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B&W BVR Two-Way

B&W Bowers Wilkins Two way BVR Short Horns Speakers

A friend brought me a pair of B&W CDM7 loudspeakers for an advice how to improve the sound performance. He complained of boomy bass out of control and lack of balance in the entire sonic spectrum.

Bowers Wilkins CDM7

To find out the reason for the slack bass, I started with the measurements of the low frequency drivers. The detailed measurements can be found here:

Woofer - B&W ZZ08613

Midwoofer - B&W ZZ10075

I was surprised, that the frequency response of the woofer is extended up to 7kHz and its linearity is not bad at all. 
The Thiele/Small parameters of both drivers are almost identical. They share a common volume with one front loaded basreflex port. With the Qts of 0.32 we could not expect  remarkable performance in lows in a classic bassreflex construction. That's why designers of the CDM7 decided to use the lower driver for lowest frequency only. 

B&W CDM7 Crossove B&W CDM7 Crossover View

The crossover schematics of B&W CDM7 is simple. The cutoff frequency for the woofer is 160Hz and for the midbass - 1250Hz.

B&W CDM7 Bassbox simulation

The BassBox simulation of the B&W CDM7 enclosure shows clearly the reason for "bummy" and monotonous bass. The woofer's range (in red) is narrow with a peak at 100Hz. The black curve is for the midbass unit. 

B&W CDM7 Midwoofer and Tweeter Responce Curves B&W CDM7 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 about 2.3 kHz. 
The tweeter in the original crossover has no attenuation. As we see its level is about 2-3dB higher than the average level in the midrange. Maybe the reason design engineers did it this way, is to compensate the roll-off in the off-axis response, given in the right graphics. Or maybe with making highs brighter, they tried to mask somehow the imperfection of the bass fundament ... who knows.

B&W CDM7 Burst Decay Responce B&W CDM7 Impedance

The burst decay looks good, and the impedance curve is normal, with an exception, that the impedance does not match the manufacturer's specification, which is 8 Ω. Since the impedance is going under 4 Ω in ranges around 30Hz and 12KHz, the impedance should be rated strictly 4Ω to avoid a damage of certain amplifiers.

My New Construction

B&W Bowers Wilkins BVR

The initial idea was to keep all the original B&W drivers constructing  an enclosure with better bass performance and improved crossover.  After a careful observation of all the drivers, I found, that one of the tweeters was damaged.  Instead of searching the original tweeter ZZ03123, which is hard to find now, I decided to use one of my favorite high-efficiency drivers:


This driver has very good off-axis response at a glance and matches very well with Kevlar drivers I tried it with.

In the new construction the so called 2 1/2 design of CDM 7 was explicitly unwanted and while I tried to avoid this configuration, my friend solved the problem, finding another pair of ZZ10075.
This drive can still be found in the Online Parts Store of the B&W Group website.
Thus we had a matched quad for a two-way d'Appolito configuration.   

After all only a pair of midbasses and the input terminals left from the original system.

The Thiele-Small parameters of the midwoofers guided me to apply my beloved Big Vent Reflex (BVR) construction.  For drivers with moderate values of Qts (0.25-0.35) this design delivers much more efficiency in the low-frequency reproduction than the ordinary bassreflex. The performance is faster and does not suffer of humdrum.


The Simulations

Driver Parameters Horn Parameters
Rdc 5 Ω Type Rearloaded
Fs 34 Hz Contour Exponential
Qes 0.34 Position Floor
Qms 4.7 Leight Difference dl 0 m
Vas 32 ltr Mouth Height h 32 cm
Pmax 50W Horn Width b 18 cm
Sd 160 cm2 Contour Leifgt l 0.5 m
Z1K 10 Ω Driver Position 0 m
Z10K 28 Ω Mouth Area At 40 cm2
Xmax +/-4 mm Chamber Volume Vfc 33 ltr
Number of Chassis 2 Coefficient β1 40
Uin 2.0 V Coefficient β2 0

The table above shows the input parameters for the AJ Horn simulation program.

B&W BVR SPL AJ Horn Simulation SPL B&W BVR SPL AJ Horn Simulation SPL MAX

The BVR improves the whole efficiency in the range up to 300Hz. The performance graph shows 40Hz at -3dB and 36Hz at -6dB

B&W BVR SPL AJ Horn Simulation Impedance B&W BVR SPL AJ Horn Simulation Acoustic Impedance

This Electrical and Acoustic impedance without the passive crossover.

The Enclosure

B&W Bowers Wilkins BVR Plans

Most parts of this enclosure were made of mid-density practical board. I do prefer this material to MDF for its better sonic performance and ability for homogenous assemblies using glues for wood.

The detailed plans and the part list of the construction can be found here:

BW BVR Enclosure Plans and Part List  Link To BW BVR Enclosure Plans and Part List

The essential of this construction are Detail1 and Detail2. They are precisely cut of practical board using CNC router or water jet cutter.
Here is the AutoCad drawing file for both details: BW BVR Details AutoCad File

They are 10 details of each kind for one enclosure with a thickness of about 18mm. It is important to mention, that this value varies for each sheet of material, even from the same manufacturer. To avoid misalignment, they have to be cut and assembled prior to cut out the other parts. After the assembly the thickness of the pack should be measured. This will be the value, market with * in the part list. 

The base and the top were made of 25mm MDF. The top is formed with radius for aesthetical appearance and it is glued to the common construction. The bottom is a different part and it is attached to the bottom using a Confirmat screws.

Parts for B&W BVR Loudspeaker systems Parts for B&W BVR Loudspeaker systems Enclosure for B&W BVR Loudspeaker systems

Some photos of the process of assembly. All the drivers were dug into the front panel. For sharper cut it consists of two layers - practical board and HDF, that can be seen in the picture and plans.

The Crossover

They are two approaches in the crossover design. They will be presented in parallel. The Version 1 on the left is a result of my attempts to give the midbass driver more air and to add some tube friendliness using small values for all coils and capacitors. The Version2 on the right is a classical design for best linearity and off-axis performance.

B&W BVR SPL Xover V.1 Schematics B&W BVR SPL Xover V.2 Schematics

The crossover schematics of both versions.

B&W BVR SPL Xover V.1 SPL Responce of tweeter and midwoofer B&W BVR SPL Xover V.2 SPL Responce of tweeter and midwoofer

The acoustical crossover frequency is about 5.5 KHz for V.1 and 2.5 KHz for V.2

B&W BVR SPL Xover V.1 SPL Responce at 1m on axis and 15 30 deg off axis B&W BVR SPL Xover V.2 SPL Responce at 1m on axis and 15 30 deg off axis

Frequency response at 1m with 2.0 V input RED - on axis, BLUE - 15 º off axis, GREEN - 30 º off axis.
Here it the main difference between two version. The roll-off in the range of 2-5 KHz of V.1 is easy to observe. The SPL graph of V.2 is excellent even at 30 º off axis. In fact it is most linear at this angle.

B&W BVR SPL Xover V.1 Second, Third and Fourth Harmonic B&W BVR SPL Xover V.2 Second, Third and Fourth Harmonic

The second, third and fourth harmonic curves measured at 2.0V input RMS. Version 2 has a bit better result.

B&W BVR SPL Xover V.1 Burst decay Responce B&W BVR SPL Xover V.2 Burst decay Responce

Burst decay response, measured at 1m on axis. Almost the same.

B&W BVR SPL Xover V.1 The impedance B&W BVR SPL Xover V.2 The impedance

The impedance curves


Some words about the sound. I liked the performance of both two versions and that's why I decided to give them as two options. The V.1 is vivid and bright, with an excellent body and scale of vocals and pianos. The V.2 is linear and tolerant to the placement and position angle with very clear and articular reproduction in the whole range. The bass fundament of whole systems is tight and strong. The midbass drivers are a bit gentle and this is not recommended for this system to be pressed hard by power.

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