A DIY PASSIVE CROSSFEED FILTER One major feature of all CORDA headphone amplifiers is their natural crossfeed filter. This filter mimics aspects of directional hearing and strongly improves on the soundstage as produced by headphones. Listening thus becomes more natural and relaxed.

The technical implementation of the crossfeed filter is relatively simple and is shown in the figure to the left below. Only 3 resistors and 2 capacitors are needed to obtain the effect required. A more detail description of this circuitry is found at the project-chapter "headamp".

For optimal performance the natural crossfeed filter needs to "see" two (low output impedance) input buffers and two (high input impedance) output buffers. If the output impedances of the source are relatively high or if the input impedances of the load are relatively low, strong interactions will occur with the filter network and the filter characteristics will change considerably. The schematics of this situation are shown to the right.
Without proof: If the source impedances (Rsource) and load impedances (Rload) are known on forehand, then their influences can be corrected for by changing the values of the filter components and by adding an extra resistor R'2 and an extra capacitor C'2 to the filter network as shown below. The main functional difference to the original network is that the output signals are now attenuated by a factor k . Time delay values, filter frequencies and channel balance (as function of frequency) do not change.

Without proof:

k = Rload / ( R1 + Rload )

V'left,out = k.Vleft,out

V'right,out = k.Vright,out

R'1 = ( ( 2.R1 + 2.Rload ) / ( 2.R1 + R2 + 2.Rload ) ).R1

R'2 = ( Rload / ( 2.R1 + 2.Rload ) ).R2

Rsource = R1.R2 / ( 2.R1 + R2 + 2.Rload )

C'1 = ( ( 2.R1 + R2 + 2.Rload ) / ( 2.R1 + 2.Rload ) ).C1

C'2= ( ( 4.R1.(R1 + Rload ) ) / ( Rload.( 2.R1 + R2 + 2.Rload ) ) ).C1		 An example:

At the low crossfeed level of all CORDA amps:

R1 / R2 = a = 0.47

C1 . R2 = 0.001 Farad.Ohm

Let us assume:

Rsource = 1 kOhm

Rload = 15 kOhm

This results in:

R2 = 10315 Ohm

R1 = 4848 Ohm

C1 = 97 nF

k = 0.76

R'1 = 3848 Ohm

R'2 = 3898 Ohm

C'1 = 122 nF

C'2= 50 nF
To optimize the filter for a specific combination of output device (Rsource) and input device (Rload) is rather simple:

The desired amount of crossfeed is entirely determined by the ratio of R1 and R2.

R1 / R2 = a

The optimal values of R1 and R2 can be easily determined using:

Rsource = a.R2.R2 / ( 2.a.R2 + R2 + 2.Rload )

Next the values of the other components can be calculated

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BUILDING THE CORDA CROSS-1
At times it's very usefull if the amount of crossfeed can be varied and if one filter can be made suited for different combinations of input and output devices. The figure below shows the schematics of such a design, the CORDA CROSS-1. The remainder of this chapter explains how this filter exactly is build.
The resistors R1..R10 can be placed in series with the output impedances of the source. The total effective output impedance as seen by the filter thus can be varied and optimized to the value desired. There are six different settings selected by a 2-pole rotary switch, S1. In practice the use of switch S1 just slightly changes the high frequency response of the filter and can be used as a fine control of the treble.

The resistors R23..R32 can be placed in parallel to the input impedances of the load and allow to optimize the total load impedances seen by the filter. Resistors are switched by a 2-pole rotary switch S3 that allows six different settings. In practice the use of switch S3 slightly changes the low frequency response of the filter and can be used as a fine control of the bass.

The level of crossfeed can be set by the 4-pole rotary switch S2. This switch changes impedance values in all three branches of the crossfeed filter simultaneously. There are three settings; low, medium, high.

The crossfeed filter can be completely bypassed with switch S0, which allows for conventional stereo listening. Please note that in stereo mode there's no attenuation of the signal as in crossfeed mode.

The capacitors C3 and C4 are not necessary for the functionality of the filter but practice is that they greatly improve on the quality of sound. Although not completely understood my theory is that they prevent the capacities and inductivities of the input and output cables to make a high-frequency LCR-oscillator in combination with the resistance of the filter. Inputs and outputs are now simply short-circuited at very high frequencies. The capacitors have no effect on the frequency response at the audio-range.

Number Part Color code / description / label
1 PCB double sided PCB-board
2 150 Ohm brown-green-black-black-brown
4 390 Ohm orange-white-black-black-brown
2 470 Ohm yellow-violet-black-black-brown
4 1 kOhm brown-black-black-brown-brown
2 2.2 kOhm red-red-black-brown-brown
4 3.6 kOhm orange-blue-black-brown-brown
2 3.9 kOhm orange-white-black-brown-brown
2 10 kOhm brown-black-black-red-brown
2 11 kOhm brown-brown-black-red-brown
2 15 kOhm brown-green-black-red-brown
2 22 kOhm red-red-black-red-brown
2 33 kOhm orange-orange-black-red-brown
2 47 kOhm yellow-violet-black-red-brown
2 1.5 nF WIMA green 1500
1 47 nF Vishay blue 47nF
2 100 nF Vishay blue ,1 uF
2 2-pole switch Lorlin 2 x 6 rotary switch
1 4-pole switch Lorlin 4 x 3 rotary switch
1 toggle switch C&K; 2x2 toggle switch
2 RCA-input black RCA connector black
2 RCA-input red RCA connector red
1 flatcable 30 cm (10") 4-fold flat cable
1 backplate aluminium backplate black-anodized
1 frontplate aluminium frontplate black-anodized
1 enclosure aluminium profile black
4 screws 4 screws M3 12 mm black
3 dial aluminium dial black
4 feet self-adhesive rubber feet black
1 user-manual user manual CORDA CROSS-1
The part list of the CROSS-1 is shown to the left.

The CROSS-1 has a double sided PCB. Layout as well as positioning of the components are shown below.

Due to limited space the metal-film resistors are to be placed in an up-right position.

The capacitors are all high-quality polypropylen versions.

The rotary switches have silver plated contacts and are made by Lorlin. The miniature toggle switch has gold plated contacts and is made by C+K.

RCA-contacts are no-name but gold-plated.

The enclosure is made of aluminium. The black-anodized front- and backplates have laser-engravings.
Assembly note 1:

It is not advised to solder the components on side A before cutting their legs. Cut first and solder next as this will greatly reduce the height of the solder contact and prevent any assembly problems of the rotary switches at the other side.

Adequate mechanical and electrical stability is guaranteed by the feed-through solder pads.
Assembly note 2:

Capacitors C3 and C4 have been added to the design after the PCBs were manufactured. They can be added by soldering their legs to the backside of the toggle switch. The picture shows which contacts are to be used.
Assembly note 3:

If the toggle switch is soldered with its back close to the PCB board, then the top of the switch will not be completely flush with the front plate.

A optically nicer results is obtained if the switch is mounted slightly elevated with the mounting pins being flush with the back of the PCB. This can be easily obtained by placing the PCB on a heat resistant surface and placing and soldering the switch from the top-side of the PCB.
Assembly note 4:

The picture to the left shows the proper wire connections. The red wires and red RCA-inputs are for the right channel signal. The blue wires and the black RCA-inputs are for the left channel signal. Black wires represent ground. In the picture colored wires are used for clarification. In practice flat-cable is used to keep wires together and thus minimize trace loops.

The two left RCA-connectors are the inputs. The right connectors are the outputs. Please do not turn the nuts of the connectors too firmly to prevent damage to these connectors.

Upon soldering do not put the wires into the soldering holes to deeply to prevent any unwanted short circuits with the contacts of the rotary switches.
Assembly note 5:

Around the axis of the rotary switches we find a nut, a milled ring and a ring with a pin that sets the number of positions that the switch can take. The pin should be placed into one of the holes around the axis. With the left and right switches the pin should be placed in hole number 6. With the middle switch hole number 3 should be choosen.

The plastic pins on the switches between holes 3 and 4 should
be removed (cut).

With the CROSS-1 the milled rings are not needed. Be carefull not to scratch the frontplate when the nuts are placed. If necessary use tape to protect the plate.

If the filter has been finished please carefully read the user manual to fully understand its functions. Press the button below for a download.

Happy buildings,

Jan
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