Modified Linkwitz Crossfeed PCB
There are several different publically-available crossfeed circuits. They all sound different, they have different levels of complexity, and some make demands on the circuits they're used with. I've settled on Chu Moy's Modified Linkwitz Crossfeed for amps I build due to its low complexity, good sound, and low requirements on the surrounding circuit. I'm happy enough with this circuit that I've made PCBs for it. Both the PIMETA and PPA amplifier boards have mounting holes that line up with corresponding holes on the crossfeed boards.
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A populated Modified Linkwitz Crossfeed PCB (v1.2)
The picture shows the kit as populated from the kit I offer. You will notice that two of the four C2 positions are left unpopulated. See below for the reason.
The current version of this circuit board is 1.85" × 1.6". The outer pair of mounting holes are 1.4" apart, and the inner pair are 0.5" apart.
I won't go into much detail about this circuit, since Chu Moy's article has all the info you need if you want to tweak the circuit. I do have a version of the schematic here.
Parts
The circuit uses four capacitors and 10-12 resistors, depending on whether you want single-level crossfeed or a high and low crossfeed setting.
In Chu Moy's article, he describes two different sets of part values, one for a low-impedance ("low-Z") version of the circuit, and one for a high-Z version. I prefer the high-Z version because I think the crossfeed circuit works better between a source and a headphone amplifier. The low-Z version is for use directly ahead of the headphones, a poor place for a crossfeed circuit, in my opinion. A bonus of using the high-Z version of the circuit is that its capacitor values are lower, allowing higher quality capacitor types to be used. If you really wanted try the low-Z version of the circuit, you would need to compromise on capacitor quality, since more compact capacitors generally sound worse. I will use the high-Z values exclusively below.
Not all capacitor lines have 0.12 µF capacitors in them, so C2 is split into 2 board positions in parallel, allowing you to use 0.1 µF and 0.022 µF to get 0.12 µF total. Since C1 is 0.022 µF, you need caps of that value anyway. (Pedants, take note: 0.1 µF plus 0.022 µF is not 0.122 µF, if you're dealing with tolerances of 2% or wider.)
The capacitor positions are sized to allow 50V Panasonic ECQP series caps (polypropylene and foil) to fit easily. Wima MKP-10 caps will also fit nicely, but they're metallized polypropylene so the Panasonics are preferred. With some careful lead bending and mounting, you can use the very large Orange Drop 715Ps, which some prefer to the ECQPs. If you want to cheap out a bit or you want to squeeze this board into a tight place, the board will accept polyester box caps with standard 5mm pin spacing.
The resistors have 0.3" pin spacing, so standard 1/4W and some specialty 1/8W resistors will fit. For the resistor values, see Chu Moy's article.
Pads
The IR, IL, and IG pads are right channel input, left channel input, and input jack(s) ground. I'm sure you can figure out what OR, OL, and OG are for. :)
The remaining pads are for the switches used to control the crossfeed. The switch pad naming scheme is:
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The first part tells you the switch, the second the pad function, and the third the channel.
Switch
The normal switch configuration for the crossfeed uses two DPDT switches. (See below for an alternate arrangement using a rotary switch.) S1 picks the crossfeed level, and S2 is for bypassing or enabling the crossfeed circuit.
If you only want one crossfeed setting, you can leave out the R1As and jumper from each S1C to the corresponding S1L.
Pad Function
If you use a standard DPDT toggle switch, the pad functions will line up so that you can put a "rainbow" of wires between the switch's solder lugs and the pads on the board. That is, the bottom-most lugs on the switch will be wired to the pads at the edge of the board, the top lugs to the pads farthest from the edge, and the middle lugs to the pads in between. See below for a picture showing this arrangement.
The functions on S1 are High crossfeed, Common, and Low crossfeed. The functions on S2 are Enable, Common, and Bypass.
Channel
This being a stereo circuit, there are two channels. It happens that channel 1 is the left and channel 2 the right, but most of the time it isn't important to know this.
Using a Rotary Switch
Although the modified Linkwitz circuit was envisioned to use a pair of DPDT toggle switches, Scott Lindeman came up with two arrangements that each let you use a single 4P3T rotary switch. The wiring is a bit more complex in each case and a rotary switch takes more room than a pair of toggles, but you may prefer the look of a crossfeed selector knob to that of a pair of toggle switches. The rotary setup also ensures that you can only pick one of 3 settings. The toggle switch method allows a fourth setting: with the crossfeed bypassed, toggling the high/low switch does change the sound, because the high setting attenuates both channels a bit.
The switches I've used from C&K and E-Switch have the "common" solder lugs in the center of the switch, labeled A thru D. Then there's a ring of 12 other solder lugs labeled 1 thru 12 surrounding the common lugs. Lug A connects to lug 1, 2 or 3 depending on the knob position, B connects to 4-6, etc. Here's what gets connected to what:
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Simple Way:
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Silent Way:
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The Simple configuration is electrically the same as using two DPDT toggle switches. The only trick is that some of the solder lugs on the rotary switch are connected to their neighboring lugs.
The Silent configuration is a variation on this which eliminates the click you get in the headphones when changing between crossfeed settings. It does this by ensuring that there is always some resistance between input and output; in the other configurations, this connection is momentarily interrupted when changing the crossfeed level, giving an audible click. The switch lugs which are not mentioned in the table are not connected. Also, you must tie S1H1 to S1C1 and tie S1H2 to S1C2 on the crossfeed board and change the R1B resistors to 6K ohms, assuming you use the high-impedance values elsewhere in the circuit. The low crossfeed setting is then this 6K resistance in parallel with the 2K high resistance, which gives a 1.5K overall resistance.
Mounting
The crossfeed mounting holes on the PIMETA board align with the outer pair of mounting holes on the crossfeed board. The holes in the PIMETA board don't have much space around them, so the diameter of the spacer you use can't be much larger than that of the screw you use; this rules out all nylon spacers I've looked at. I recommend Keystone's #4 aluminum spacers, distributed by Mouser. I include a pair of the 3/4" ones (Mouser stock #534-418) with each board, but you may want to get a different length, depending on your space requirements. I recommend using a round-head #4-40 1" long screw, available at any hardware store. The length assumes you're using a 3/4" spacer; there isn't much screw shaft left over in this setup. A 3/4" spacer allows just enough room below the board for 4× stacked buffers. I recommend that you put the nut on top of the crossfeed board, so that the nuts don't touch signals and pins on the bottom of the amp board.
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On the PPA board, you have considerably more flexibility. There are three mounting holes, which you use with the inner pair of holes on the crossfeed board; the two combinations allow you to choose which parts on the PPA board the crossfeed board covers. There is enough space around the holes in the PPA board to use nylon spacers, if you want. The tallest parts underneath the crossfeed on the PPA are generally 12.5mm C4 caps, so you can use shorter spacers than the ones I provide.
Although the board is designed to be mounted with the parts facing upward, you could mount the board upside down for a more compact arrangement. The downsides are that the switch wires won't make the neat "rainbow" to the front panel if you use DPDT toggles, and it will be harder to get at the parts underneath the crossfeed board.
Another alternative is to turn the board around so it hangs out over the space behind the amp like a diving board. This gives you maximum access to the parts on the amp board but it's the least compact arrangement and it requires very long wires to get to the front panel.
Hooking it Up to an Amp
I first build the amp without crossfeed, and I make the wires from the input jacks to the board a little long. Once the amp is working well, I cut the input wires roughly in half. Then I hook the wire halves going to the input jacks to the crossfeed board's "I" pads, and the remaining wires to the crossfeed's "O" pads. If the amp doesn't work right after this surgery, you know the amp itself is working, so the problem must be on the crossfeed board.
Depending on the enclosure setup, I sometimes don't do the toggle switch wiring until after I have the board mounted. This way, I can cut the wires exactly to the right length. If you wire the toggles to the board before you mount the crossfeed, you can end up with hookup wires that are too short or too long.
Circuit Notes
Some things about the circuit's behavior that you might want to know:
Attenuation
The Modified Linkwitz crossfeed circuit attenuates the signal going through it by roughly 6-10 dB, depending on the source you use. (The variance is probably due to different source output impedances.) You need to set the amp's gain 2 to 3× higher than you would if there were no crossfeed in the circuit to counteract this attenuation. Try 2× first, and only go up to 3× if the amp still won't drive the headphones loudly enough in your setup.
Bypassing
The term "bypass" is misleading, since the crossfeed isn't completely bypassed. If it were, the amp's output voltage would jump by 2-3× when you flip the switch into the bypass setting. Instead, what the bypass setting does is just makes the impedance between the channels much higher than the path that connects the each channel's input to the output. Therefore, the signal "prefers" the direct path from input to output. There is still a tiny bit of channel mixing, but it's a necessary compromise to avoid the volume jump.
How to Get It
I offer the board plus standoffs or a more complete kit that also includes the passive parts in my Parts Shop.