NHT 2.5i review part 4

 

Part 4 of the NHT 2.5i review contains the dissection of the cross-over and some cross-over simulations.

The cross-over:

 

The cross-over components is mounted on a double sided circuit board attached on the backside of the 5-way binding post.

The cross-over schema:

 

2.5i Original cross-over

The cross-over is rather minimalistic with few components.

The tweeter cross-over filter section consists of one resistor (R1) together with an electrical second-order (C1 + L1) high-pass filter that shapes the cross-over slope to a LR4 roll-off with a targeted 3.3kHz cross-over point.

The mid-woofer cross-over filter section have an electrical first-order (C2) high-pass section and an electrical first-order low-pass section, which consists of a coil (L2) paired with an impedance equalization circuit (R2+C3) that shapes the cross-over slope to a LR4 roll-off with a targeted 3.3kHz cross-over point.

The sub-woofer cross-over filter consist of an electrical second-order (L3+C4) low-pass section together with a resistor (R3) in parallel with the sub-woofer. The sub-woofer is connected with reverse polarity.

The cross-over components:

 

2.5i Original cross-over components

(click on picture to zoom)

The cross-over components consist of ceramic resistors, electrolytic caps, steel laminate coils (L1+L3) and an air-core coil (L2). They are of simple, but fair quality and are in fine condition. The caps (C1+C3) have a +/- 5% tolerance, while the caps (C2+C4) have a +/- 10% tolerance.

They all seem to be within acceptable tolerances even though more expensive components would probably have better compliance with the specification.

I can’t see any other reasons but space and cost saving reasons why the (L1) uses a steel laminate coil rather than an air-core coil. It wouldn’t fit on the circuit board if it used an air-core coil.

Cross-over simulation:

 

The cross-over simulations below are made using the raw in box driver measurements and simulates the on-axis frequency response at a 3m listening distance.

The sub-woofer frequency response are made of the summed near field response of the sub-woofer and port. The shown sub-woofer frequency responses don’t include the influence of the room.

SIM - Freq on-axis +11cm full
SIM - Freq on-axisSIM - Freq on-axis +11cm
SIM - Freq on-axis RPSIM - Freq on-axis RP +11cm

(click on picture to zoom)

Top: Simulated on-axis response @ 3m, 11cm above mid-woofer (Sub-woofer nearfield below 275Hz)

Middle left: On-axis response @ 3m, mid-woofer height.

Middle right: On-axis response @ 3m, 11cm above mid-woofer.

Lower left: On-axis response @ 3m, mid-woofer height, tweeter reverse polarity.

Lower right: On-axis response @ 3m, 11cm above mid-woofer, tweeter reverse polarity.

The optimal listening height is about 10-12cm above the mid-woofer center. The reverse polarity simulation shows a deep reverse null indicating a good phase tracking between the tweeter and mid-woofer. The cross-over point between the tweeter and the mid-woofer seems to be spot on the specified 3.3kHz.

Sub-woofer nearfield frequency response:

 

SIM - Sub NF x-over vs no x-over
SIM - Sub NF x-over vs no R3SIM - Sub Imp x-over vs no R3

(click on picture to zoom)

Top: Summed sub-woofer + port nearfield response. Red without x-over and Blue with x-over.

Lower left: Summed sub-woofer + port nearfield response. Red without R3.

Lower right: Sub-woofer impedance. Red without R3.

As can be seen on the above frequency responses the cross-over adds about 3dB centered at 75Hz. If R3 is omitted, additional 2dB is added, but the minimum impedance is also lowered.

Go to NHT 2.5i review part 5