Let me be the first to start this thread. I know there is much to discuss about Crack mods, as many of them they sell directly. I will start this thread to discuss different mods to this or any similar designed OTL cathode follower amp. Mods I have done to my Darkvoice 336: 1. Disconnect the line output. The 0.47uf series cap and the 2K resistor to ground cause some type of loading to the output. If you look at the Crack schematic: [REDACTED] You will see there is no line out, but it has a similar headphone output with a decoupling cap of 100uf in series and a 2.49K resistor to ground. For the DV circut, the high pass cutoff frequency would be about 0.53 Hz. For the Crack circuit, the HPF cutoff frequency would be about 0.64 Hz. Both use the same design, but the Crack uses a higher value cap and a lower value resistor, but achieves the same result. But somehow the extra capacitance and resistance to ground on the DV moves up the HPF to 1.2Hz, but also somehow contributies to somehow loading the output. So for mine I disconnected the Line Out all togehter, but one could simply remove the connector to ground essentially taking that part of the circuit out of the equation. This change simply lifted the "veil" on this amp. I compared it to a Crack initially and the DV was much darker. After removing the Line Out loading issue, they were close to the same given the same tubes. The Crack might have been slightly better given better components and design overall (better transformer, correct heater voltage, and less resistance on headphone out, even if it did have an electrolytic cap on the output). 2. LED cathode bias on the preamp tube This is a fairly simple mod and also one I borrowed from the Crack schematic. I simply removed the 1K cathode biasing resistors on the 6SN7's and replaced with LED's just like the Crack. This allows for a much more steady biasing voltage. This change made it even closer to the Crack. It retained much more detail and clarity and less sag. Not as big a change as lifting the Line Out, but much improved. ================================= Other ideas for DV Mods: 1. Fix the high 7V heater rails. It seems this is the biggest cause of the hum in the preamp tubes. Some 6SN7's are more sensive to high heather voltages and why different tubes as well as "burning in" or leaving it on can cause the hum to go away. One person fixed it by converting the AC heater rails to DC. This would work and would actually likely knock it down closer to 6.3V, but might cause some extra strain on the transformer as the specs on the transformer are unknown and may not have suitable amps for this section of the transformer. 2. Take out the LED on the heater rail and replace with a lamp. There is a safer way to incorporate an LED on a heater rail, but this is a bad implementation. There are better configurations, but I would almost rather take it out the LED all together. 3. Drop the 220 ohm resistor in series with the plate of the 6SN7. This will only increase the plate voltage and increase the headroom. There is already a 30K resistor in line there that will do just fine. Still not sure if taking this 220 would change the bias or operating point of the 6SN7. There are other mods too, but most of those are replacing components (Alps pot, MKP caps, etc.). Any other ideas?