AFW1 / Active Crossover

AFW1 / Active Crossover Network

Table of Contents



With the beginning of 2006 I started to design an active crossover network system in parallel to the design of a subwoofer. First of all I read the Pass Labs XVR1 manual and try to analyse the technology behind this device. However I was not very satisfied with the results of different simulations I’ve made with this informations. A plain frequency response and a similar phase response at the cross over point was one of my main goal in the design. But the use of a discrete operational amplifier in the XVR1 has amazed me.

After studying different topologies for active filters it was clear that I only can use State Variable or Sallen-Key filter. The amount of needed operational amplifiers for the Sallen-Key design is lower than that of the State Variable filters and therefore I decided to build a design around the first topology. A lot of very nice informations about such a crossover design you can find at Elliott Sound Products.

2 Way Crossover Network with Integrated Operational Amplifiers


Before I start working on a complex crossover network with discrete operational amplifiers I want to collect some experiences with an easier to handle design. Therefore I have build an active 2 way crossover network with integrated operational amplifiers. This crossover I want to use as an intermediate step and as a test arrangement in my audio system.

Also for this design I looked first at the XVR1 and took over the idea to change the frequency of the filters with jumpers. Besides that I limited this crossover to 2 ways. Additionally however I have integrated a stage to sum the 2 low frequency signals of both channels – a mono signal for a subwoofer – with the possibility to change the phase and the frequency response. Both channels inclusive the needed power supplies – as always in a dual mono construction – are placed on the same PCB.

Butterworth 12dB Sallen-Key Filter

Butterworth 12dB Sallen-Key Filter

The filter consists of a normal Sallen-Key filter structure which you can find in every textbook about active filters. For a Butterworth filter both resistors in the low pass filter are equal and the capacitor in the feedback loop has a value twice as much as the one at the positive input of the operational amplifier (see above on the left side). In the high pass filter the places of the resistors and capacitors are changed. Both capacitors have the same value and the resistor at the positive input of the operational amplifier has a value twice as much as the resistor in the feedback loop (see above on the right side). It is wise to contain first the values of the capacitors (E12) and thereafter to calculate the values of the resistors (E96) out of it. Below you find the calculation for the resistor out of the value of the capacitor :

    \[ R = \frac{1}{2\sqrt{2} \, \pi{} f C} \]

The twice as much values of the components you get for the resistors with a series connection of two equal resistors and for a capacitor with a parallel connection of two equal capacitors.

Two 2nd order Butterworth filter in a serial connection gives a 24dB 4th order Linkwitz-Riley filter which is my prefered filter topology.

The complete considerations guide me to the following requirements :

  • active Linkwitz-Riley 2 way crossover
  • 6/12/18 and 24dB filter selectable (Linkwitz-Riley filter only with 24dB)
  • 3 decades and 10 crossover frequencies per decade selectable
  • differential input and output signals
  • low frequency mono signal with phase shift and changeable frequency response
  • both channels incl. the power supplies on one PCB

With the thoughts and the requests from above I have developed the following circuits :

With the resistor and capacitor values in use I got the following crossover frequencies of the filter :

 R [kΩ]
C [nF]5,627,158,8711,014,018,022,128,035,745,3
Freq. [Hz]


2 Way Crossover Network with Integrated Operational Amplifiers

2 Way Crossover Network with Integrated Operational Amplifiers

Assembly of the Crossover Network


I bought a Slim Line chassis with a 10mm front panel and the knobs from HI-FI 2000 for my crossover network. The mounting plate and the complete mechanical machining I made myself on a Datron M35 milling machine.

AFW1 - Front View

AFW1 – Front View

Here you see the front with the operational controls of the crossover. On the right side you find the gain control for the high and low pass channels. The controls for the mono subwoofer channel – gain, changeable frequency response, switch for a 180° phase shift and a control for changing the phase continuously – is located at the left side of the front.

AFW1 - Look Inside from Behind

AFW1 – Look Inside from Behind

On the picture above you can see the assembly of the electronic. The PCB on the right side of the big audio board is a remote control.

AFW1 - Back Side

AFW1 – Back Side

Here you can see the connectors on the back side of the crossover chassis. On the left side you find the two symmetrical inputs, then the outputs of the high and low pass channels and the connector for the mono subwoofer. On the right side there are the power connector, the power switch and the fuse. The earth jack connects the signal ground with earth – I use a earth rail in my equipment. The remote connectors are used in my system to switch the whole equipment on and off from the preamp.

Replicas of the AFW1


Every now and then I build a crossover for an audiophile “fellow sufferer”. I don’t want to hide the pictures.

The AFW1 by Volker W.

I put this crossover in an enclosure with aluminium bottom and top plates, so the mounting plate could be omitted. The cabinet is also shorter. On request, the electronics for remote switching are missing. For the input and output sockets I put other connectors on the PCB. In addition, other rotary knobs have been added to the front panel.