I would like to design a simple crossfeed circuit to go between my sources, DAC and phono preamp, into my Bottlehead Crack headphone amp.
I am going off of the circuit and design on of the Corda Cross on this page https://web.archive.org/web/2009052...-audio.homepage.t-online.de/passivefilter.htm
My plan was a simplification of this design using two switches, one with three settings: bypass, input for DAC, input for phono preamp; and another with two or three different levels of crossfeed.
The circuit is relatively simple, and the page gives a set of equations and an example for optimizing the circuit for known source output impedance and amp load. The page says that the ratio between R1 and R2 sets the crossfeed level, and states that R1/R2 = 0.47 is the "low level crossfeed of most corda amps". It also states that R2*C1 = 0.001 Farad.ohm.
That's all well and good, but for someone who wants a design with variable crossfeed, nowhere on the page does it say HOW adjusting these values changes the actual crossfeed level and what are a reasonable range of values. I was hoping for some insight on how changing these values might affect the level of crossfeed and what a reasonable range of crossfeed values might be for open headphone listening?
I apologize if this is a newbie question but I am very much an amateur with circuits and have almost no formal training, so any education is appreciated. Thanks in advance!
I am going off of the circuit and design on of the Corda Cross on this page https://web.archive.org/web/2009052...-audio.homepage.t-online.de/passivefilter.htm
My plan was a simplification of this design using two switches, one with three settings: bypass, input for DAC, input for phono preamp; and another with two or three different levels of crossfeed.
The circuit is relatively simple, and the page gives a set of equations and an example for optimizing the circuit for known source output impedance and amp load. The page says that the ratio between R1 and R2 sets the crossfeed level, and states that R1/R2 = 0.47 is the "low level crossfeed of most corda amps". It also states that R2*C1 = 0.001 Farad.ohm.
That's all well and good, but for someone who wants a design with variable crossfeed, nowhere on the page does it say HOW adjusting these values changes the actual crossfeed level and what are a reasonable range of values. I was hoping for some insight on how changing these values might affect the level of crossfeed and what a reasonable range of crossfeed values might be for open headphone listening?
I apologize if this is a newbie question but I am very much an amateur with circuits and have almost no formal training, so any education is appreciated. Thanks in advance!
But "low, medium and high" aren't exactly resistor ratios. I suppose I was trying to avoid asking a stupid question about the schematic. I tried calculating the ratios myself but obviously I'm doing something wrong.
In that circuit, S2 sets the crossfeed level. The first part of S2 is a resistor-capacitor combo, and the second part is a resistor which then meets with a capacitor mutual to both channels which goes to ground.
With S2 in its first position and looking at the left channel, it looks to me like the resistance is just 390 ohm (R13) and the resistor is in series with the capacitor C1 rather than being in parallel with it as in the original crossfeed circuit (C3 and C4 are stated as not necessary for the functioning of the crossfeed). On the other half of S2, in its first position there is a 3k6 ohm resistor. This gives a ratio of 390/3k6 = 0.108. Okay, no problem. That sounds reasonable.
With S2 in its second position, there are two 390 ohm resistors R11 and R13 in parallel. Since they are equivalent, Reffective = 1/2* R or 195 ohms. On the other half of S2, there is an 11k resistor and a 3k6 resistor in parallel, so 1/Reffective = 1/11k + 1/3k6 = 2712 ohm. 195/2712 = 0.07, which is an even smaller value than the 0.108 from before and quite far off from the 0.47 value quoted on the page.
With S2 in its third position, we then have a 3k9 ohm resistor in parallel with the capacitor and a 390 ohm resistor in series with it. Because the resistors are in parallel with each other, the effective resistance is 354 ohm. The other half of S2 at position 3 uses two 3k6 ohm resistors in parallel, with an effective resistance of 1800 ohm. 354/1800 = 0.197. This, even being a "high" level of crossfeed, is still well below the quoted 0.47 ratio.
What am I doing wrong? Any help is appreciated.
In that circuit, S2 sets the crossfeed level. The first part of S2 is a resistor-capacitor combo, and the second part is a resistor which then meets with a capacitor mutual to both channels which goes to ground.
With S2 in its first position and looking at the left channel, it looks to me like the resistance is just 390 ohm (R13) and the resistor is in series with the capacitor C1 rather than being in parallel with it as in the original crossfeed circuit (C3 and C4 are stated as not necessary for the functioning of the crossfeed). On the other half of S2, in its first position there is a 3k6 ohm resistor. This gives a ratio of 390/3k6 = 0.108. Okay, no problem. That sounds reasonable.
With S2 in its second position, there are two 390 ohm resistors R11 and R13 in parallel. Since they are equivalent, Reffective = 1/2* R or 195 ohms. On the other half of S2, there is an 11k resistor and a 3k6 resistor in parallel, so 1/Reffective = 1/11k + 1/3k6 = 2712 ohm. 195/2712 = 0.07, which is an even smaller value than the 0.108 from before and quite far off from the 0.47 value quoted on the page.
With S2 in its third position, we then have a 3k9 ohm resistor in parallel with the capacitor and a 390 ohm resistor in series with it. Because the resistors are in parallel with each other, the effective resistance is 354 ohm. The other half of S2 at position 3 uses two 3k6 ohm resistors in parallel, with an effective resistance of 1800 ohm. 354/1800 = 0.197. This, even being a "high" level of crossfeed, is still well below the quoted 0.47 ratio.
What am I doing wrong? Any help is appreciated.
You're forgetting that looking at the schematic only gets you the R'1, R'2, C'1 and C'2 values. You'd still have to calculate R1/2 and C1/2 from there by solving the formulas given for them.
Looking at these things with known source and load impedances may prove more enlightening:
A DIY Headphone Amplifier With Natural Crossfeed
(It's a pity that Headwize was relaunched only to drop off the map once again less than 3 years later.)
Looking at these things with known source and load impedances may prove more enlightening:
A DIY Headphone Amplifier With Natural Crossfeed
(It's a pity that Headwize was relaunched only to drop off the map once again less than 3 years later.)
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