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 ) ).R1R' 2 = ( Rload / ( 2.R1 + 2.Rload ) ).R2R source = R1.R2 / ( 2.R1 + R2 + 2.Rload )C' 1 = ( ( 2.R1 + R2 + 2.Rload ) / ( 2.R1 + 2.Rload ) ).C1C' 2= ( ( 4.R1.(R1 + Rload ) ) / ( Rload.( 2.R1 + R2 + 2.Rload ) ) ).C1 |
An example: At the low crossfeed level of all CORDA amps: R 1 / R2 = a = 0.47C 1 . R2 = 0.001 Farad.OhmLet us assume: R source = 1 kOhmR load = 15 kOhmThis results in: R 2 = 10315 OhmR 1 = 4848 OhmC 1 = 97 nFk = 0.76R' 1 = 3848 OhmR' 2 = 3898 OhmC' 1 = 122 nFC' 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 R 1 and R2.R 1 / R2 = aThe optimal values of R 1 and R2 can be easily determined using:R source = 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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||