Archive for Field Coil Loudspeakers

Field Coil Conversion JBL 2220H

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Hello,

There are many vintage drivers that posses a great timbre, used in many legendary speakers but after many years their motor is demagnetized to a point that they lost their output and the impact on midrange and high frequencies.

This is true especially with Alnico magnets since they are prone to demagnetization. Revitalizing these magnets can be very hard, it is for sure not something a DIY-er can do.

In these cases converting the motor to a field coil motor can be a very sensible solution. It will never demagnetize and it can offer other advantages such as adjusting the motor force for specific applications.

Someone has a pair of JBL 2220H which he loves and uses them in a Front Loaded Horn. However, in this horn, the midrange above 400Hz is not very detailed and asked if somehow a field coil motor can spark some life in that band.

Output in this region is influenced primarily by mass of moving assembly (cone, voice coil, surround, spider, dustcap etc) and the strength of the motor. Since changing the cone mass would mean altering the timbre of the driver the only option is to change motor strength.

If we increase the electro-mechanical coupling factor BL, we will notice a rising response characteristic with frequency. This is because the output of the driver will rise but lower frequencies will be tamed down by an increased impedance in that region.

This is a very good characteristic for drivers intended to be used in Front Loaded Horns because it will compensate for the natural low pass filter of such horns. This LPF is created by the air trapped between driver’s cone and the throat of the horn. At low frequency this air has mass and all of it is pushed into the throat but as frequency rises the air becomes compressible acting like a compliance and not all the air will be pushed into the throat. This compliance is a reactive element and acts as the low pass filter. Fortunately our rising frequency response characteristic will compensate for this effect and equalize the system to a flat response.

JBL 2220H has a strong motor already with a BL of 22.5 Tm, Re of 5.7 ohms and B of 1.15 T. Being a 15 incher however, there is room for a bigger one, a field coil motor. In the JBL specs sheet we see that the gap height is 9 mm  and voice coil height is 7.2 mm so we are dealing with an underhung design with a ceramic magnet.

The motor i am proposing is a 180mm in diameter and 80mm in height. It has the same 9 mm gap height but it can reach a flux density B of 1.6T with a 1010 steel structure. As we will see, this will boost the midrange frequency of the driver so that it will respond better in a FLH.

JBL2220H_2 Field Coil Conversion

With a 4 inch (100mm) voice coil i considered a 40mm cooling vent with large flange to eliminate any noise. It also features a T shaped pole piece for field symmetry around the gap as you can see in the image below.

Field coil motor for JBL 2220H

The below graph shows the theoretical flux density curve with a 13 mm range, that’s +/- 2 mm  outside the gap. In reality the value will be a little lower @ 1.5T due to imperfection of the joints between metal parts as opposed to the 1.7T on the graph.

Flux density of field coil JBL 2220H

To determine the new BL we have to know the length of wire in the gap on the JBL 2220H. The original BL is 22.5Tm and a B of 1.15T, this means that L=22.5/1.15=19.5m. With the new motor we have a B of 1.5T and this results in a BL of 29.25Tm Very Powerful!

The field coil used is a low voltage one <30V so that power supplies should not pose any problems. The graphs above were acquired with 26W delivered to the field coil. Considering careful ventilation this should not pose any heat problems.

JBL2220H_1 Field coil Motor

JBL2220H Field Coil Exploded View

 

I simulated the new driver in a 100L closed box to see how the response compares to the original. The result is shown below. The response rises indeed to about 5-6dBs from 220Hz up.

Simulated Response field coil JBL2220H

As you can see a field coil motor not only will solve the problems of old weak motor but it can also give you the ability to tune the driver to a specific application. By lowering the current in the field coil you can get to the original parameters of the driver. By not modifying  the mechanical side, the cone, the suspension system we also maintain the original sound characteristic, the timbre of the driver.

I hope it was interesting and thank you for visiting.

 

 

About Field Coils and Magnetic Circuits

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Hello,

I’ve been studying the past couple of months some ways to improve the field coil performance and in a way to make my tools better, to sharpen them. My goal is to get the most flux density in gap for a given current in field coil.

I tried many magnetic circuits meant to take advantage of the shape of a field coil because being a solenoid it will work better having a length per diameter ratio above unity. Works better means a more uniform field in the core.

I managed to get a simulated result of 1.5T in a 1mm wide and 12mm high gap using standard 1010 steel and a field coil consuming 7.231 Watts!

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

 

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

The field coil is made with 1 mm EC wire so it will be able to handle a lot more current. You will get more flux density this way but the curve will get more and more bell-shaped.

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

As you can see for 1 Amp you get a very nice increase in flux density. Its not as linear but the difference is small.

There is a balance, hard to achieve, between linearity and maximizing flux density. And this has to do with the magnetic circuit and saturating the pole plates. I usually want to saturate them right at the gap. This way it’s less prone to modulation.

In placing the saturation area, the central pole piece and the top plate geometries have a large role. Always remember that the thin parts of steel saturate faster and zones with transitions from one dimension to another or sharp angles are also prone to saturation.

Below you can see a motor with a linear flux density curve over a 20 mm travel. The central pole piece is of the same diameter as before. You can see that making the top plate thicker killed the saturation around the gap. To bring it back, we must use a larger diameter for a voice coil. But more on that in a future article.

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

Field Coil Loudspeaker Design フィールドコイルスピーカーデザイン

 

Thank you for visiting

[Project Ryu] Ryu Spiders

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A spider for high X-max drivers.

The progressive spider allows for higher excursion. Used with Ryu concept it can have high X-max and keep good linearity of stiffness.

Here is a picture.

Project Ryu Speaker Spider

[Project Ryu] Field Coil Loudspeaker Mid-Bass Driver

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Hello,

Recently i have finished a pair of field coil mid-bass drivers. The driver has a Fs of 50Hz and a frequency response up to 7kHz. It is a dual voice coil design features a 160mm diameter motor with a max of 1.4T in the gap, Ryu spider, non-treated paper cone, triple roll cloth surround and bullet shaped phase plug.

 フィールドコイルスピーカー field-coil loudspeaker

 フィールドコイルスピーカー field-coil loudspeaker

 フィールドコイルスピーカー field-coil loudspeaker

The voice coil construction was presented in an earlier article here. Inside the gap set at around 1T flux density, the voice coil inductance was measured at 0.52mH @1kHz and 0.15mH@10kHz.

The high compliance of the Ryu spider was measured at around 3.4mm/N for this mid-bass and makes the compliance of the cloth surround the determinant factor in the system’s total compliance value.

 フィールドコイルスピーカー field-coil loudspeaker

 フィールドコイルスピーカー field-coil loudspeaker

Below you can see the impedance measurement and frequency response of these mid-bass drivers. The frequency response is measured with the voice coils connected in parallel. you can see a nice constant 6db/octave rise in response. You can use this characteristic when designing the loudspeaker if you consider baffle step or horn loading.

 フィールドコイルスピーカー field-coil loudspeaker フィールドコイルスピーカー field-coil loudspeaker

I have tested the unit and played Isao Tomita’s Snowflakes are Dancing and The Police Synchronicity. The sound is thin with the voice coils connected in parallel and no baffle but with the voice coils in series even without a baffle you can reach a good tonal balance. This indicates the unit can be used successfully in an Open Baffle system or in a horn loaded enclosure with the voice coils in parallel.

 フィールドコイルスピーカー field-coil loudspeaker

Thank you for visitng.

Project Ryu New Spider Concept Video

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Project Ryu New Spider Concept Test スピーカースパイダーテスト

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Hello, こんにちは,

私の日本語があまり上手くありませんが許して下さい.

 

Today i could do a first test of the new spider for my 12 inch field-coil loudspeaker. I attached the spider to a voice coil and to a frame. Without any added mass i applied variable DC voltage on the voice coil to determine how much it will move at a specific voltage.

この記事では、リュ·フィールド·コイル·ラウドスピーカー用の新しいSPIDERをテストしています。結果は、新しいコンセプトが良いliniarityを有することを示しています。私はそれがさらに良くするためにもっと勉強します。

 

First I had to create a variable DC Voltage generator.This can be done with op amp but since the voice coil has low resistance (Rdc = 5.1 ohms) the op amp will need to deliver high current. This is why i chose LA6520 which has a 500mA output current. In the picture below you can see the schematic. Please note that LA6520 has different pinout like in the next picture so keep it in mind when soldering. Also this circuit needs symmetric +/-15V power supply.

回路図では、LA6520 pinoutをご参照ください。

Small Voltage Generator Schematic 小さな電圧発生回路図   LA6520 pinout   The test setup includes a series resistor. We will measure the voltage drop on this resistor in order to calculate the current through the voice coil. It is advantageous to display excursion vs current as the force that pushes against the spider’s restoring force is defined by F=BL*i where BL is the motor strength factor and i is current through voice coil. The series resistor was formed from two 30W 3R3 resistors connected in parallel. This way temperature effects are negligible. Below you can see the setup. Spider test setup スピーカースパイダーのテスト·セットアップ

Spider’s behavior proved to be quite linear. Shown below is the results measured for the voice coil’s up direction. Above 3.5mm the voice coil is getting out of the magnetic gap so force becomes smaller. As can be seen though Project Ryu’s magnetic circuit is strong enough to use the fringe field to its advantage increasing this limit to about 5 mm.

テスト結果:

Spider x vs i field-coil loudspeaker project ryu SPIDERフィールドコイルスピーカープロジェクトリュウ

 

Below are some images of the test:

フィールドコイルスピーカースパイダー  Field-coil speaker Spider フィールドコイルスピーカースパイダー  Field-coil speaker Spider フィールドコイルスピーカースパイダー  Field-coil speaker Spider Voltage generator 電圧源

 

And a small video on youtube:

Thank you for visiting,

ご覧いただきありがとうございます。

New spider for field coil loudspeaker フィールドコイルスピーカーのための新しいSPIDER

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Hello, こんにちは

私の日本語があまり上手くありませんが許して下さい.

For a while, we have been discussing on diyAudio the influence of the spider. Ideally we want a low mass, low radiating area compliance for this. Also a very high stiffness towards side-to-side movement to avoid rocking modes.

フィールドコイルスピーカープロジェクトRYU用SPIDER新しいタイプ。低質量、低放射面で、それは布スパイダーよりも優れています。すぐに私は、そのパラメータをテストします。

I have studied this for a couple of months and now the concept has materialized. Tests will follow shortly to determine the qualities of such a spider.

Project Ryu Fieldcoil loudspeaker spider compliance

 

Thank you for visiting,

ご覧いただきありがとうございます。

New Paper Cones For Ryu Field Coil Loudspeaker

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Hello, こんにちは

私の日本語があまり上手くありませんが許して下さい.

Work on Project Ryu continues with a new model. This one will be a mid-bass by design but as always i will try to push the limits in bandwidth and sensitivity. The new cones are a bit on the heavy side weighting 18 grams but i think it will balance out very well in the mass controlled region.

プロジェクトの作業は、新しいモデルを継続します。このモデルは、MID_BASSスピーカーとなります、しかし、私は、SENSITIVITYとBANDWIDTHを高めるためにしようとします。フィールド·コイル·モータは、18グラムのスピーカーコーンとのバランスになります。

12 inch Cone for Ryu Field Coil Loudspeakers

12 inch Cone for Ryu Field Coil Loudspeakers

The 12 inch cone has a curved profile and concentric rings to reduce breakup effects.

12 inch Cone for Ryu Field Coil Loudspeakers

 

12 inch Cone for Ryu Field Coil Loudspeakers

 

Voice coil was designed using my Excel spreadsheet tool. You can find a link to download it at the end of the article. Double coil, each with estimated 8 ohm impedance allowing for 4-8-16 ohm operation.

私は私のExcelのスプレッドシートツールでボイスコイルを作りました。あなたは、記事の最後にそれをダウンロードするためのリンクを見つけることができます。ボイスコイルは、4または8または16オームとして使用することができます。

12 inch Cone for Ryu Field Coil Loudspeakers プロジェクト龍のフィールドコイルスピーカーのための12インチのコーン

 

voice coil design tool

 

You can download the spreadsheet here:

Voice Coil

Thank you for visiting,

ご覧いただきありがとうございます。