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It has been a long time since my last Blog update, since I’ve been busy and haven’t had much time over to update the website, but you can rest assure that I haven’t given up! 😉

Finally I have published the Concentric Three – M5 design. It has actually been finished for some time now and I enjoy and like it very much.

If comparing with the commercial KEF loudspeakers, the Concentric Three – M5 speaker most closely resembles the 2600$ KEF R500 design in size and driver unit layout.

The KEF SP1632 co-axial driver is truly an amazing driver and delivers top notch performance to a budget price.

I have a couple of times listened on the KEF R500 and they instantly caught my interest, but I haven’t actually listened to my design side by side with the KEF R500 so I can’t comment on the exact sonic differences between them.

One thing I can say is that as a DIY designer I don’t have to compromise with the cross-over design in order to cut costs as most commercial loudspeaker manufacturer have to do, at least in this loudspeaker price range.

It would however be interesting if anyone out there has the cross-over schematics for the KEF SP1632 driver used in the KEF loudspeaker “R series” in order to compare the design choices made.

If you do please let me know!

For further details on my design see: Concentric Three – M5

I’ve now finished the cross-over simulation and I’m moving on to the next stage of building the first cross-over prototype and starting the listening tests.

The cross-over:

(click on picture to zoom)

The cross-over is very simple and uses a LR4 filter topology (acoustical) between the tweeter and mid and a LR2 topology (acoustical) between the mid and woofer. The cross-over consist of only 12 parts and all sections are second-order electrically, besides the tweeter section which is third-order electrically in order to achieve an acoustical LR4 frequency roll-off.

Note! The woofer’s are connected with reverse polarity.

The tweeter level can be adjusted by changing the value of the single resistor R1. R2 and R3 can be replaced with one resistor if it can handle >20W.

I don’t reveal the actual component values until I’ve evaluated the cross-over prototype. The cross-over schematic is only used here to inform how the cross-over looks like. Depending on the listening tests and further measurements it can be changed little or completely.

Frequency response simulation:

As in most of my designs the cross-over is optimized from the 15deg off-axis measurements and thus, the loudspeaker is intended to be used with none or little toe-in when setup.

Using the 15deg off-axis measurement generally means that you have a less pronounced baffle diffraction influence in the frequency response when optimizing the cross-over. Generally I’m optimizing the speaker cross-over to have equally good phase behavior between 15-30deg off-axis frequency response, rather than having a good phase response at the on-axis measurement and a deteriorating phase behavior as you go off-axis.

When optimizing around the 15deg off-axis measurements, the on-axis frequency response tends to be a little “hot” in the mid and upper treble. By using the appropriate amount of toe-in, I think this slightly added frequency response gives a more presence and airiness to the sound and are usually beneficial and desired.

However, each design and the drivers used in it are unique and sometimes it might not work out properly. The cross-over simulations and measurements alone doesn’t tell if it’s good or not and I consider this and the tweeter level adjustments to be the voicing and fine tuning part of the loudspeaker design.

(click on picture to zoom)

Left: 15deg off-xis frequency response (no smoothing applied).

Right: 15deg off-xis frequency response with mid-range reverse polarity.

The mid-range driver has a built in “BBC” response dip between 2-3.5kHz. The tweeter has a very extended and smooth frequency response, especially considering that this is a co-axial driver unit. The loudspeaker has a system sensitivity of about 88dB @ 2.83v/1m.

When reversing the mid-range polarity we can see very deep notches around 590Hz and 2.8kHz at the cross-over frequencies. The deep notches indicate a very good phase response tracking and summation between the driver units around the cross-over frequency.

(click on picture to zoom)

Left: Frequency response 0-60deg (no smoothing applied).

Right: The frequency response curve is an average of four measurements (blue): on-axis, 15, 22.5 & 30deg and six measurements (red): on-axis, 15, 22.5, 30, 45 & 60deg.

The loudspeaker has a smooth power response with a slightly downwards tilting response at higher frequencies. The tweeter has an extended frequency response with very wide off-axis dispersion.

(click on picture to zoom)

Left: Four different tweeter levels.

Right: System impedance simulation

The resistor R1 can be changed to fine tune the tweeter level. The level 2 is the “reference” level used in the above frequency and cross-over simulations. The minimum impedance occurs @ 120Hz and is slightly below 3 Ohms. The system sensitivity of 88dB @ 2.83v/1m is mainly achieved by the low impedance driver units used in the loudspeaker. A 4 Ohm stable amplifier is recommended.

Summation:

As I said before, I don’t intend to make a clone of any of KEF’s loudspeakers and I don’t have a clue how the cross-over for KEF’s loudspeakers looks like, even though my chosen cross-over frequencies seems rather similar to those used in the R500 loudspeaker. It just happens to be very suitable frequencies to cross-over at. I would say that a suitable cross-over window is about +/- 100Hz from that I have chosen, at least the drivers seems to be engineered in that way.

With that said, the KEF driver is really great and I’m eagerly waiting for the cross-over components I’ve ordered for the first prototype and I can’t wait to have a listen to the Concentric Three – M5 loudspeaker! 🙂

For further details of the driver unit measurements see:

“KEF SP1632”

“SB Acoustics SB15NRXC30-8”

The KEF R500 loudspeaker using the same mid/tweeter and similar sized woofer’s:

“KEF R500”

Independent KEF R500 loudspeaker measurements:

“KEF R500 Measurements!”

Here’s a short update of the project progress!

It’s progressing slowly, but I’ve finally finished building the front covers. The cross-over components has arrived and It’s time to bring out the solder iron and build the first cross-over that will be used in the first listening tests.

The front grills are only used as protection covers and isn’t intended to be used while listening to the loudspeaker. The front grill covers are fastened with small magnets attached to the grill cover frame and snaps on to the baffle screws.

Some quality cross-over components from Mundorf.

Hopefully, I can start the listening tests in a couple of weeks! 🙂

I’ve started the construction of the Concentric Three – M5 loudspeaker design. The main feature in this design is the KEF SP1632 co-axial driver unit.

The KEF SP1632 is an aluminium/magnesium 5” mid-range driver paired with a 1” aluminium dome tweeter mounted in a wave-guide like setup. There are a lot of co-axial PA drivers available to DIY:ers, but there aren’t many so called HiFi drivers available to choose from. SEAS has a couple of drivers, but in my opinion none of them come close to the features and performance of this new KEF driver.

Since the driver is a pure mid-range and cannot play bass, it has to be paired with a woofer or mid-woofer in a three-way design. In this first setup I chose to pair the KEF SP1632 with two SB Acoustics 5” mid-woofers in a Dáppolito configuration mounted in a Dayton 28 liter M-T-M enclosure.

The KEF driver is mounted in a closed box with a net volume of about 2.5 liters. In the Dayton enclosure this leaves about 12 liters each for the two mid-woofers and is about optimal for these drivers. A port tuning between 38-40Hz seems feasible and will be tested.

This loudspeaker could be built as a stand-mount monitor like I do or as a floor-stander, perhaps paired with an active sub-woofer in the excess volume of the floor-stander.

One could say that this design very much look like the KEF R500, but let me point out that I don’t intend to build any sort of clone of the KEF “R” loudspeaker series. I don’t have a clue how the filter design used in this loudspeaker series look like besides the official specification about cross-over frequencies etc. In fact it’s not interesting at all, since all I want to do is to find a truly nice co-axial driver for my loudspeaker design and in this case the KEF SP1632 was the best driver I could find for my design needs.

I will base my design on the measurements I’ve done on the SP1632 and that alone together with my own preferences will decide the cross-over topology cross-over frequencies and so on. As a DIY:er I have the advantage not having to consider making design choices based on reducing cost for e.g. the filter design. Consequently I will use the quality and the amount of cross-over components needed for the intended design.

I don’t know if KEF has done such design choices with its R series designs, but for a commercial loudspeaker manufacturer like KEF they always have to weigh in production cost and often in this price range at the expense of a non-optimal performance.

The details of the driver unit measurements can be found here:

“KEF SP1632”

“SB Acoustics SB15NRXC30-8”

Currently I’m running the Concentric Three – M5 with my DEQX HDP-3 pre-amp in an active cross-over configuration using 48dB/octave phase-linear FIR filters with a cross-over frequency of 500Hz and 2.8kHz.

I use my DEQX setup to burn-in the drivers and to get a general feeling of the driver’s characteristics and usable frequency ranges etc.

Even in this early stage of construction and evaluation the sound from these KEF drivers is really nice and I’m looking forward to start with the passive cross-over design and simulations.

Regards

/Göran

I’ve published a full set of measurements on the very nice KEF Uni-Q co-axial driver unit used in the KEF “R Series” loudspeakers.

This co-axial driver is very well built and have average to good distortion performance and an outstanding frequency measurement performance.

Details can be found here: “KEF SP1632”.

I’ve published a full set of measurements on the very nice and affordable SB Acoustics SB15NRXC30-8 mid-woofer.

Details can be found here: “SB Acoustics SB15NRXC30-8”.

 Overview:

I’ve used concentric (co-axial) drivers from SEAS in the past and liked them, but unfortunately SEAS hasn’t refined the concept much the last 15 years and there are not generally speaking many good concentric drivers available to choose between for the DIY enthusiast.

One company that certainly has improved the concentric driver concept during the last two decades is KEF and the relativity new Uni-Q drivers from the “R Series” are no exception. The “R Series” concentric driver inherits its technology in a simpler variant from the KEF flagship loudspeaker the “Blade”.

Top: KEF “Blade”

Left: KEF R300

Right: KEF R500 and R900

Here’s how KEF describes the new “R Series” concentric driver unit:

“The new Uni-Q MF/HF driver array is a masterpiece. Midrange response is amazingly fast and clean, thanks to a braced magnesium/aluminium alloy cone, with KEF’s Z-flex surround smoothing the transition to the front panel and a sturdy die-cast aluminium chassis that prevents resonance being transferred to the cabinet.

At the acoustic centre of the midrange cone is the sophisticated vented tweeter that endows KEF Blade with such startlingly pure HF response. With a large neodymium magnet and a computer-optimised dome structure so stiff that it operates pistonically over its entire working range, the upper registers are articulated with flawless precision – and to capitalise on Uni-Q’s point source characteristics, KEF’s ‘tangerine’ waveguide helps to disperse the higher frequencies evenly across a wider angle to flood the room with an intricate and refreshingly natural soundfield.

Operating together as a single, perfectly integrated voice, the array as a whole delivers a sweet, accurate sound of unprecedented clarity.”

The KEF SP1632:

(click on picture to zoom)

Top left: With design ring

Mid left: The”tangerin” waveguide

Lower left: The “Blade” Uni-Q exploded driver view (picture borrowed from the KEF website)

Lower right: The SP1632 Uni-Q exploded driver view (picture borrowed from the KEF website)

The KEF SP1632 driver unit used in the “R Series” is a 5” mid-range driver paired with a 1” tweeter placed in the centre of the cone. This driver can’t be used for mid-woofer duties, since it’s a pure mid-range driver and consequently it has to be used in a three way loudspeaker design.

I intend to use the KEF SP1632 in a small three way called “Concentric Three – M5”.

Note! The KEF SP1632 driver isn’t generally available to DIY:ers. Either it has to be stripped from a loudspeaker in the “R Series” or ordered as a spare part, but this requires a valid “R Series” serial number in order to be able to obtain the driver.

T/S Parameters:

Even for a 4 Ohm driver unit it has a low voice-coil DC resistance (Re). Slightly different free-air resonance frequency (Fs) between the two samples.

Impedance:

(click on picture to zoom)

Top: Mid-range sample 1 vs sample 2

Bottom: Tweeter sample 1 vs sample 2

It looks like the tweeter is using ferrofluid in the magnetic gap and it has a suppressed impedance peak at the driver resonance. The two tweeter’s impedance doesn’t match each other perfectly, but it shouldn’t be a problem in the cross-over design.

Stay tuned for frequency and distortion measurements!

/Göran

I visited the High End Mässan 2013 and Sweetspot 2013 HiFi Shows last week and some pictures and comments can be read here:

HighEnd Stockholm 2013
Sweetspot Stockholm 2013

Regards

/Göran

It’s time for a short update on the ”Sequence Three – Grand Reference” construction!

Unfortunately the build has progressed slowly because I’ve been busy with other designs, but I did a lot of measurements during the Christmas holidays and I have now done some cross-over simulations.

The two cross-over simulations below are two prototypes that I will actually build, test and evaluate before proceeding further with the build.

The frequency response isn’t spliced with the near-field measurement and the response is valid down to approximately 270Hz.

The cross-over schematics doesn’t contain any component values because I don’t want to publish them in the first prototype. The values will probably change anyway and the schematics are only shown to get a general feel of the complexity of the cross-over.

Prototype 1:

Click on picture to zoom:

Left: Tweeter height @ 15deg off-axis

Right: Tweeter height @ 15deg off-axis, tweeter-mid reverse polarity.

Bottom: Cross-over schematics (hybrid)

This is a hybrid cross-over with a second-order Linkwitz-Riley (LR2) filter topology between the woofer and mid-range and a fourth-order Linkwitz-Riley (LR4) filter topology between the mid-range and tweeter.

The cross-over frequencies are 350Hz (LR2) and 3000Hz (LR4). The C7 cap is optional and is used to raise the tweeter response above 15kHz. The same thing can be done by changing the position of the ribbon “foam deflection” pads.

The R1+C1 is a response shaping circuit for the woofer and the R2+L2+C3 is used to flatten out the mid-range’s sharp impedance peak @ 75Hz. Both these circuits are needed in order to hit the targeted LR2 topology slopes.

Note the woofer reverse polarity!

Prototype 2:

Click on picture to zoom:

Left: Tweeter height @ 15deg off-axis

Right: Tweeter height @ 15deg off-axis, tweeter-mid reverse polarity.

Bottom: Cross-over schematics (LR2)

This is a cross-over with a pure second-order Linkwitz-Riley (LR2) filter topology. The cross-over frequencies are 350Hz (LR2) and 3200Hz (LR2).

The R1+C1 is a response shaping circuit for the woofer and the R2+L2+C3 is used to flatten out the mid-range’s sharp impedance peak @ 75Hz. R3+L4+C4 is a response shaping circuit for the mid-range. All these circuits are needed in order to hit the targeted LR2 topology slopes.

Even though the RAAL ribbon is extremely robust, this shallow sloped filter will most likely not work since the tweeter isn’t attenuated enough at lower frequencies, at least not at higher listening levels.

With the LR4 filter the tweeter is down -35dB @ 1kHz and with the LR2 filter it’s down about -20dB, which may not be enough, but the LR2 filter is interesting and will be evaluated and compared against the LR4 prototype.

As can be seen in the cross-over schematics the mid-range doesn’t have any resistor in series for attenuation. If it works out, it’s great news being able to avoid any sound degrading resistors in the signal path.

The mid-range frequency response ripple @ 900 and 1.5kHz will be investigated further and part of it is likely some back wave reflections from the box stuffing.

Note the mid-range reverse polarity!

It’s time to order some cross-over components and start the listening tests! 🙂

Regards

/Göran

I’ve updated the T/S parameter section with the official specification sheet for the “Satori” MW16P-4 mid-woofer.

For details see:

SB Acoustics Satori MW16P-4