GLM wrote:
adding a bypass cap works up to a point. you seem to have removed all
the ripple a cap could remove.

From the waveform, it sounds like a bad isolation issue: adding caps
will help but you'll need much more than nanofarads to get that slow
ripple out. Use bigger caps if you want to keep it to simple cap
patching (something like 1100uF or 2200uF is nice.


The slow ripple is definitely not via the HT - one a scope after the RC
filters it is as quiet as a mouse.

Personally, I prefer the capacitor array method to the simple addition
of single poles.

Power supply would go as such:

XFO - DIODE BRIDGE(tube rect for purists..)- Pi Filter (Shunt cap
+choke + shunt cap)

At equal main's frequency attenuation, the tradeoff is the filter's
inductance vs instant current draw. I usually lift off that limit by
using bigger chokes and a few (BIG) tank capacitors right after the pi
filter (I like arrays of 1100uF,2x2200u,3300u...you can make a nice
little square with 1-sided pcb routing if you put 2 together...little
footprint, nice instant draw compensation).


Don't forget this is a mic pre not a power amp so current draw is less
than 10mA and as its class A it is also constant.


Cheers

Ian