SpeakerBuilding.com - Making of M1

Making of M1

By Man-Sun Huh
20 Jan 1998

Printed from SpeakerBuilding.com, 09 Sep 2010 06:00
URL: http:///content/diy/1021/

System Introduction

2-Way closed box speaker with Scan-speak units. This speaker system is designed for near field monitoring in small rooms ("M" for M1 means Monitor) and for reproducing clear images. I selected a closed box system. Though "M1" has restricted bass, it can deliver more articulate bass response than any other bass-reflex system. In my room (about 20 square meters), it can play chamber music, jazz and pop with satisfaction Fig. 1. Picture of the M1.

Units

Woofer: Scan-speak 18W/8545 7" paper cone with carbon fiber

This ia an amazing mid-bass unit from Scan-speak Fig. 2. The drivers. It has a sufficiently low bass response and makes me possible to make a closed box system. Its sound is very clear and wide in midrange but need some network technique to control some gentle bump (300-1000 Hz) in the midrange. This unit and some revised versions are used in many well known commercial speaker systems (Wilson WATT5, ProAc 2.5 etc.). The Thiele-Small parameters of specification and test results are:

Parameter                    Listed    Measured
-----------------------------------------------
Sensitivity (dB/1m 2.83V)      88.0        87.8
Free air resonance (Hz)        28.0        31.6
B x L (Tm)                      7.2         7.9
Moving mass (g)                19.0        21.7
Qms                            2.41        2.35
Qes                            0.35        0.39
Qts                            0.31        0.34
Vas (Liter)                    54.0        37.1

Tweeter: Scan-speak D2905/9300 28mm soft dome

It's another amazing tweeter from Scan-speak Fig. 2. The drivers. And It's updated version of the legendary D2905/9000. Though not cheap, I can say it has very high cost performance. It's dynamic range is beyond expectation for soft dome and well match with 18W/8545.

Cabinet and network

Cabinet drawing showing the front and side Fig. 3. Cabinet drawing .

I wished system Qtc around 0.577 (Bessel Alignment). This low Qtc can give very detailed response what I like. The optimum volume was 16 liter but I designed internal volume 20 liter and Qtc is measured 0.53. The network will raise Qtc somewhat.

Network

The crossover network Fig. 4. The crossover network Fig. 5. Picture of the crossover.

Here are some criteria for network design:

1. The simpler, the better. Only 8 elements are used. In tweeter signal path, single capacitor and resistor are used for pure sound.
2. Low crossover point (1800Hz). It can deliver a wide dispersion. But it need reliable tweeter such as D2905/9300.
3. Acoustically 4th. order Linkwitz-Riley filter slope around crossover point.

Test Results

Anechoic response of system Fig. 6. Anechoic response. Sensitivity is around 84dB/2.83V and -6dB point in bass is about 47Hz. It is very satisfactory result for small speaker. It's bass roll-off is about -8dB/Oct. With room enforcement in bass, this speaker can reach down to 40Hz.

Impedance of system Fig. 7. Impedance. Minimum impedance is 7 Ohm and it's a very kind load for amp. No wrinkle in graph shows good restriction of cabinet vibration.

Anechoic response of each unit Fig. 8. Anechoic response of each unit. Anechoic response when tweeter polarity is reversed Fig. 9. Anechoic response with tweeter polarity reversed. Well designed crossover must show steep dip at crossover point and M1 also shows steep dip.

Sound

For my experience, It need more than 2 weeks of burn-in. And It's bass improves dramatically during burn-in. It's sound is well balanced. It make me feel comfortable and I can't hear any booming sound in bass. Finishing my project, I sold my $2,000 speaker system.

Part List

Units
Tweeter Scan-speak D2905/9300: 2 ea.
Woofer Scan-speak 18W/8545: 2 ea.

Capacitor
Cresendo 5.0 uF: 2 ea.
Zen (10+2uF): 2 ea.

Resistors
Ohmite 10.0 Ohm 10Watt: 2 ea.
Ohmite 5.0 Ohm 10Watt: 2ea.

Coils
North Creek Air-core 2mH 14AWG: 2 ea.
North Creek Air-core 1.2mH 14AWG: 2 ea.
North Creek Air-core 0.7mH 18AWG: 2 ea.

Miscellaneous

Dupon Dacron: 24 oz. (12 oz. per box)
Back cup: 2 ea.

Specification

System: Closed box 2-Units 2-Way system
Drivers: 28mm soft dome tweeter (Scan-speak D2905-9300)
7" Paper cone with carbon fiber (Scan-speak 18W/8545)
Crossover Frequency: 1800 Hz
Sensitivity: 84 dB (1m/2.83V)
Impedance: 8 Ohm (Minimum 7 Ohm)
System Resonance: 47 Hz
Frequency range: 47 Hz - 20 KHz
System Damping: 0.53
Size: 470 x 230 x 350 mm (H x W x D)
Weight: approximately 16Kg/each

Man-Sun Huh