I bought a couple of autoformer modules for my line stage (schematic attached). Before, I was using a 47k resistive attenuator. I replaced it with the autoformers. The first problem is that it picks up a tiny bit of hum. I guess that I could cure that with some mu-metal shielding.
Worse is that when I turn the switches on the attenuators, there are loud pops and swooshes. Also, sometimes the swooshes appear without turning the autoformers. Some kind of self-oscillation? I have been trying to use two different tube phone stages before the line stage, but with the same result.
I tried to put the autoformers on the output of the line stage. No pops and swooshes, but the first step above silent is pretty loud! Sure, I can reduce the sensitivity with a couple of resistors on the input of the stage, but I am not sure what I have gained compared to the resistive attenuator.
Any advice would be appreciated!
Problem with Autoformer Modules
Problem with Autoformer Modules
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- Line stage schematic
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Hey,
The hum suggests a bit of shielding is needed. The clicks and pops when switching is either a bad switch or DC on the switch. The swooshing sounds like an oscillation and a 100 ohm resistor at the grid pin of the D3A might help.
I think the real question here is why the d3a for a linestage? If when you place the autoformer at the output your first click (-42db) is too loud, it means you are already attenuation by a ratio of 100:1. This suggests you have way too much gain considering the d3a+4.5:1 stepdown nets you a gain of around 17.
have you tried the autoformers as a passive?
dave
The hum suggests a bit of shielding is needed. The clicks and pops when switching is either a bad switch or DC on the switch. The swooshing sounds like an oscillation and a 100 ohm resistor at the grid pin of the D3A might help.
I think the real question here is why the d3a for a linestage? If when you place the autoformer at the output your first click (-42db) is too loud, it means you are already attenuation by a ratio of 100:1. This suggests you have way too much gain considering the d3a+4.5:1 stepdown nets you a gain of around 17.
have you tried the autoformers as a passive?
dave
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