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Speaker Projects : the BasicBass

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The BasicBass speaker-kit project..

(new retail price : 383 euros a pair, VAT incl. effective from March 1st 2006)

darkgrey or cherrywood vinyl finish

BasicBass1tmp.jpg (38081 bytes)

 

Introduction

In the beginning all we wanted to do was a bass engine : a small sized speaker for small rooms (about 9-15 m2), capable of delivering plenty of clear undistorted bass at a very, very low price !

We had to find a strong motor at the 6.5'' woofer class and we came up with P17REX/P by SEAS. Its 39mm voice coil proved more than just capable of handling large amounts of low-frequency power. It actually gave no indication of compression or mid-frequency coloration while reproducing very low frequency acoustic bass 'bursts'.

P17REX_P_Asv.jpg (102849 bytes)

 

On the other hand we wouldn't want anyone to say 'ok what about the tweeter drivers ? Are they the usual 3/4'' flat-magnet cheap drivers everybody uses in cheap speakers ?'

For this we selected SEAS, chambered 27mm precoated fabric dome tweeter that would definitely match the qualities of the 6.5'' woofer. Tweeter model 27TDC will make anyone forget this kit's price and wonder!

27TDC_Bsv.jpg (93307 bytes)

 

The enclosure design..

Many speaker designers tend to choose increased volumes even for small 6.5'' woofers in order to achieve an extended low frequency response (i.e. a very low cutoff frequency). In this way power handling capacity of the speaker-under-design is severely compromised.

BasicBass was given a 18lt net volume only, thanks to P17REX/P Thiele Small parameters. The reflex-port was tuned at 44Hz and the resulting  cutoff frequency (-3dB) was placed at 40 Hz.

High-density 19mm thick MDF was selected for the enclosure material.

FV29NEWsv.jpg (76086 bytes)

Driver and cabinet data were input to our simulation software (LFA v5.0) to derive the optimum driver and port positions over the baffle board. Vent emissions were kept to a minimum outside the tuning frequency as illustrated in the following picture:

 

LFA_SHOTsv.jpg (110804 bytes)

BasicBass was designed to deliver clarity and exceptionally low distortion in the frequency range 120-900 Hz where enclosure standing waves and vent resonances dominate.

A near field SPL measurement in the port's mouth verified the results of our efforts (although measurement is degraded by nearby woofer interference) :

BB_VENT_near_field_sv.jpg (69148 bytes)

 

The crossover network..

We have said it before : we believe that crossover networks are intended to manipulate, compensate and align driver responses. And in that sense we certainly do not promote minimalism in this area of electronics because we definitely do not encourage the excitement of driver's nonlinearities and cone break-up waves that many DIY designers induce through their oversimplified crossover network topologies.

The idea has always been simple as that : Intermodulation distortion artifacts never appear in SPL measurements. Crossover networks are not merely 'frequency dividing' circuits. It takes more than a desired SPL curve to really design a speaker system.

BBSPL.jpg (114935 bytes)

The picture above depicts the (simulated in CALSOD) anechoic SPL response of a BasicBass speaker on tweeter axis, at a distance of 1m, for 2.83Vrms input.

Sensitivity was estimated at 86dB SPL and the upper cutoff (-3dB) frequency at approx. 32kHz !

The small 'valley' in the mid-frequency range compensates for early reflections in typical living room environment.

A circuit schematic follows right below:

BasicBass__2sv.jpg (64179 bytes)

A variable value of resistance in series with the tweeter driver (selected freely in the range of 22 to 56 Ohms) will give everyone the opportunity to adjust mid-to-high frequency reverberation field in typical small sized rooms. The whole circuit has been carefully optimized so that this will be a trouble-free feature.

Low pass filter's inductor embodies a mild-strength (inductance multiplication factor 1.2 approx.) hollow core for saturation-free operation and decreased resistance : only 0.3 Ohms for more than 2mH !  

 

A few words on impedance..

MTZ Audio staff has got an extensive experience on amplifier design hence regards the nature of a speaker's complex impedance very critical for amplifier feedback operation. We all know that amplifier specs are measured across laboratory resistive loads of 4 or 8 Ohms. IEC 268 standard was never revised despite the strong criticism by many researchers and designers involved in the amplification case.

A special impedance correction circuit was engaged in parallel to BasicBass' crossover network, to render the latter's impedance as resistive as possible in a wide frequency range. This sub-circuit is shown within the crossover network schematic just above. Its 'healing' properties can easily be understood in the following impedance graph:

BBIMP.jpg (89043 bytes)

In a very wide frequency range (400Hz to 40kHz) a nearly flat impedance for the BasicBass speaker was achieved.

At lower frequencies where large impedance resonant peaks occur to absolutely all loudspeaker systems (up to 40 or more Ohms !) such correction circuits demand extremely large inductance and capacitance values and therefore are not feasible. BasicBass' impedance correction circuit is surely a decisive step towards sonic perfection, especially for those who do not own highly expensive amplifiers.

 

The assembly procedure..

Sometimes small details make the difference.

One piece of tar-sheet glued behind the woofer driver..

BB_TARsv.jpg (93035 bytes)

Lamb wool by Visaton for the difficult task of internal absorption (120gr per cabinet)..

BB_WOOLsv.jpg (203112 bytes)

Robust welding joints on the driver terminals..

FV34.jpg (48428 bytes)

..are some of BasicBass's small secrets.

Some useful links (in Greek) in AVforum's discussions concerning the BasicBass project :

http://www.avforum.gr/showarticles.php?topic=11&subtopic=24675&st=0


wpe1.jpg (16992 bytes)

 


BasicBass speaker kit


Technical Specifications

two-way, two-drivers' design,

elegant 19mm-MDF enclosure in cherrywood or darkgrey vinyl finish

vented-box low frequency alignment, port tuned at 44Hz,

high freq. driver: 27TDC, fabric 27mm dome by SEAS,

bass-mid driver: P17REX/P, polypropylene 6.5'' cone by SEAS,

enclosure volume: 18lt net,

speaker ext.dimensions in mm: 218x458x278 (WxHxD),

SPL response cutoff frequencies (-3dB): 40Hz-32kHz,

sensitivity: 86dB SPL / 1m / 2.83Vrms input (anechoic),

power handling capacity: 100W rms per amplification channel,

impedance: 8 Ohms,

crossover frequency 2.3 kHz.

complete kit inclusive of screws, absorbing materials, SEAS drivers, crossover components, dust frame assembled with black cloth, vent tube and mouth, cabling, printed lab report..

 


Drawings and Assembly Instructions..

Hint : images below, link to downloadable pdf files.

BasicBass_Enclosure_Y02-118_00-003vAs.jpg (34308 bytes) BasicBass_Final_Assy_Y12-318_00-005vAp1s.jpg (39960 bytes)

BasicBass_Final_Assy_Y12-318_00-005vAp2s.jpg (58438 bytes)   BasicBass_Final_Assy_Y12-318_00-005vAp3s.jpg (66935 bytes)

BasicBass_Y12-318_00-001vAs.jpg (53393 bytes)

 

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PLEASE READ CAREFULLY THIS DISCLAIMER : MTZ Audio staff do their best to ensure that all material -technical or not- presented in this website is accurate and correct; however MTZ Audio holds no responsibility for this. In a similar manner, should anyone claim to have been harmed directly or indirectly by the use of technical information presented in this website, MTZ Audio will not be liable for such claims.