I’m seriously considering building a Crack-based preamp to feed my solid-state power amp. I’m trying to understand how the basic Crack circuit works so I can make reasoned changes to it to optimize it for preamp duties. Since I’ve never tried to understand the Crack circuit before, I’m scratching my head a bit. I have some questions about the redesign that I need some help answering.
1) In the normal Crack build, there are two 2.49k 1/8W resistors soldered across the headphone jack itself. From looking at the schematic, I conclude that these are the resistors shown connecting the load-side of the output coupling cap to ground. So the reason there’s two of them on the jack is that they’re connecting the “hot†for each channel to ground on the jack which is electrically equivalent to connecting the load side of the coupling cap to ground. To re-create this with RCA jacks, I would simply connect these resistors between the hot and ground on each channel’s jack, right?
2) Since the input impedance of the solid state amp is 47k Ohms, I know I can seriously downsize the output coupling caps. Searching the Bottlehead archives, I saw a recommendation to use the V-Cap online coupling cap calculator and plug in a desired -3dB frequency of 5Hz. The calculator says 0.68uF for 47kOhm and 5Hz. Does that really mean I can go all the way down to a 0.68F cap so long as I meet or exceed the 160V requirement called out in the Crack schematic? I certainly hop so as a cap that small will be both much easier to fit and MUCH easier to afford!
3) I’m trying to confirm which is the final output capacitor in the power supply, since I intend to use a film cap. Comparing the schematic to the build instructions, it appears as though the B+ wire from the tube sockets connects to the power supply at terminal 13U. This would mean that the final power supply cap is the one that spans 12U and 13U. It’s the “one of these things that’s not like the others†in that it lays over parallel to the base plate rather than sticking out from the plate at 90* like the other two, correct?
4) I’m not sure there’s easy room to fit 3 pairs of input RCAs, 1 pair of output RCAs, a selector switch, and the Khozmo attenuator I’d like to use into the stock Crack layout without getting uncomfortably tight with the wiring in several places. Because of this, I’m considering building a small box to house the attenuator and selector switch. I guess this would effectively make the small box a passive pre with the Crack functioning as a buffer between the passive and the power amp. Is this at all a good idea? If so, do I continue to use the 100kOhm-nominal impedance attenuator value that the Crack uses or do I switch to a lower impedance attenuator because I’ve moved it into an external box with a set of short interconnects between?
Thanks guys
