KS-2125

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Introduction
Prototypes
Enclosure
Crossover
Simulations
Measurements
Sound
Summary


Introduction

Bass unit is over-dimensioned with 2x18" to enable higher sound pressures without compression and over-excursion. Box is classical resistance enclosure. Leaking area of the ports is more than 160% Sd to reduce non-linearity of resistance material. Only speciality is two holes between upper and lower sections. Effect of the holes could be zero but it's good to have some tools for adjustments if needed.

Midrange box is variable volume resistance enclosure, but the port between chambers will be blocked if polar response is not hypercardioid enough. Idea is to produce some dipolic back wave to avoid excessive dryness and forwardness in the sound. Vertical directivity is not increased with many drivers or height of radiating area. Normal directivity in the front maintains some features of common 3-way box speaker. Excessive midrange directivity may cause unnaturality, loss of presence and floating (floorless) sound. In addition, bad step in polar response may cause thinness and aggressiveness. This happens if midrange is more directive than upper midrange and treble. Bad step could be compensated with on-axis response, but it will gain effects of listening distance and acoustics.

Tweeter is horn-loaded compression driver. Smaller horn is located on the back, and tilted 45 degrees up. It has reversed polarity and mild high pass shelving filter smoothing the power response. Back tweeter increases air, space and openness to the sound. Double tweeter emulates far field omni sound of live instruments. Single front horn can't handle different kind of recordings so well.

Directivity target as room and anechoic responses:
KS-2125_FR_targets.png

Technical specifications

General 3-way, resistance enclosure W+M,
horn-loaded tweeters
Dimensions W: 480 x 940 x 528 mm (W x H x D)
MT: 280 x 500 x 399 mm
Volume (90+90) + (17+15) liters
X/O frequencies 220 Hz and 1800 Hz
acoustically LR2 and But18 'LeCleach'
Drivers BMS 18N840v2
18Sound 10NMB420
18Sound NSD1095N + XT1086, front
18Sound ND1050 + XT120, back
Impedance 4 / 8 / 4 ohm (W / M / T)


Prototypes

See Resistance enclosure 2


Enclosure

Material is 24 mm birch plywood called "kilovaneri" in Puukeskus. There's one layer of SPO Yhtymä Oy's Lido Ural synthetic felt carpet in the resistance ports. Thickness of felt carpet is about 2.5 mm. Felt is clued onto outer surface with PVAc and stapled with 11x6 mm.
Inner surfaces of bass enclosure are covered with 50 mm E30 plastic foam (Etola).

Lower section of MT enclosure is 100% filled with polyester wadding. Back part of upper section is filled with E30 plastic foam, thickness 50...100 mm. There's no polyester filling or plastic foam inside the ports.

Enclosure drawing: KS-2125.pdf

MT enclosure

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Img_0890.jpg Img_0902.jpg

WW enclosure

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Crossover

Crossover is Behringer DCX2496, modified with Selectronic I/O-card.
Parameter file: KS2125.xpc (13.03.2011)
Download Behringer DCX-Remote16a.exe to investigate this file.

KS-2125_DCX-Display.png

Gain of power amplifiers:
- woofers +27.6 dB (Apoll Acoustics MA-1.0)
- midrange +18.1 dB (Arcam FMJ A38, Vol=57)
- tweeters +16.0 dB (Accuphase A-20V, Gain=-12 dB)

Connection of drivers

KS-2125_wiring.gif

Simulations

None.


Measurements

Frequency response

Measuring distance 270 cm, 0°
- red: time window 300 ms, smoothing 1/6 oct.
- green: time window 3.33 ms, smoothing 1/24 oct.
KS-2125_FR_270cm_300+3ms_1p6+24oct_R.png

Individual frequency responses
- measuring distance 50 cm, 0°
- time window 300 ms, 20 ms, 5 ms
- smoothing 1/6, 1/6, 1/24 oct.
KS-2125_IFR_50cm.png

Power average of MT-unit, ±90° horizontal red, on-axis green
- measuring distance 99 cm
- time window 6 ms
- smoothing 1/12 oct.
KS-2125_FR_hor-avg_180deg-front.png

Power average of MT-unit, ±90° vertical red, on-axis green
- measuring distance 99 cm
- time window 6 ms
- smoothing 1/12 oct.
KS-2125_FR_ver-avg_180deg-front.png

Step response

Measuring distance 75 cm, 0°
KS-2125_SR_55cm_R.png

Harmonic distortion

Measuring distance 100 cm, 0°
- SPL 100 dB, smoothing 1/6 oct.
<200 Hz on woofers' axis
>200 Hz between mid and tweeter
KS-2125_HD_100dB_1m.png

Impedance response

2 pcs 18N840 parallel in resistance enclosure
18N840x2_ZR.png

10NMB420
10NMB420_ZR.png


Sound

Bass range is clearly the best I've ever heard; very traceable, accurate (tune), balanced, fast and moderately dynamic. It's not very pressurizing, slamming and kicking because of leaking enclosure, but cone area of 2x18" compensates capacity loss quite well.

There's not too much resolution at midrange. Mild directivity step from midrange to tweeters keeps harshness away, but sometimes midrange sounds pronounced. Height of speaker increases this feature because room height is only 248 cm and roof is reflecting (painted pine panel).

Upper octave drops after 15 kHz, but otherwise there's not much to complain about tweeters.

Over-all sound is slightly mechanical - not extremely open, airy, lively, microdynamic, esoteric and so on by speaking in High-Endish. Maybe because of (originally) low-cost DSP and complexity of electrical part of the system.


Summary

Shelving high-pass of back tweeter should be slightly higher because XT120+ND1050 combination has clear peak at 5 kHz. Other 1" drivers - for example BMS - would be interesting to compare against 18Sound.

Placement:

Listening distance 280 cm
Distance between speakers 280 cm (center to center)
Shortest distance from front baffle to front wall 105 cm
Aiming 5° toe-out.

Page updated 13.03.2011