Due to the weight and size of my 65 Vibrolux, I decided to replace it with a Princeton 65 Solid State. It works well considering it's a disposable amplifier; however, the distortion channel is rather hissy, especially at higher gain levels, where the amp shines somewhat.
Looking at the schematic, two problem sources are evident. The first one is high resistance into the non-inverting input of U2A. The other problem is that U2 is a bipolar opamp. Perhaps a JFET input type may help, but the total of all those resistors will still be an issue. I know this from past experience.
To remedy the high resistance is not as simple as reducing resistance values and increasing capacitance values for the equivalent roll-off, because of the U1A circuitry. I have never seen anything like this before. It is not readily intuitive, at least for me.
Would anyone have an idea that doesn't require completely redesigning the input section? I would consider substantial modifications to the board, however. Here's a blowup of the input area:
Here's the entire schematic:
View attachment Princeton_65_Schematic.pdf
Any suggestions are appreciated.
Looking at the schematic, two problem sources are evident. The first one is high resistance into the non-inverting input of U2A. The other problem is that U2 is a bipolar opamp. Perhaps a JFET input type may help, but the total of all those resistors will still be an issue. I know this from past experience.
To remedy the high resistance is not as simple as reducing resistance values and increasing capacitance values for the equivalent roll-off, because of the U1A circuitry. I have never seen anything like this before. It is not readily intuitive, at least for me.
Would anyone have an idea that doesn't require completely redesigning the input section? I would consider substantial modifications to the board, however. Here's a blowup of the input area:

Here's the entire schematic:
View attachment Princeton_65_Schematic.pdf
Any suggestions are appreciated.