Bowers & Wilkins 685 review part 4

Disclaimer!

 

In this final review part I suggest some modifications to the B&W 685 loudspeaker cross-over. Even though I think these mods improve the sound of this loudspeaker I can’t guarantee that everyone likes it better than the original cross-over.

If you had this loudspeaker for a long time, you might have got used to the sonic profile of it and might not like the mods. I suggest listening to the chosen mod for a longer period of time before making final judgement. If you swap back and forth between the original cross-over and the mod you might be influenced by the “loudness” character of the original cross-over compared to the more neutral and balanced mods.

If you haven’t owned this loudspeaker before and like DIY experimenting, go find a pair second hand for a good price, test the mods and explore the world of DIY loudspeakers. The B&W uses high quality drivers and is a good speaker even though if you in the end keep the original cross-over.

Choose your mod level and have fun!

Cross-over Mod 0

This is the simplest modification suggestion, hence the name Mod 0. Actually it’s only a tweeter level adjustment. Thanks to the bi-wiring capabilities of the binding post you don’t even need to open the enclosure. Connect a high quality MOX resistor of 1.0-2.2 ohms between + terminals as shown in the below picture – that’s it!


Make sure you connect your loudspeaker cable to the lower terminal when using the above Mod.

(click on picture to zoom)

Left: On-axis, 2.2 ohm resistor
Right: On-axis, 2.2 ohm resistor, reverse tweeter polarity.

The 1.0 ohm resistor dampens the tweeter 1.5-2dB between 8-20kHz and the 2.2 ohm about 2.5-3.5dB. You can use any resistor value between 1.0-2.2 ohm in order to fine tune the tweeter level to your own liking. The frequency plot shows a slightly deeper notch at the cross-over point for the reverse polarity check, indicating that the phase behavior is unaffected by the mod.

Cross-over Mod 1

This mod is a bit more complex and requires you to open the enclosure and remove the original cross-over. This mod suggests that you keep the coil in the mid-woofer section from the original cross-over, in order to reduce the cost for the new cross-over. If you want to keep the original cross-over intact and don’t want to unsolder the coil from the cross-over board – Go to the mod 1.5 instead.

If you can’t find an 18uF cap, use a 10+8.2uF cap connected in parallel instead.

Note! For a perfect phase behavior and driver time alignment, it’s required to tilt the enclosure backwards about 5 degrees. Listening height is slightly below tweeter level (10-20mm).

 

(click on picture to zoom)

Upper left: On-axis frequency response.
Upper right: On-axis frequency response vs. original cross-over.
Lower left:  On-axis frequency response, reverse tweeter polarity.
Lower right: Individual driver unit phase tracking.

Mod 1 uses a third-order Bessel topology (acoustical) and the cross-over point is at 2.7kHz. This cross-over is a bit “hotter” in the 0.5-2kHz region, but flatter and more linear from 2kHz and upwards compared to the original cross-over.

The tweeter reverse polarity check show a deep notch at 2.7kHz and the individual driver unit phase response show a close phase tracking, indicating a good phase behavior at, below and above the cross-over frequency.

Cross-over Mod 1.5

Choose the mod 1.5 if you don’t want to reuse the 1.35mH coil from the original cross-over or/and don’t want the “hotter” frequency response between 0.5-2kHz compared to the original cross-over.

The 1.35mH (17AWG) coil is replaced by a 1.50mH (15AWG) coil.

If you can’t find an 18uF cap, use a 10+8.2uF cap connected in parallel instead.

Note! For a perfect phase behavior and driver time alignment, it’s required to tilt the enclosure backwards about 5 degrees. Listening height is slightly below tweeter level (10-20mm).

 

(click on picture to zoom)

Upper left: On-axis frequency response.
Upper right: On-axis frequency response vs. original cross-over.
Lower left:  On-axis frequency response, reverse tweeter polarity.
Lower right: Individual driver unit phase tracking.

Mod 1.5 uses a third-order Bessel topology (acoustical) and the cross-over point is at 2.5kHz. This cross-over is flatter and more linear from 2kHz and upwards compared to the original cross-over.

The tweeter reverse polarity check show a deep notch at 2.5kHz and the individual driver unit phase response show a close phase tracking, indicating a good phase behavior at, below and above the cross-over frequency.

Cross-over Mod 2

Mod 2 is the full mod and gives you the flattest frequency response as well as the most balanced sound.

If you can’t find an 18uF cap, use a 10+8.2uF cap connected in parallel instead.

Note! For a perfect phase behavior and driver time alignment, it’s required to tilt the enclosure backwards about 5 degrees. Listening height is slightly below tweeter level (10-20mm).

 

Mod 2 uses a third-order Bessel topology (acoustical) and the cross-over point is at 2.7kHz. C3+L3+R2 notch out the slightly “hot” frequency response between 0.5-1kHz and reduces the level about 2-2.5dB in that region. Its frequency response is flatter and more linear compared to the original cross-over.

The tweeter reverse polarity check show a deep notch at 2.7kHz and the individual driver unit phase response show a close phase tracking, indicating a good phase behavior at, below and above the cross-over frequency.

Happy DIY!