living sounds
Well-known member
I've modified the inputs on my console per the attached schematic.
AC 2 in this case denotes the output of a 4:1 input transformer.
R4 sets input impedance to 10k ohm.
R3 is the gain pot wired as a variable resistor.
C6 makes sure the circuit is stable, but also adds a bad sheen to the high end (that doesn't show up in THD measurements but is nonetheless unacceptable. Leaving C6 out I get oscillations in the following EQ stage (with gear connected to the balanced input). Increasing R1 to a higher value can again mitigate that, but doesn't sound good either (and affects frequency response). I also don't like the idea of RF getting to the op amp input in the first place.
My guess would be that C6 interacts with the transformer. I tried mitigating that by experimentally increasing R2 and lowering C6, but the result is pretty much the same as with the circuit in the schematic.
So how to you actually add an effective RF filter to a transformer input without it audibly affecting performance? I guess I could add an addtional balanced input stage, but that would be a massive overhaul on an already badly beaten board...
Thanks!
AC 2 in this case denotes the output of a 4:1 input transformer.
R4 sets input impedance to 10k ohm.
R3 is the gain pot wired as a variable resistor.
C6 makes sure the circuit is stable, but also adds a bad sheen to the high end (that doesn't show up in THD measurements but is nonetheless unacceptable. Leaving C6 out I get oscillations in the following EQ stage (with gear connected to the balanced input). Increasing R1 to a higher value can again mitigate that, but doesn't sound good either (and affects frequency response). I also don't like the idea of RF getting to the op amp input in the first place.
My guess would be that C6 interacts with the transformer. I tried mitigating that by experimentally increasing R2 and lowering C6, but the result is pretty much the same as with the circuit in the schematic.
So how to you actually add an effective RF filter to a transformer input without it audibly affecting performance? I guess I could add an addtional balanced input stage, but that would be a massive overhaul on an already badly beaten board...
Thanks!