VCL EX34


March 2026 3-way studio monitor concept using Purifi and Bliesma transducers



The VCL EX34 is a concept study of a high-performance studio monitor loudspeaker. It is not conceived as a mass-market loudspeaker. It is a reference-level design study for serious DIY builders, recording engineers, and demanding listeners who value tonal neutrality, micro-detail resolution, and dynamic realism.

In this study, the loudspeaker is built around the Purifi aluminum 8″ woofer PTT8.0X08-NAB-01, the Bliesma beryllium 3″ dome midrange M74B-6, and the Bliesma beryllium 1″ dome tweeter T25B-6. These drivers were selected for their exceptionally smooth SPL response without resonances within their operating range, their extremely low distortion, and their wide dynamic capability.
By choosing aluminum and beryllium cone materials, all drivers in this configuration feature rigid diaphragms. A configuration using soft-cone drivers is also possible. For example, one could consider the Scan-Speak paper 8″ woofer 22W8557T00, the Bliesma silk 3″ dome midrange M74S-6, and the Bliesma silk 1″ dome tweeter T25S-6.

When determining the cabinet dimensions, careful consideration was given to allowing a very robust construction with thick enclosure walls.

A closed box cabinet design was chosen to achieve a a tight, controlled, and acoustically truthful low-frequency reproduction.

With these characteristics, this loudspeaker is particularly well suited for nearfield monitoring in studio environments, where accurate evaluation of vocals and instruments during recording and mixing is essential.




Contents




Chapter 1 Specification Headlines

  • System: 3-way, closed box system, passive X-over
  • Woofer: Purifi PTT8.0X08-NAB-01
  • Midrange: Bliesma M74B-6
  • Tweeter: Bliesma T25B-6
  • Low frequency response: F3 = 45 Hz
  • Sensitivity: 81 dB at 1m, 2.83 Vrms, full space
  • SPL at maximum excursion, 45 – 20000 Hz: 99 dB for a flat input spectrum and 108 dB for a music input spectrum, at 1m in full space
  • Crossover: LR4 at 550 and 2750 Hz
  • Impedance: value between 5 and 20 Ohm
  • Cabinet dimensions, exclusive stand: W x H x D = 43 cm x 27 cm x 38 cm


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Chapter 2 Speaker System


2.1 Woofer Purifi PTT8.0X08



The Purifi PTT8.0X08 is an 8 inch woofer with an aluminum cone. It is placed in a closed box system.
This is a very well performing driver. One of the reasons we have chosen this Purifi driver for this application, with its rather high cone mass of 48 gram, it can be used in a rather small cabinet and a low F3 value.
The 4 Ohm version of this woofer has been measured by HifiCompass, see in following link.


Woofer Closed Box System – Small Signal Analysis

The 8 inch Purifi woofer is mounted in a closed box of 20 L filled with wool. VCLLabs recommends the long tail sheep wool.
For this woofer: fs = 24 Hz; Vas = 74.5 L; Qts = 0.35 on infinite baffle (datasheet values).
Placed in the 20 L cabinet filled with wool: fc = 45 Hz and Qtc = 0.72.
The amount of filling can be determined by measuring the maximum impedance of the transducer at the frequency fc. It has to be about 35 Ohm for Qtc to be 0.70. Then Qmc is around 3.87.


Impedance woofer Purifi PTT8.0X08 in the cabinet


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2.2 Midrange Bliesma M74B-6



The Bliesma M74B-6 has been measured by HifiCompass and it is performing very well, see following link.


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2.3 Tweeter Bliesma T25B-6



The Bliesma T25B-6 has been measured by HifiCompass and it is performing very well, see following link.


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Chapter 3 Cabinet


Cabinet Mechanical drawing

These are the cabinet dimensions as used for this concept study.

At the rear side there is an internal chamber to mount the crossover filter.
All panels are 22 mm thick, the front panel is 30 mm.
The transducers are mounted flush with the front panel.


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Chapter 4 Responses of the transducers in the cabinet


4.1 SPL and impedance of the transducers in the cabinet


These are the SPL and impedance curves of the transducers placed in the cabinet in full space at 1 meter, 2.83 Vrms . The SPL responses on infinite baffle are also shown to see the impact of the cabinet on the response when placed in a full space domain.


SPL woofer Purifi PTT8.0X08 in the cabinet in full space (blue curve) and on infinite baffle (pink curve)


Impedance woofer Purifi PTT8.0X08 in the cabinet


SPL midrange Bliesma M74B-6 in the cabinet in full space (green curve) and on infinite baffle (pink curve)


Impedance midrange Bliesma M74B-6 in the cabinet


SPL tweeter Bliesma T25B-6 in the cabinet in full space (red curve) and on infinite baffle (pink curve)


Impedance tweeter Bliesma T25B-6 in the cabinet



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4.2 SPL off axis, Polar diagrams, Power and Directivity Index of the transducers in the cabinet


Horizontal SPL off axis response of woofer Purifi PTT8.0X08 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Vertical SPL off axis response of woofer Purifi PTT8.0X08 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Horizontal SPL off axis response of midrange Bliesma M74B-6 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Vertical SPL off axis response of midrange Bliesma M74B-6 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Horizontal SPL off axis response of tweeter Bliesma T25B-6 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Vertical SPL off axis response of tweeter Bliesma T25B-6 in the cabinet
at 0, 15, 30, 45, 60, 75 and 90 degrees at 3m in full space


Horizontal polar diagram woofer Purifi PTT8.0X08 in cabinet in free space
at 80 – 160 – 320 – 640 – 1280 Hz


Vertical polar diagram woofer Purifi PTT8.0X08 in cabinet in free space
at 80 – 160 – 320 – 640 – 1280 Hz


Horizontal polar diagram midrange Bliesma M74B-6 in cabinet in free space at 320 – 640 – 1280 – 2560 – 3840 Hz


Vertical polar diagram midrange Bliesma M74B-6 in cabinet in free space at 320 – 640 – 1280 – 2560 – 3840 Hz


Horizontal polar diagram tweeter Bliesma T25B-6 in cabinet in free space at 1.2 – 2.5 – 3.8 – 5.1 – 6.4 – 12.8 kHz


Vertical polar diagram tweeter Bliesma T25B-6 in cabinet in free space
at 1.2 – 2.5 – 3.8 – 5.1 – 6.4 – 12.8 kHz


SPL on axis (black) and Power (blue) of woofer Purifi PTT8.0X08 in the cabinet in full space


SPL on axis (black) and Power (green) of midrange Bliesma M74B-6 in the cabinet in full space


SPL on axis (black) and Power (red) of tweeter Bliesma T25B-6 in the cabinet in full space


Directivity Index DI in full space of woofer (blue), midrange (green) and tweeter (red) in the cabinet


The Directivity Index plot shows that, to keep the DI of the sum response flat, the crossover frequencies can be chosen at 550 and 2750 Hz.


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Chapter 5 Crossover Filter


It’s possible to design various types of crossovers for this speaker. As an example, we’ve designed a passive filter. A copy of it using DSP or an active filter with op amps is always an option.

Filter Targets

LR4 at 550 and 2750 Hz

Sensitivity is 81 dB. The -3 dB cutoff frequency (F3) is 45 Hz.


Acoustic centers of the transducers

For this concept design, the acoustic center of the woofer is assumed to be 30 mm behind the acoustic center of the midrange and tweeter. The acoustic centers of midrange and tweeter are assumed to be the same.


Schematic configuraton


Filter components

For a good quality crossover, air core wire coils, MKP capacitors and 10W MOX or carbon resistors, are recommended.


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Chapter 6 Responses of the total speaker with crossover filter


SPL of the filtered drivers and the sum at 1m, 2.83 Vrms, full space


Impedance


SPL of the sum on and off axis at 3m, 2.83 Vrms, full space
0 degrees in black, 30 degrees in green, 60 degrees in blue


Power response of the filtered drivers and the sum at 3m, 2.83 Vrms


Directivity Index in full space of the filtered drivers and the sum
woofer in blue, midrange in green, tweeter in red, sum in black



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Chapter 7 Maximum excursion and maximum SPL with crossover filter


Maximum excursion

With the designed Linkwitz – Riley 4th order filter, the woofer has a maximum excursion of 9.3 mm peak at 45 Hz for a voltage of 24.2 Vrms at the input of the filter.
The peak excursion of the midrange and the tweeter have been calculated also at 24.2 Vrms.

Excursion peak at 11.3 Vrms of woofer (blue), midrange (green) and tweeter (red)


The midrange excursion is maximum at 451 Hz and equal to 0.117 mm peak.
The maximum specified linear excursion of the Bliesma M74B-6 is 1.2 mm peak.

The tweeter excursion is maximum at 3152 Hz and equal to 0.041 mm peak.
The maximum linear excursion of the Bliesma T25B-6 is 1.0 mm peak.


Maximum SPL

For a flat frequency spectrum at the input:
at maximum excursion of the woofer at 45 Hz, the maximum SPL for the operating frequency range 45 – 20000 Hz is 99 dB, at 1m and 24.2 Vrms at the input of the crossover filter.

for a mean music frequency spectrum at the input:
at maximum excursion of the woofer at 45 Hz, the maximum SPL for the operating frequency range 45 – 20000 Hz is 108 dB, at 1m and 24.2 Vrms @ 45 Hz at the input of the crossover filter.


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