RCA phono stage

[Janos]

I have been working on the RCA phono stage (got the schematics from
the web, forgot from which site) for quite a while. I've been plagued
by bad luck, loads of it, partly because I have been using so many
scavenged items.
I decided to rework it substantially, and kept its "cabinet" (a
euphemism for that Swiss-cheese looking shack) for the power supply,
and built a new one for the amp itself.

The PS is a 5T4 rectifier paralelled by ultrafast solid state diodes,
filtering is 165uF-500R-165uF-4H-165uF*-2H-165uF**-1.5H-165uF***.
*: takeoff for cathode folower B+
**: takeoff for second section of 12AX7 B+
***: takeoff for first section of 12AX7 B+

It took a while to come up with this PS. I had originally a big choke
in the PS, that was acquired by dumpster-diving, so I had no idea
about the inductance. I have decided to ditch this choke, and after
several Duncan amps simulations I arrived to this filtering scheme. I
had to put the 500R in the first section, otherwise I saw mA-range
oscillations of several second length, though the ripple was still
below nA at the last stage. Using 500R took care of the slow
oscillation - at least in the simulation.

I have decided to add an LC section with big chokes between the amp
stages, for good decoupling. I've used the 330uF Aerovoxs (two in
series) to get the 165uF. I had to put two in series, because they are
rated @ 400V, and my B+ is around 430V.

The PS looks very crude on the inside, as it's wired with ga 12 copper
wire, and the ground is ga8 copper wire. The wires are so tough they
hold in place all the chokes and the caps!

I'll post later the original schem, the schem with the cathode
follower, and the PS schematics - in case I manage to put it
together...


Janos