PP logo

XO14

2 channel 2-way or 1 channel 3 or 4 way Linkwitz-Riley cross-over network.

Contents


Introduction.

XO14 is a PCB with the basic building blocks for a 2-,3- or 4-way loudspeaker cross-over network.
There are 2 input amplifiers with a high-pass filter, 3 fourth-order Linkwitz-Riley cross-over filters, 4 output amplifiers and an output mute circuit.
The input amplifiers can be balanced or unbalanced. The balanced has some common-mode rejection while the unbalanced has lower noise.
The high-pass filters can be 2.order or 4.order and component values are given for both maximum flat and linear-phase filters.
The Linkwitz-Riley filters use the Sallen-Key topology for lowest noise and identical component values for best accuracy.
The output amplifiers can be balanced or unbalanced. They can have fixed or adjustable level. The level adjustment can be a trimpot on the PCB or a potmeter on the front panel.
The mute circuit sense the PCB supply voltage and turns the outputs on some seconds after the supply is sufficient and turns them off immediately when the supply voltage drops.
A switch on the front panel can be used to mute the outputs.
A mono-link allows the low-frequency outputs to be summed to mono. This can be to 2 low-outputs from a 2-way cross-over on one PCB or from a 3 or 4-way cross-over on 2 PCBs.


Block schematic.

XO14 block schematic.
Fig.1: XO14 block schematic.

The Input Amplifiers block is 2 balanced or unbalanced input buffers followed by a second or fourth order high-pass filter.
Component values for both maximum flat response and linear-phase are shown in the calculations.
RH1..RH3 selects the operation mode for the left channel and RH4..RH6 for the right channel.
Only the left channel input is used for single-channel operation.

X-over A..X-over C are 3 fourth order Linkwitz-Riley filters.
The resistors RM1..RM4 selects the mode of operation:

Table 1: XO14 operation modes.
ModeXOAXOBXOCRM1RM2RM3RM4OUT1OUT2OUT3OUT4
2ch, 2wLO-HINULO-HI100 ΩNUNU100 ΩLeft HILeft LORight HIRight LO
2ch, 2w, mono LOLO-HINULO-HI3 kΩNU0 Ω3 kΩLeft HIMono LORight HIMono LO
1ch, 3wMID-HINULO-MIDNU0 ΩNU100 ΩHINUMIDLO
1ch, 3w, mono LOMID-HINULO-MIDNU0 ΩNU3 kΩHINUMIDMono LO
1ch, 4wMID-HILO-MIDSUB-LONUNUNU100 ΩHIMIDLOSUB
1ch, 4w, mono SUBMID-HILO-MIDSUB-LONUNUNU3 kΩHIMIDLOMono SUB

NU: Not Used. The outputs of a NU filter is high-impedance.
The mono function for 1 channel filters is implemented by connecting the ML terminals of the 2 channels together.

The output amplifiers are 4 balanced amplifiers with a level control.
The level control can be trimpots on the PCB, potmeters on the front panel or bypassed.

The Mute Control prevents noise during turn on or off from reaching the outputs. It consist of a voltage detector and a 4 second turn on timer.
The E+ and E- allows the relays on 2 PCBs to be driven from the same control circuit.
The MU terminal is for a Mute key. The outputs are muted when this terminal is connected to GND.


Input amplifier and high-pass filter.

Input amplifier and high-pass filter schematic.
Fig.2: Input amplifier and high-pass filter ( left channel, add 50 to designators for right channel ).

The circuit around U101B can be configured as an unbalanced or a balanced input.
It should be noted that the balanced configuration has around 2 dB higher noise ( measured on the summed output of a 4-way cross-over ) than the unbalanced.
The Unused mode is used for single-channel filters where the right input is unused ( the OP-AMP is shared between channels ).

Table 2: XO14 input amplifier modes.
ModeC101C102C103C104R101R102R103R104R105R106
Unbalanced22 µFNUNUNU100 kΩ0 Ω100 ΩNU100 kΩ0 Ω
Balanced47 µF47 µF100 pF100 pF100 kΩ100 kΩ5.1 kΩ5.1 kΩ5.1 kΩ5.1 kΩ
UnusedNUNUNUNUNUNUNUNU100 Ω100 Ω

Unbalanced mode:
If you for any reason want gain in the input stage, increase the value of R106 to around 3 kΩ and select suitable values for C102 and R104 ( noise will increase too ).

Balanced mode:
C103, C104 sets the upper cut-off frequency around 300 kHz and C101, C102 the lower around 1 Hz.
As the tolerance of C101 and C102 is 20%, their value must be higher than expected to maintain CMRR at low frequencies.
R101 and R102 keeps the input DC free to avoid a big bang if you plug a connector in while the circuit is powered ( of course you don't do that ).
If you for any reason want gain in the input stage, increase the value of R105 and R106 ( noise will increase too ).
The resistor pairs R103, R104 and R105, R106 should be from the same tape for best CMRR.

The circuit around U102 is a 2.order or 4.order high-pass filter.

Input amplifier and high-pass filter schematic.
Fig.3: Amplitude response ( solid lines ) and group-delay ( dotted line ) for the filters.

Lime: 2.order maximum flat ( Butterworth ) filter.
Blue: 4.order maximum flat ( Butterworth ) filter.
Red: 2.order linear phase ( Bessel ) filter.
Cyan: 4.order linear phase ( Bessel ) filter.

Table 3: XO14 HP filter parameters.
FilterFirst section fFirst section QSecond section fSecond section Q
2.order maximum flatf3dB0.707
4.order maximum flatf3dB0.541f3dB1.31
2.order linear phase0.785 * f3dB0.577
4.order linear phase0.658 * f3dB0.5220.738 * f3dB0.806

The component values for the HP filter is shown in table 4 below.
C105 = C106 and C107 = C108 ( they can often all be same value ).
For best noise performance, the capacitors should be as large as possible while maintaining the resistors >1.4 kΩ.
To design the filters, start by selecting a capacitor value. The most common values available today is the E3 series ( 10, 22 and 47 ).
Calculate the resistor values. These should be between 1.4 kΩ and 10 kΩ. If not, adjust the capacitor values.
E24 series resistors should be sufficiently accurate here ( the design examples you can download uses E96 as I have them ).

Table 4: XO14 HP filter component values.
FilterR107R108R110R111
2.order maximum flat1 / ( 8.886 * f3dB * C105 )1 / ( 4.464 * f3dB * C105 )0 Ω
4.order maximum flat1 / ( 6.801 * f3dB * C105 )1 / ( 5.846 * f3dB * C105 )1 / ( 16.42 * f3dB * C107 )1 / ( 2.441 * f3dB * C107 )
2.order linear phase1 / ( 5.698 * f3dB * C105 )1 / ( 4.246 * f3dB * C105 )0 Ω
4.order linear phase1 / ( 4.315 * f3dB * C105 )1 / ( 4.083 * f3dB * C105 )1 / ( 7.468 * f3dB * C107 )1 / ( 2.901 * f3dB * C107 )

If you use linear phase filters, their cut-off frequency should be set 25% to 50% higher than you would normally set a Butterworth filter.

R109 and R112 are HF stoppers for the OP-AMPs.
I have used these ever since I was involved in designing mixing consoles for OB vans.
Without these resistors it is impossible to keep the audio circuits stable when the mixing console is located within 10 cm of a 100 W VHF transmitter.


Cross-over filters.

The cross-over filters are standard Linkwitz-Riley filters using a unity-gain Sallen-Key filter circuit.

Cross-over filter schematic.
Fig.4: Cross-over filter A. Add 20 to designators for X-over B and 40 for X-over C.

RF = 1 / ( 4.443 * CF * fx ), where fx is the cross-over frequency.
For best performance, capacitors and resistors should be from the same tape.
The maximum summation error is -1.25/+1 dB with 5% capacitors and 1% resistors and ±0.35 dB with 1% capacitors and 1% resistors.
For best noise performance, the capacitors should be as large as possible while maintaining the resistors >1.4 kΩ.
This will keep the load on the OP-AMPs >600 Ω.
To design the filters, start by selecting a capacitor value. The most common values available today is the E3 series ( 10, 22 and 47 ).
Calculate the resistor values. These should be between 1.4 kΩ and 10 kΩ. If not, adjust the capacitor values.
E24 series resistors should be sufficiently accurate here ( the design examples you can download uses E96 as I have them ).


Output Amplifiers.

Cross-over filter schematic.
Fig.5: Output amplifier. Output 1 is shown. Add 10 to designators for Output 2, 20 for Output 3, 30 for Output 4.

The two resistors RMx are only present in outputs 2 and 4. They are used for mono-summation with a value of 3 kΩ.
If mono summation is not used, only one of them is used ( 100 Ω ).
R300 is used for an amplifier without level control. R301 can be left out.
A level control can be connected to terminals P1T, P1W and P1G or a trimpotmeter can be installed directly in the PCB.
A linear potmeter will generally be the best for this application ( it is not a volume control ).
C301 and C302 keeps the potmeter DC free. If you use a trimpotmeter and can accept some adjustment noise, the capacitors can be left out.
The gain in the output amplifier can be set with R303 and R305. this is useful for adding additional gain after the level control or for compensating the voltage division between RMx and a potmeter.
For 0 dB gain in the output amplifier, R303 is left out and R305 is 100 Ω. For 6 dB gain, R303 and R305 are both 3 kΩ.
C304 is the output DC blocking capacitor. 470 µF gives a lower -3 dB frequency below 1 Hz into a 600 Ω load. A smaller capacitor is sufficient for most unbalanced applications. To acheive a 1% output balance below 50 Hz, 470 µF is required.
As the mute circuit can be active continuously, with signal on the amplifier, R307 is included to reduce the power dissipation in U351 during mute.
Short R307 if you do not use the mute-relay.
R308 sets the output resistance. R309 is used if the output is used to drive a balanced input. For an unbalanced input, short R309.


Output Mute Circuit.

Cross-over filter schematic.
Fig.6: Output Mute Circuit.

R360, D360, R361 and Q360 is a voltage detector with a threshold around 24 V.
When the voltage drops below the threshold, Q361 discharges C360 and Q363, Q364 turns off.
When the voltage is above the threshold, R367 charges C360 until D362 and the output transistor turns on.
The turn-on delay is around 4 s with the values shown.
R363, D361, R365 and Q362 is an input for a mute-switch. This can be connected to GND or ( preferably ) VCC.
If you use a switch with a minimum current rating, you can put a resistor between MU and VEE to achieve the required current.
If you have an external drive circuit for the relays, install the relays, D363 and R370 and leave the rest of the components out.


Component Selection.

Resistors

Use 1% metal film resistors.

Film capacitors

Use 5% MKT capacitors.
Some people feel better if they measure and match the capacitors. Feel free to do so.
If you try to match 10% or 20% capacitors to within 1% to 5% of their nominal value, you may not be able to.
The ones within 5% are probably already labeled and sold as 5% types.
Just measure what you have to obtain identical values and adjust the resistor values to fit.

Electrolytic capacitors

Use 20% general purpose types with sufficient voltage rating.

Ceramic capacitors

For values <1nF, use 5% NP0 (COG), 100V types.
For OP-AMP decoupling, use 50 V X7R types.

OP-AMPs

Use NE5523 or NE5532A.
The only requirement is that they have reasonable low noise, are unity gain stable and have sufficient slew-rate ( >3 V/µs ).
Do have a look at capacitor prices before you start saving a few cents on the OP-AMPs.

Relays

The relays suggested in the parts-list are sealed, silver contact relays with gold-flash plating.
These should last "for ever" in this application although they are used below their minimum recommended contact loading.
If you want the best, go for relays with bifurcated contacts with a specified gold plating thickness ( 10..30 µm ). They cost 2..5 times more than the type suggested.

Potmeters

If you use trimpots, use sealed metal-film types.
If you use panel potmeters, use metal-film or conductive plastic types.
If you insist on using carbon potmeters, set them to the proper setting, measure them and replace them with metal-film resistors.

Transistors

Anything with a current rating of around 100 mA, a voltage rating of >40 V and a hfe of >100 will probably work.
If you drive more than 2 relays, Q364 should be a higher current type. If you use a type with a hfe <200, reduce the value of R368.

Specification.

This is the specification for the XO14ABA prototype.
This is a single channel 4-way cross-over with cross-over frequencies of 90 Hz, 800 Hz and 7 kHz, a 4.order maximum flat amplitude response high-pass filter at 40 Hz and a maximum of 6 dB output gain.
Supply is ±15 V from a low-noise lab power supply (<-92 dBu (18 µV) noise in the 22 Hz to 22 kHz bandwidth).
The unit dBu is dB referred to 0.775 V.
More detailed measurements are in the file XO14A_Measurement.ods in the design file download.

Table 5: Specification for XO14ABA.
Supply current, no signal, no load ( Note 1 )120 mA
Under voltage detection threshold±12 V
Output relay turn on delay5 s
Gain error, 20 Hz..20 kHz, all outputs summed,
level adjustments in maximum position ( Note 2 )
+0.05 dB / -1.3 dB
Gain error, 200 Hz..20 kHz, all outputs summed,
level adjustments in maximum position
+0.05 dB / -0.4 dB
THD+N, 1 kHz, 22 Hz..22 kHz BW, all outputs summed,
14 dBu input level, level adjustments in maximum position
0.0005%
THD+N, 20  Hz..20 kHz, 10 Hz..80 kHz BW, all outputs summed,
14 dBu input level, level adjustments in maximum position
<0.01%
IMD, SMPTE, 60 Hz / 7 kHz, 4:1, all outputs summed,
14 dBu input level, level adjustments in maximum position
0.0011%
Output noise, 22 Hz..22 kHz, input shorted, all outputs summed,
level adjustments in maximum position
-98.3 dBu ( 9.4 µV )
Output noise, CCIR-Q-peak, input shorted, all outputs summed,
level adjustments in maximum position
-87.2 dBu ( 34 µV )
Output noise, A-wgt, input shorted, all outputs summed,
level adjustments in maximum position
-100.5 dBu ( 7.3 µV )
Output noise, 22 Hz..22 kHz, input shorted, all outputs summed,
level adjustments in 0 dB position
-99.8 dBu ( 7.9 µV )
Output noise, CCIR-Q-peak, input shorted, all outputs summed,
level adjustments in 0 dB position
-89.2 dBu ( 27 µV )
Output noise, A-wgt, input shorted, all outputs summed,
level adjustments in 0 dB position
-102.3 dBu ( 5.9 µV )
Output noise, 22 Hz..22 kHz, input shorted, all outputs summed,
level adjustments in minimum position
-103.1 dBu ( 5.4 µV )
Output noise, CCIR-Q-peak, input shorted, all outputs summed,
level adjustments in minimum position
-93.4 dBu ( 16 µV )
Output noise, A-wgt, input shorted, all outputs summed,
level adjustments in minimum position
-106 dBu ( 3.9 µV )
Board size (length / width / height)158 mm / 99 mm / 15 mm

Note 1:45 mA is the output relays. Use lower power relays for lower supply current.
Note 2:This is the high-pass filter frequency that is 41.5 Hz in stead of 40 Hz

PCB.

XO14 PCB photo.
Fig.7: Photo of mounted PCB for a 4-way, 1-channel filter. Dimension is 158x99 mm.

I have boards available for this project. See the PCBs page.


Downloads.

XO14A design files.

XO14A simulation files.


Known Issues / updates.

XO14A:
No known issues.


References.

[1] Comparison of Sallen-Key and State variable topology for a Linkwitz-Riley cross-over network.

Copyright and disclaimer.

Copyright Notice.
This web-page, including but not limited to all text, drawings and photos, is the intellectual property of Poul Petersen, and is Copyright ©.
Reproduction or re-publication by any means whatsoever is strictly prohibited under International Copyright laws.
The author grants the reader the right to use this information for personal use only.
Any commercial use is prohibited without express written authorization from Poul Petersen.

The information is provided on an "as-is" basis and is believed to be correct, however any use of the information is your own responsibility.

This web-site may contain links to web-sites outside Poul Petersen domain ( www.poulpetersen.dk ).
Poul Petersen has no control over and assumes no responsibility for the content of any web-site outside Poul Petersen own domain.

Poul Petersen does not use cookies to "enhance your experience" on this website.


Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: diy@poulpetersen.dk

Poul Petersen DIY index

Poul Petersen notes index


Copyright © Poul Petersen 2019. Last update: 20191125. Valid HTML!