diytube.com

Min -

It's always good to keep the AC away from the inputs
as far as possible when doing a layout. Since you have
a small box, everything is pretty close. Aluminum isn't
much of a shield against magnetic fields. You notice in
the original Shure picture, the transformer was on the
outside of a steel chassis, and the tubes had shields.
You could probably easily rework your box to move the
transformers outside, then move the diode/cap strip
over behind the transformers, instead of near the
input connector. I might also move the outputs to
the far set of jacks, proximity of input to output is too
close for me. Try Ed's "unplug and listen" suggestion too.

Preamps are probably the trickiest ... and I do mean
"trickiest" since the ubiquitous Lafayette stereo preamp
which sold for about $ 12.95 when I was a kid was
a gutless wonder, but of course they are bringing $$$
on eBay these days.

There are two good choices:

1: Bottlehead PT-1 (which was specifically designed for their "seduction" phono stage usage).

120 VAC primary, secondaries are 250VDC @ 20 mA and 6.3VAC @ 1.2A RMS. It has magnetic shielding. $40 plus shipping.


scroll down this page:

http://www.bottlehead.com/et/parts/parts.htm


I've used these in a Seduction phono stage and in a modified "foreplay2" linestage, with DC filament power, and they are very good!



Hammond 162C6, available from AES as PT-C261C6:

115VAC primary, Secondaries are 250VDC @ 45 mA and 6.3 VAC @ 1A.

Somewhat larger and heavier than BH's PT-1, and it has no magnetic shielding. $31 plus shipping.

HTH

/ed B in NH

Thanks, Tom and Ed. Yes, boy I am glad that my first project wasn't a preamp. I probably would have quit making amps and wouldn't be so broke. I did juggle around to have the circit board above and trannies inside. I grounded, all except the inputs, at where caps are grounded. I also ran pin 9 to the ground for the filament PS. Cleaned up all around with AC lines tucked away. To my dissapointment the hum got worse. The hum increases again w/ volume and my power amp started to buzz when it got to a certain volume. I did what Ed suggested. The hum goes dead while caps were discharging. Where can look next? I am going to go over the wiring again meanwhile. -Min

One possible approach is to build the PSU in its own separate chassis, and then bring the B+ and DC filament voltages over to the phono stage via a 4-wire shielded cable (B+, Power Ground, and two for the DC filaments, and maybe a fifth chassis ground wire or shield). This cable can have a plug and socket, or just run it direct.

This keeps all of the AC power out of the preamp, and any hum after that has to be an external ground loop or interconnect problem.

This is where those cast-aluminum chassis boxes come in real handy, they're relatively inexpensive, readily available and very easy to drill and punch.

I have some more related info over in the "confusing schematic" topic.

HTH

/ed B in NH

Apparently the diodes in the filament PS were bad. I recycled those from other project and I guess they got soldered often enough that the heat finally go to them. The amp run pretty quiet with new diodes. I hear what sounds like hum or noise from the cartridge at max volume. Even then it's not too bad. No hum at max tolerable listening volume. And yes, I added some cosmetic changes. With a wood base, the amp might actually look nice.

http://bp2.blogger.com/_i08-ibvZ150/Rv2rMVVajdI/AAAAAAAAANQ/twt8bKu9Idg/s1600-h/S5032911.JPG