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In Reply to: Phono preamp - odd capacitor question - picture posted by PeterI on March 19, 2007 at 09:06:53:
Pete,Can you provide a topside picture of your preamp? I'm pretty sure that I've got the "same thing" 'cept some of the parts seem to be spaced a bit differently and mine uses "King" brand oil caps.
If we've got the same unit then your's has only a 1/2 wave rectifier (1 diode) with 60 Hz ripple - changing this to a full wave bridge will more or less double your filtering. Also the tube heaters aren't referenced to ground nor to a positive source. Adding a "synthetic center tap" - that is, two matched 100 ohm resistors in series across the heaters with the mid point where the resistors are soldered to each other either connected to ground or to a positive source through a voltage divider should help your noise level a bit.
Otherwise there really isn't much more you could do with this chassis 'cept perhaps add a choke to the power supply bolted onto the output side - but you'd have to experiment to determine whether the choke radiated more hum than it eliminated.
Follow Ups:
This is a minor phono preamp that was sold under different brands - I've see a Calrad version.There isn't a lot of room to work underneath.
I just thought if I got rid of the hum and replaced/upgraded caps it might sound decent. I will leave the silver mica caps alone.
Thanks
Pete
Hey Pete,Yep, the same wee beastie! So I stand by my previous recommendations - there's lotsa room to put a bridge rectifier in there - either one made up of discrete diodes or a prepackaged one. And creating a virtual centertap on the heater supply should help also. The side panel opposite the one pictured is where the output leads exit the chassis. This where you might be able to use a small choke to smooth the B+ some more - but if there is some hum from the choke you and if you don't use both the "tape/phono" functions you could move the output to one of the pairs of RCA jacks, making the unit "tape or phono" only, and there wouldn't be anything for the choke's field to couple to.
Per both his pics, clearly THIS one has input on the right front & output on the left front, only power cord & chassis wire on the rear side.
The outputs are on the back. They are RCA male end leads. The 'front' (shown) has inputs for both Phono and Tape, with a switch in the middle. An odd little affair.
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