Diy AMT

[solhaga]

I used REW very successfully when I evaluated my FAs.
Very easy to pinpoint resonances and distorsion as you can sweep manually.
You must be a member of Home Theater Shack though, but that´s at a reasonable fee.

 

[solhaga]

Another thought is to use a DC current to see if the pleats folds evenly or if you have the possibility to run a stroboscope.

 

[båndsei]

Thank you for all the idea´s for measurements.
I have tried impedance measurement :

Green is a 2" Aurasound, and yellow diy AMT.
I am testing with DC before i hook it to the amp.I use app. 2Amp,and you can hardly see any movement.

This one is 170x40x6,5mm on axis and a larger horn.
I am not sure wich kind of horn thats the best for low increasement? 120' 90' 135' Or hyberpol konical .
My latest measurements are made with REW. Maybe i can post. a REW file
Bernt

 

[RayCtech]

I am testing with DC before i hook it to the amp.I use app. 2Amp,and you can hardly see any movement.

With hardly any movement with 2 Ampere it is not strange that the bass response are restricted.

When I test the polarity with a 1.5 volt battery (ca. 150mA) the movement are the half of xmax.

The total mass / weight of my diaphragm (20 x 27 cm = 540 square centimeter) are 2 gram - the moving mass are ca. 1.5 gram.

Converted to the 17 x 23 cm you use the total mass would be 1.45 gram and moving mass 1.1 gram.

I would expect the diaphragms you use may be closer to 15 - 20 gram and have possibly 10 times as stiff / hard suspension.

 

[solhaga]

I also had my doubts but didn´t dare to say anything only having practical experience of ordinary ribbons.

If the diaphragm is light and the suspension soft, it should be too weak motor?

 

[RayCtech]

If the diaphragm is light and the suspension soft, it should be too weak motor?

I use the most powerful magnetic system I practically and mechanically can fit.
Neodym N52 custom ordered magnets.

I have also developed and designed my own 8" drivers (560 gram N52 neodym magnets). Proprietary tools made for dust cap, cone and surround, magnetic system CNCed. Surround made of pure silk and several one, two and three layer cones. 24mm magnetgap and several custom voice coils, up to 30mm xmax (18mm with voice coil inside the magnet gap), spider less, corian basket etc..

With three 8" (up to 300Hz) and AMTs in waveguide above in a OB dipole system I have 15Hz to 50kHz bandwidth and need (and use) only a 1 watt JFET class A amplifier for optimum SPL. The speakers are actually so effective that I have headroom for a variable bass EQ (made for use when playing at low SPL levels) variable 0 to +6dB bass boost and variable frequency.

 

[solhaga]

@RayTech: I meant båndsei´s motor of course.
Impressive 8"s, have you measured the TS-parameters? Bl alone must be well over 20 Tm.

Do you have any information on your AMTs you would like to share with us?

 

[båndsei]

Thank you for the replies.
I have tried to weigh my active part of the diaphragm, moving mass, and it is 1-1,5gr.
I have to work on my motor.
Sensivity is app. 90db. at 2,8v.
I think my results are OK regarding my first goal:to reach 700 hz. at low distortion.

Bernt

 

[båndsei]

I might go for this motor:

It is from Kithara and it is crossed at 700 hz.

Bernt

 

[solhaga]

Yes båndsei, you are definitely doing a great job.
I think the diaphragm is the hard part to get right and you have come up with a working way for a DIY:er to manufacture one. Thanks for sharing!

Any ideas for making the motor stronger, stronger magnets?

 

[båndsei]

My choice of the current magnetsystem was: nothing in front of diaphragm, and easy to manufactor.
I think I know how the system used in Kithara is done.My magnets might be strong enough,but have to be focused in the gap.

Bernt.

 

[båndsei]

I have just found this picture:

It seems like they use a U shaped large ferit magnet.
They reach 700 hz without any horn or baffle.

Bernt

 

[solhaga]

So an U-shaped pole piece with some Neos at the top of the U?:

The AMT betwen the dots.

 

[solhaga]

Of course will the field not be that strong. The iron over the AMT is not solid.

Just an example to start with.

 

[båndsei]

I tkink the Kithara magnet is a one piece ferite magnet.
My idea is something like this ,seen from above:

Iron pieces made from 3x15mm iron.

Bernt

 

[solhaga]

"I think the Kithara magnet is a one piece ferite magnet."
Yes of course. I just did a quick sketch on how one can do it DIY.

Your idea is much better though, much easier to get a rigid construction.
Just drill holes through the two pole pieces between the magnets.

So now your are going to slaughter some transformers in order to make the pole pieces?

 

[solhaga]

So why not double sided?

 

[solhaga]

It might look as the equivalent to the motor in post #136, but the motor in the post above has a stronger, and more focused and even field in the gap.

 

[solhaga]

I´ve been trying to increase the field in the gap by having a larger pole piece etcetera, but the pole piece saturates (2 T) where the magnets ends. I also narrowed the gap.
It is also important that the pole piece over the gap has a slope.

 

[båndsei]

I have just ordered 9meter 3x15mm iron to make my new magnetsystem.
I will do it my way.
Your latest is better ,but I will copy the Kithara.
I have been searcing for 8mm thick neodymiun magnets.I found some 30x30 x7,5mm in Poland.http://www.magnet-magnete.eu/ndfeb-magnete-quader-mittel-g1-235.html
10 pcs.60 EUR incl. shipping

Bernt