Rythmsandy, there are lots of possible reasons you may be measuring some residual hum.
Just applying a scatter gun list of changes, or coming across a thread where someone says they improved hum by doing such and such, and then copying that change, may achieve the solution you want - but imho if any of those changes you list actually did lower hum and noise then it would appear to be by luck.
Can I suggest you do some simple tests to try and localise the source of hum, and provide a schematic and photos of your amp and measured results. That my allow others some awareness of what you have in front of you, as you appear to have hijacked this thread and are using a significantly different power supply scheme than the OP.
Just applying a scatter gun list of changes, or coming across a thread where someone says they improved hum by doing such and such, and then copying that change, may achieve the solution you want - but imho if any of those changes you list actually did lower hum and noise then it would appear to be by luck.
Can I suggest you do some simple tests to try and localise the source of hum, and provide a schematic and photos of your amp and measured results. That my allow others some awareness of what you have in front of you, as you appear to have hijacked this thread and are using a significantly different power supply scheme than the OP.
Hi Robbins,
Thank you very much for the reply. The problem what the OP has the same one as what I have, hence I see both might have done same mistake and have the solution.
Infact the trick happened to be with .47uF across the rectifier reduced the problems to a very substantial level. the slight hum might be because of transformer is coupling into the circuit. I have all connected on the test bench just in open air. PCB of the preamp stage is just as in one piece.
Thank you very much for the reply. The problem what the OP has the same one as what I have, hence I see both might have done same mistake and have the solution.
Infact the trick happened to be with .47uF across the rectifier reduced the problems to a very substantial level. the slight hum might be because of transformer is coupling into the circuit. I have all connected on the test bench just in open air. PCB of the preamp stage is just as in one piece.
Something else that was only mentioned briefly in one post herein (and yes, I did scan the whole 7 pages 
) -- the Quasimodo/Cheapomodo threads. Mark continues to support them, even several years on and with humor, as new DIY-ers discover the technique.
No need for high voltage probes or insulated gloves; but you do still need an o'scope. And it arrives at an exact solution for each transformer/rectifier combination, without so much as a molecule of math.
trobbins point is good, too -- IMHO: The cap-input filter poses a different-enough challenge to merit its own thread, despite the similarity of symptom.
Cheers,
-R
) -- the Quasimodo/Cheapomodo threads. Mark continues to support them, even several years on and with humor, as new DIY-ers discover the technique.No need for high voltage probes or insulated gloves; but you do still need an o'scope. And it arrives at an exact solution for each transformer/rectifier combination, without so much as a molecule of math.
trobbins point is good, too -- IMHO: The cap-input filter poses a different-enough challenge to merit its own thread, despite the similarity of symptom.
Cheers,
-R
the problem is solved as there was slight hum induced in the heaters because of the ac heater. Moved to DC heater with lm317 now and the whole hum is gone on the scope I dont see the waves anymore but a 0.25mv or so noise band. I guess that will also gets reduced once everything is put in chassis.
Will start a new thread as it would be useful for many.
Will start a new thread as it would be useful for many.
Heavy filtering and DC heaters solves the day again!
I would have suggested moving your probing of the DC far away from the setup to see if the hum was EMF induced. This can be a problem in phono stages and other low-level sources with induced interference from power transformers and initial filter chokes.
Also, to alleviate the buzz (not hum) issue completely, you can try Schottky rectification. I've had very good luck recently using the low voltage standard (SB5XX series) and high-voltage SiC schottky (Vishay and others) rectifiers in designs. I continue to use bypass capacitance, but it may not be necessary with these devices.
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