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B&W DM601-S3 Crossover Upgrade

B&W Bowers Wilkins DM601-S3

A friend asked me for an advice about bookshelf loudspeaker system at affordable price, with a natural tonality and good overall sound balance. The first model that came to my mind was B&W 601 - a highly rated by many users as they deliver a great sound for a budget price.
My friend bought a pair of second hand B&W 601 S3. His first impressions were very favorable. However after extensive listening sessions he noticed some recession in the midrange and bass with too much detailed upper end. The midrange itself is slightly muffled with a lack of consistence. That's why I decided to take a look at the construction and, if it is possible to try to improve the overall sound.

Bowers Wilkins DM601-S3

My first step, of course, was to perform a detailed tests of both drivers. The measurements can be found here:

Tweeter - B&W ZZ13137

Midwoofer - B&W ZZ12831

Both drivers show very good linearity. The tweeter unit has an excellent off axis frequency response.
The midwoofer has too high electrical factor Qts for bass-reflex enclosure like this. It results a peak in the 80-100Hz area, which is not too offensive and needs no special treatment.

The crossover PCB is directly attached to the terminal panel by two nuts and can be easily unmounted.  
To detach the cables from the loudspeakers, the midwoofer must be removed and through its hole there is an access to the tweeter's connector. The crossover upgrade, described below, can be performed even without any driver disconnection. 

B&W DM601-S3 Crossove B&W DM601-S3 Crossover View

Here is the schematics of the crossover. The tweeter section is the usual one B&W apply in most of the systems in this class with crossover frequency . The woofer was cut with 6dB/Oct low pass filter. We can see two empty component places for more complex midwoofer crossover, marked C3 and R3, which probably were abandoned in the development process or intended for other models.
Here are the factory specifications of the B&W DM601-S3 Link To B&W DM601-S3 User's Manual

The Measurements

B&W DM601-S3 Midwoofer and Tweeter Responce Curves B&W DM601-S3 SPL Graph at 1m

The left graphics show the measurements of the upper(tweeter) and lower (woofer and midwoofer) section of the system, where the test signal is applied to the input terminals with enabled bi-wiring (external junctions were removed). The acoustical crossover frequency is 4 kHz. Although the tweeter has third-order filter, its slope was lowered by adding the 1.8 Ohm resistor and the curves of the midbass and tweeter has almost the same declivity in the crossover point.
On right we can see the off-axis response, which is very good with a huge ultrasonic peak of the metal dome tweeter.

B&W DM601-S3 Burst Decay Responce B&W DM601-S3 Impedance

The burst decay looks good, and the impedance curve is excellent up to about 10khz.  The lowest impedance point is about 3 Ohm at 14 kHz, which is mentioned in the manufacturer's brochure.

Crossover Upgrade

B&W DM601-S3 Crossove B&W DM601-S3 Crossover View

Here is the schematics of the upgraded crossover. The low-pass filter was changed to second order Bessel, tuned at about 2 KHz. The resistor R2 was removed from the high-pass section for a slope improvement . Important: the tweeter is connected with inversed polarity.
The conversation procedure is simple:
- Replace the resistor R1 with 3.3 Ohm one. Thee quality of this component is important. I used Mundorf M-Resist 5W.
- Remove the C1 4.7 uF capacitor and put it in the C3 place. At the C1 slot put the best 3.3 uF film capacitor you can afford for this upgrade
- Replace R2 with a junction. The removed 1.8 Ohm Resistor solder into R3 place.
- Change the places of the black and red wire 

The Result

B&W DM601-S3 Midwoofer and Tweeter Responce Curves B&W DM601-S3 SPL Graph at 1m

Although the crossover modification doesn't look significant, the way how it work has changed dramatically.  Both midwoofer and tweeter no longer work in the area of high distortion nor unwanted resonances. In the factory crossover we can see, that there is an insufficient decay at the main tweeter's resonance of 2 KHz. New crossover improves this decay with 10dB.
The off-axis measurements show an excellent linearity in the whole frequency range. 

B&W DM601-S3 Burst Decay Responce B&W DM601-S3 Impedance

Reducing the unwanted midwoofer's response above the crossover point decreases significantly the mess at about 10KHz. The impedance unevenness was improved too - the lowest impedance is 4.4 Ohm at 200Hz.

Listening Impressions

The most important, of course, is how the improved crossover sounds like. If I must describe it in one word, it will be "clarity". The vocals are full-blooded without cheap adornment. The descants are tempered and musical. The soundstage is realistic plausible.
All the above at the cost of four components and one hour of work ...

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