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The Monitor 2023
December 2022

 

INTRODUCTION

After the good result obtain with ceramic drivers in my Monitor 4 and the new test on Monitor 2023 with Mundorf AMT the idea was to explore the possibility of using a different technology also for the mid-woofer to increase the transient response and decrease the distortion in order to have more the details.

If we start looking for a mid-woofer capable of competing with a ceramic driver it is not easy but then I noticed this new model with an aluminum cone of Purifi-Audio which, on the datasheets, has a much lower distortion than the others of the same company.

This project born to exceed the performance of all my previous loudspeakers and create a compact full-range luodspeakers without any compromises.

TWEETER

 Mundorf  AMT U110W1.1

The Mundorf  AMT U110W1.1 is avalable with an optional faceplate which forms a small horn that shifts the offset and slightly modifies the frequency setting.

So as a first check I had milled panels made with the two configurations to verify this difference.

 

As show in the article of the Monitor 2023 with this AMT driver it is necessary compensate the trend of frequency response with a choke.

 

 

MID-WOOFER

 Purifi-Audio PTT6.5X08-NAA-08
  • 6.5 inch bass-midrange for small bookshelf monitors or slim floorstanders
  • small
  • Negligible Force Factor Modulation and Surround Radiation Distortion
  •  Low Magnetic Hysteresis Distortion
  • “Real” long-stroke performance: distortion remains low over full excursion
  • Uncompromised midrange performance
  • Designed and Manufactured in Denmark

datasheet

This photo shows the particularities of this driver with an optimized engine with an unprecedented gap geometry and the neutral surround partially visible only in certain high-end Fostex.

As already mentioned in the introduction we have in our hands one of the drivers with the lowest distortion which was obtained using various details in the construction and a rigid aluminum cone.

Like all driver with very rigid membranes also this has a frequency response characterized by high frequency peaks that are not easily controllable with normal filters.

Normally RLC cells are placed on the various peaks but these cause lowering of the impedance which in some cases are not tolerable by many amplifiers or in any case put them in crisis.

Here some test about these cells

The details of the costruction of this driver are described in the article "Voice Coil, August 2021." published on Voice Coil, August 2021 and visible also here.

Follows the measure of impedance and parameters using the Dayton DATS V3 Computer Based Audio Component Test System.

There are some differences on parameters that probably depend by the DATS errors, I will make other tests about.

Parameter Parameter Typ Value Measured Unit
Zn  Nominal impedance  8 8 ohm
RDC DC resistance  5,9 6,65 ohm
Le Voice Coil inductance 0,5 0,287 mH
fs Resonance frequency 34 34 Hz
Qms Mechanical Q factor 7,7 5,7
Qes Electrical Q factor 0,42 0,49
Qts Total Q factor 0,4 0,45
Vas Equivalent volume 20,4 17,2 L
Sd Effective piston area 132,7 132,7 cm2
D Effective piston diameter 13 13 cm
Bl Force factor 9 N/A
Rms Mechanical resistance 0,75 kg/s 
Mms Moving mass 27,2 29 g
Cms Suspension compliance 0,82 mm/N

Follows the simulation with WinISD using the declared and measured parameters on the same bass-reflex box.




MEASUREMENTS AND CROSSOVER

Here I am in my living room where the reflections on the walls are controlled with 10 panels model 3D by Acustica Italia.

I will use a miniDSP UMIK-1 Omni-Directional USB Measurement Calibrated Microphone and the Dayton DATS V3 Computer Based Audio Component Test System for the impedance measurement.

The box is an old Philips about 27lt used only for this first measurements.

This crossover is the result of a long series of measurements of the frequency response and several listening sessions with a my friend who makes live recordings.

This is followed by the first response of the filter in the near field where the remaining 2 peaks are visible but at -25dB and -50dB below the overall response of the system does not actually change.
I must specify that the efficiency in the graph should not be considered.


 

 

 

Here the new filter response at 50cm with an increment on high frequency obtain with the new 0.68uF not present in previous filter.
The little problems on 150-600-800Hz visible in one plot are caused by the room environment and these vary with the location of the microphone and speakers.
I must specify again that the efficiency in the graph should not be considered.


 

 

 

 

 

 

Follows a first measurement with the Extreme H-frame subwoofer

 

 

 

REFLEX TUBE

I think to use the HP 50/145 reflex tube and make some test with different lenght.

In my first box 27lt has been used the HP50/145 but cut 2cm to get about 120-125mm

Folllows two measurements in two methos with reflex and with the closed box.

  

 

THD COMPARISION

In a listening session in which we compared this new loudspeaker with my Monitor 4 I noticed that the sound was much more distorted with the ceramic drivers so I wanted to check if this thing was instrumentally verifiable.
 
I made various measurements at different volume levels and I confirmed that at any level the ceramic drivers have much higher distortion.

  

 

BOX DESIGN

I am thikinng about 4 possible configurations to use these drivers.

1. Bass reflex about 25lt with port like show above, crossover at the bottom of the cabinet, calculated for 19mm MDF.

 

2. Large Dipole 30+30+30 x 120 cm (Ft about 100Hz) + H-Frame subwoofer

 

3. Closed box 30+ 30 x 50 cm + Extreme H-frame subwoofer (the final design)

 

 

FINAL CONSIDERATIONS AND LISTENING SESSION

 I started this project without great expectations because in any case a 2-way bass-reflex with a 6.5" would not have been a full range system.

So the target was something more than a near field monitor to be used in small rooms around 12-15 square meters.

All my ideas were involved from the measurements and the first listens in two aspects.

We have incredible low frequency dynamics and extension for a 6.5" that normally can only be achieved with an 8" so we can consider this a true full-range with a frequency range 50Hz - 20KHz.

The second aspect concerns the distortion which is considerably less than that present on my ceramic drivers and I'm not talking about something just measurable but a substantial improvement in listening.

The Purifi mid-woofer integrates perfectly with the AMT tweeter returning a fast and detailed mid-range with plenty of separation between the instruments.

If we add a subwoofer and a simple capacitor as high pass filter on the signal path of the amplifier dedicated to the mid-high frequency the sound is cleaned even more.

After a first test with a closed box for the Purifi I have build a large dipole panels and the sound obtained was freer and more natural but obviously the problems related to the environmental placement began.

For this new 3 ways system I have used a H-frame created with a suitably equalized 10" Wavecor and after a bigger H-frame with a 18" woofer to reduce the distortion in low frequancy range and to increase the crossover frequency.

I left the dipole because having a tweeter closed at the back, the mid-woofer range was too evident compared to the high frequencies and in any case the rear reflections reduced the definition in the mid range.

So the final design is the version 3.

 

PHOTOS

Here I am in my living room where the reflections on the walls are controlled with 10 panels model 3D by Acustica Italia.

Follows some photos of the definitive closed box build with Routech R200K CNC by

Fill the volume with Dacron Hollofil II got from La Redoute in "Cuscino alla francese" cod. 9567160.