each front and rear.

***Design of Yoke and Pole Piece***

Yoke Front Main Plate
Yoke Front Sub Plate
Yoke Rear Plate
Yoke Side Plate
Pole Piece

***Production process of Yoke and pole piece***

Trial assemble of Yoke and Pole Piece

31mm diameter hole for yoke front plate was processed by professional fabricator, but other holes are done by myself.
Accuracy was very poor, then I needed additional adjustment for alignment.
Disc shape plate with 3mm thickness is purposed to get Deep magnetic gap. Fixed 2 pcs of this plate to Front yoke plate, Total thickness of front yoke is 15mm.

Finaly managed to align center hole to pole piece.

I used 9mm and 3mm thickness SS400 plate with oxide layer. I Painted with Zinc epoxy spray.

***Design and Production of cone paper***
Please see drawing below.
cone cut.jpg

Apex angle is 120 degree.
VC diameter is 29mm, cone max diameter is 228.6mm,
Draw 2 concentric circles, inner circle diameter is 33.5mm, outer is 264mm, cut out sector with 311.8 degree.
Black dotted line is cone's outer end, fold back cone paper 8mm with this dotted line to make cone firm.
Outer circle size including folding back is 280mm.

Half circles dia-158mm and 140mm are cutting line and over wrap width to make cone shape.
Material for cone paper is sshibukami with 160um thickness.
Use circle cutter to cut, and use divider to scribe folding back line.
Glue for cone paper, I chose 3M Scotch "Super multi purpose".
Effective weight of cone paper is 5g, weight of bobbin is 2g. Total weight is 7g only.
For comparison, spider damper weight is 3.5g per piece.
Photos of cone after processing

***Design and production of Spider damper***
Concept of this unit is edgeless, so,there is no support in cone edge. Unit needs more than two dampers to hold cone or voice coil bobbin. I designed double damper to hold both end of voice coil bobbin.
It's not gathered type, spider type. Material is bakelite, 1mm thickness.
Outer size is 100mm diameter, inner hole has 28.6mm diameter, same as voice coil bobbin.
Shape of spider follows one of famous European old unit.
spider design.jpg

Print out the damper shape to copy paper, and attach with starch paste. Use 3.4mm drill bit to cut out shape roughly.
Detail will be finalized by file.
Damper holder is 50mm length of M4 screw without head. Place 3 points on the dia-95mm circle line.
Tentertively fixed damper alone and checked damping function. It looks to hard, cannot get enough stroke distance.
Then file damper until around 0.5mm thickness.

***Progress situation of Production 22 Jan. 2014***

1) Made 10 units of voice coils and bobbins including spare
2) 2 pcs of field coil were completed production. Length of Polyurethane Enamel wire for field coil is calculated more than 4,000m. Suppose 1 turn length is 22cm (Ave winding diameter 7cm), then total length of 1kg dia-0.18mm Polyurethane Enamel wire needs more than 20,000 turns. 2 to 3 hours per day, It took 3, 4 days to wind 1 field coil. Have to make 2 units, it was very tiresome.
Lead wire is solderd, wrap coil with isolation paper (same as cone material). This is also good for cosmetic purpose.
Total resistance of field coil is around 2.7 to 2.8k ohms as expected. Resistance value is not stable, due to temperature condition.
3) Changed current out put from 7-Pin connector to terminal block. 7-Pin connector for Magnavox field units moved to exclusive box.

Photo shows field coil with Yoke and spider

***Remaining process***
Fix damper and Bobbin
Fix cone paper and bobbin
Fix Voice coil lead wire to voice coil Polyurethane Enamel wire
Fix terminal Block for signal and power supply

It takes about 8 months, project status is final stage now.

***Last things to do are***
1) Align the voice coil lead wire
Fix voice coil lead wire with capton tape in 10mm distance.
2) Fix cone side damper tentatively.
Front (cone side) damper must be attached before fixing cone and voice coil bobbin.
Make 2 small notches in front damper for voice coil lead wire.
3) Make holes for voice coil lead wire and flexible wire on cone paper.
4) Adhere voice coil bobbin and cone.
for the cone paper, make about 5mm wrap zone at the time of paper cutting.
Use "cemedain C" glue for fixing cone and bobbin.
5) Fix Cone & bobbin assembly to front yoke plate, tentertively. Screw front damper nut.
6) Fix rear (magnet side) damper, tentertively, screw nut.
7) Adjust damper height with screws.
8) Adjust voice coil position in the center of front yoke,
9) Threading flexible wire to the cone paper. And solder voice coil lead wire and flexible wire.
10) Fix terminal block.

All precess is completed.

Check the static specifications.
DCR of voice coil is 5.6/5.9 ohms OK
DCR of Field coil is 2.7/2.7 k ohms OK (within Expected range for using 1 kg of dia-0.18mm wire)

Connect to the power supply, and measure DC voltage and current.
DC Voltage = 210V both channel
Field current = 62mA both channel

***Sound performance check (Unit alone)***
Re-connected UV-845S-A1 Amplifier from Multi-amp use to single-Amp use for fullrange unit.
Feel bit of over damp in low frquency range, but It operates perfectly, it's almost trouble free .
No smokes, no vibration sound, no voice coil touch.
But I still feel over damping in low frequency.

***Consideration about over damping in low frequency***
Although I don't know about the reason of over damping, when I touch to the cone and tried to move cone paper to stroke.
I still felt some resistance in it's action. It might be reduced.
Checked the dampers pattern, there are 3 points of bridge shape that hold inner circle(bobbin).
Each bridge has 2 branches, cut 1 branch from each bridge for reducing registance for stroke movement.
I felt much smooth movement.

***Sound performance check 2 (Unit alone)***
I'd checked unit again with music. Low frequency sound has more volume, Welbalanced overall.
Cannot play until lowest end, but it's enough for practical use. I can change sound expression with adjusting field supply current. Sound tight or loose, speed high or low.
This check is done without box and just put on the floor, I think that put this unit to flat baffle or rear pressure free box, then I can get better sound.

***Points that I noticed***
1)1.5mm magnetic gap is manageable to fix Voice coil & bobbin inbetween, even for amateur. I will try 1mm gap for next trial

2)Processing accuracy is very important. Don't be stingy. Better to ask professional to drill hole to metal parts. You can get accuracy and will not waste time.

3) Stiffness of damper is still under observation, if still need to make it softer, I will cut out damper bridge and change material.

4) 160um cone paper might be too thin, my unit is still ok, because it does'nt have edge, so far from problem.
If change thickness sound might have changed..

5)This time I designed field coil follow the specs of my Magnavox woofer. If make with thick magnet wire, then easier to wind field coil, because number of winding is lesser.

6) I didn't have any experience to make speaker unit before, but could get result. I'm happy if my article can help audio extinct species who want to design and produce own speaker unit.
This project has ended succesfully, even the person who has no experience designed, processed, purchased parts. Many many information in WEB helped me, I hope I also can help the person who is going to design and produce own hand made speaker.

If I have chance to make field coil speaker for 2nd time, I want to correct and change design for few points.
1) Make front yoke plate thicke to 15mm, remove 3mm sub plate.
2) yoke side and rear plate change to 12mm thickness.
3) Change hole position because of yoke plate thickness change.

***Installation to Rear pressure free box***
This unit has field coil magnetic circuit, it is very old fashioned style.
Aim is light and high speed sound.
Although this is first unit for my experience, I could follow my concepts through designing and production, I satisfied with sound.
Through sound testing, I put bare unit on the floor without box.
I cannot deny the low frequency sound was offset by reflection of the sound from behind the unit.
To prevent this offset, I want to install this unit to either box or buffle board.

There are many reports that Old field unit was installed to flat baffle board,
Flat baffle board needs certain size to prevent back sound reflection.
This is very inconvenience for me, because my main speaker is Altec A5 "type", cannot exist together in my room.
Another idea came to me was free back pressure type, Although position is slightly too high, it still can put on top of Altec 805B. Suddenly image of the barrel came to my mind. There are several choices of Barrel for coffee, wine....
My unit's outer diameter is around 250mm, then I chose barrel with the size of 395mm bottom circle board, height is 490mm.
Price is about US$65 for oilstain finished type.

Cut the 250mm diameter circle to bottom board to fix unit. but there is no frame with the unit because of frameless.
Then need to fix magnet yoke to the barrel instead of front frame, magnet yoke is sandwiched between wooden boards and wooden board is fixed to the barrel.
This is the wooden frame for the unit, I fixed Speaker terminals (for field coil and voice) to
this wooden frame.
Made 250mm diameter hole to Barrel bottom board and fixed wooden base for wooden frame.
Field coil Speaker unit installed to the Barrel
Set up on top of Altec A5 "Type" box (I made spcial 370mm height speaker stand)
And I made special barrel holder, too.

***Overall Sound Impression with Barrel box***
Basically sound impression has not changed compare to bare unit. Very light sound, high speed.
Volume of low frequency sound became rich. I wanted to have high frequency sound more.

***Attach sub cone***
To get high frequency sound, I attach sub-cone to the unit.
Sub-cone diameter is 80mm.

***Add Tone-control circuit to pre-amplifier***
I'll right araticle separately.

***End of the article***
After attach sub-cone and add tone-control circuit, sound is not wide range but very well balanced. Good for 50's-70's music, especially for vocals, Jazz combo.
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