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