The value of C40 is way too low and is allowing the output of C40 to have a high impedance at hum frequencies allowing wiring to pickup 60Hz mains hum. If the hum is at 120Hz then the filtering of the bridge rectifiers will affect it.
affect it through . . . which wires ? op amp input? output? power rails??If the hum is at 120Hz then the filtering of the bridge rectifiers will affect it.
Subbed on board PS for external bench supply.Capacitor ESR is also an issue in filter circuits.
Try using a very low ESR capacitor(s). If that doesnt help, then the problem is with something else.
Did you mention how you are noticing the problem if you cant measure the ripple?
What made you think the ripple was a problem in the first place?
Both but 120 is stronger looking at the analyzer.Maybe the main filter capacitor following the rectifier bridge is low value or defective and is allowing a 120Hz level that is so high that the 12V regulator drops out of regulating at 120Hz.
Maybe the high level of 120Hz is feeding into nearby low level audio wiring.
It is important to know if the hum is 60Hz or 120Hz. Which is it?
Oh sorry.. connected the amplifier to a spectrum analyzer to try to find where the audible hum was coming from.Well no, i meant how did you know there was ripple not how you corrected it.
How did you measure the original ripple or know it was there before you subbed in the bench supply. What made you want to sub the bench supply.
Ok then if you can discover the sensitivity of the spectrum analyzer then you should be able to build a little op amp amplifier to amplify the AC component and measure it with a meter or scope.Oh sorry.. connected the amplifier to a spectrum analyzer to try to find where the audible hum was coming from.
Can you explain the small resistor purpose?Your GND goes to TF tap
Your Full Period Rectifier goes immediately to "2.2mF" ** . . .
. . . and to "3-terminals"
** -- there might be some resistance - either before and/or in series - to protect the rectifier diodes and
possibly the components "down the flow" + *** . . . or combined filters
In addition to that the electrolytic cap.-s require ceramics for High frequency range improvement...
...there ought to be *** a small resistor before the 3-terminals and
another .1u ceramic just at it's input to improve the transient response
nothing serious - just the potential noise sourcses . . .
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by Robert Keim
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