Tuning / Tweaking of BP 115 A

Hello,

I bought some nice two way Bluetooth loudspeaker with accumulator (battery).
Its a Thomann cheapo named Fun Generation BP115A with 15 inch bass and big tweeter. It has 93db efficiency (I found the repair driver PDF spec), stiff paper bass diaphragm and I guess - as far as I could see - a tweeter with a decent sounding mylar diaphragm. Only the tweeter seems to have a 12 db filter with 4 Ohm resistor. Bass seems to be directly driven by the amp.

In original condition it sounds as most PA loudspeakers o.k.. But in my ears - like in most PA systems - some typical hifi faults are built in. In order to bring efficiency up with a trade off in sound quality.

It is easy to open the plastic loudspeaker enclosure. The box is pretty undamped (only just a little little bit black BAF wadding and a highly tuned bass reflex. I want to turn this speaker in a good sounding hifi speaker capable of deeper bass than the 60 Hertz lower cut off frequency.

So tuning - tweaking of the FunGeneration BP 115 A loudspeaker was done:

1. putting some damping on all surfaces and fixing it
2. a method I prefer to get good sound is covering the bass diaphragm on the front side with aluminium foil.
3. Tuning the bass reflex to 30 Hertz as I want to hear the lowest octave - even if it is surely sloping.


You can see some fotos of the loudspeaker which are self explaining.
The aluminium foil is just simple foil used in the household. Glue is simple glue which fits for paper and metals.
You cut the foil into pieces (form of cake pieces) put thinly the glue on the backside of the foil an then fix it with the use of your fingers to the diaphragm beginning from the outer edge. I am used to do this since the year 2000. I had for ten years a website online exactly describing how this is done with thinner foil from the gold smith but it seems to be too much work for the most Do it yourselfers to try this tweak out. As I (nearly) never found anyone reproducing this method.

From theory it is known that the combination of metals and paper can bring a lot more stiffness to the diaphragm (known since the 60ies, but history of loudspeaker building describes this in patents since the 30ies of the last century - it means right from the beginning of loudspeaker production of the dynamic type). For germany it was Dr. Emil Podzus (aluminium foil and hard foam), later it was english speaking D.A. Barlow in the 60ies with duraluminium sheets and expanded polystyrene of thick quality. In germany these aluminium foiled loudspeakers are known as Podzus-Görlich loudspeaker drivers. Mr. Görlich was helping out as a young man at the factory of Mr. Podzus. He later produced these diaphragms with aluminium foil and hard foam. He does it until today.

I measured the loudspeaker diaphragms paper + aluminium very often. One sided and both sided (last means it is a sandwich-cone). Coating of the driver on just one side brings resonances down - makes the speaker sometimes more linear - but always the loudspeaker driver becomes much more stiff and has therefore a significant better impulse reproduction.

Best is this checked by ear while covering one driver with aluminium foil and leaving the second stereo speaker undoped. You can then use your balance knob on your amplifier for an A / B listening test.



Here about the theory:

D.A. Barlow: The Development of a Sandwich-Construction Loudspeaker System. Page 159-171. From the AES anthology of articles on loudspeakers, article written 1970.

Citation begin (page 160):
"As paper cones are thin, these resonances are bending modes. The stiffness of a material in bending, for any given geometry and edge condition, is proportional to Young`s modulus and the cube of the thickness, ignoring variations in Poisson`s ratio. ... On this basis, paper is much stiffer than metals (except beryllium) in spite of its much lower modulus, and lower density materials such as expanded plastics are even stiffer, as may be seen in Table. A method of obtaining still greater stiffness is sandwich construction. ... In bending, the maximum stress and strain occur at the outer fibers, the material at the neutral axis being unstressed. Better use of material can thus be made by concentrating it at the outer fibers. A familiar example is the tube. In the case of large areas, the same effect is obtained by using a thin high-modulus material for the outer surfaces, and a light-weight material or form of construction for the core. ... This sandwich will of course be much stiffer than the same total weight of either material used separately. The skin material should have the maximum ratio of modulus/density. Beryllium, the best material, is impractical due to difficulty of rolling and possible toxicity, so aluminium is the obvious choice. The core should be as stiff as possible in the thickness direction and have minimum density. Honeycomb aluminium or impregnated paper are frequently used in aircraft construction and could be used for flat diaphragms."
Citation end.


The Thomann bp115 a becomes after this tuning much more listenable and precise. I use it with the loudspeaker put on the side as the tweeter and the bass seem to be better time aligned than with the tweeter upside. (Listening position loudspeaker on the floor and listener standig upright).

The mylar tweeter sounds quite good, I like it.

Dragan from Germany
 

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some more fotos - covering diaphragm with aluminium

some more fotos

with the original black baf wadding I closed one bass reflex opening (much too big for low tuning). Optically the black wadding is ideal. Behind it is a piece of plastic glued in order to air tighten the opening. Behind this is some yellow damping fixed with glue.

I used a toilet paper roll painted black - approximately tuning the 70 liters of enclosure to 30 Hertz.
As the bass driver has a stiff suspension and only a very short travel the bass reflex is not underdimensioned. At full loudness there is no chuffing to be heard.

The BP 115 a could play a bit louder (the wattage could be more) but as a tweak the Fane 15-300TC could be brought in (has more efficiency) and the tweeter could be put out of use. But I leave it like this.
 

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The BP115a has this size: 67cm x 42cm x 25cm - the volume is about 65-70 liters.

The polypropylene enclosure has only 2-3mm thick plastic but due to the curvy built and stiffening ripples it has low resonances. The accumulator is on lead acid basis and can be changed and approached via screws on the outside.
 

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I got an answer from Thomann concerning the BP115a Bluetooth Loudspeaker:

The accumulator is of a lead acid type - 12 Volts. And it has no undervoltage protection. You can see by some LEDs on the backside how much energy has the loudspeaker left for operation. If you reach the red LED you have to connect the power supply to the box.
 
I added now a same size magnet fixed with glue to the bass driver (in enhancing manner for the efficiency). This was very audible and I guess the driver got more than 1 db louder. Bass is now better defined. The lower Q allowed me to take little bit of the damping out of the box so that the reflex port is working better.

All in all bass and treble are better integrated now, the bass sounds "faster" and more agile, the box got louder and has more bass output.

Alternatively I have a Fane Sovereign 15-300tc loudspeaker. But I want to cover the diaphragm with aluminium foil before putting it into the enclosure of the Thomann BP115a for testing it. The Fane Sovereign has nearly 100db efficiency and in the treble even more than that. The tweeter would be deactivated then and the portable speaker would become a true point source loudspeaker.

As there is a five band equalizer integrated the frequency response of the Fane could be adjusted to taste / linear.
 
I have put out some of the yellow damping where the tweeter is. There the damping was in a double layer what damped the enclosure too much for a driver with lower Q it had to be taken out. Now there is more bass.
 

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I have now sent a question to Thomann Germany to ask for the schematic of the amplifier in this loudspeaker BP115a. Maybe I can turn it into current drive.

This would bring distortion much down.

Like here:
https://www.diyaudio.com/community/threads/chip-amp-modification-to-current-drive.389985/

However since I have put a double magnet onto the bass driver the bass and mids are more prominent. I will try to make the tweeter louder by bypassing the 4 ohms on the high pass for the tweeter.

More efficiency is desireable for a portable speaker.
 
Double magnet.

Extra magnet made the bass reproduction more tight.

As a result I took some damping out of the box and got a little bit more from the bass reflex output.

Now I will take the 4 ohms resistor from the tweeter.
 

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I have put a switch for bypassing 4 ohms 10 watt resistor in the tweeter high pass.

I checked the wiring:

The bass if without frequency devider put directly to the amp.

Tweeter has resistor 4 ohms, 2,25 mfd and a coil for 12 db filter.
 

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Another tweak for better undistorted sound is:

glueing some damping textiles on the metal chassis on the side towards the paper diaphragm of the bass.

This minimizes early reflections on the metal chassis coming back through the cone.

Where the sheets of damping textile are lying they are glued to the backside of the metal.
 

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Some fotos of the amp.

Do not want to disassemble it for examination.

Hope to get the schematic of the amplifier. If it is BTL type - implementation of current feedback would not be done. Because its too complicated.
 

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