Have you looked at the rails on a scope to see if there is a problem or not?
I suspect something more like grounding/layout/wiring configuration tbh rather than ripple on the rails. The LM317/337 should get you 99.9% of the way to a pretty near perfect supply.
Identify the problem by measurement first would be my advice.
I suspect something more like grounding/layout/wiring configuration tbh rather than ripple on the rails. The LM317/337 should get you 99.9% of the way to a pretty near perfect supply.
Identify the problem by measurement first would be my advice.
I would say it should not be necessary tbh, at least not to get an audibly hum free design.
It is sometimes very difficult to get a measurement set up that is hum free. Do you see zero hum if you connect your measuring probe tip and ground lead together? with no connections to anything else. Just the test leads shorted together.
If you now connect the still shorted tip to the ground point of the power supply (lets say C5/C6 junction) you should still see absolutely zero noise. If you see noise at this point then you have a measurement artefact.
(The LM series regs should remove all low frequency 100Hz ripple to the point of almost nothing being present but it all relies on perfect layout. If you have any reservoir cap charging currents then that will dominate the results and cause a problem)
It is sometimes very difficult to get a measurement set up that is hum free. Do you see zero hum if you connect your measuring probe tip and ground lead together? with no connections to anything else. Just the test leads shorted together.
If you now connect the still shorted tip to the ground point of the power supply (lets say C5/C6 junction) you should still see absolutely zero noise. If you see noise at this point then you have a measurement artefact.
(The LM series regs should remove all low frequency 100Hz ripple to the point of almost nothing being present but it all relies on perfect layout. If you have any reservoir cap charging currents then that will dominate the results and cause a problem)
The reservoir cap size really depends on the overall current draw. For a preamp taking little current the caps can be surprisingly small. 1000uF is probably plenty.
That pretty much rules out excess ripple and suggests more of a layout construction problem. Those kind of things are very difficult to diagnose at a distance... you really need to be looking with a scope and seeing exactly what is going on both with the PSU and the preamp itself.
That pretty much rules out excess ripple and suggests more of a layout construction problem. Those kind of things are very difficult to diagnose at a distance... you really need to be looking with a scope and seeing exactly what is going on both with the PSU and the preamp itself.