It seems this is the biggest cause of the hum in the preamp tubes. Not as big a change as lifting the Line Out, but much improved. It retained much more detail and clarity and less sag. This change made it even closer to the Crack. This allows for a much more steady biasing voltage. I simply removed the 1K cathode biasing resistors on the 6SN7's and replaced with LED's just like the Crack. This is a fairly simple mod and also one I borrowed from the Crack schematic.
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). After removing the Line Out loading issue, they were close to the same given the same tubes. I compared it to a Crack initially and the DV was much darker. This change simply lifted the "veil" on this amp. 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. 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. 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.įor the DV circut, the high pass cutoff frequency would be about 0.53 Hz.įor the Crack circuit, the HPF cutoff frequency would be about 0.64 Hz.īoth use the same design, but the Crack uses a higher value cap and a lower value resistor, but achieves the same result. The 0.47uf series cap and the 2K resistor to ground cause some type of loading to the output. I will start this thread to discuss different mods to this or any similar designed OTL cathode follower amp. I know there is much to discuss about Crack mods, as many of them they sell directly. Let me be the first to start this thread.
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