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In Reply to: RE: SET Headphone amp posted by OldNuke on August 08, 2016 at 01:39:34
A little bit closer to being done.
Cured the buzz by using a .47uf ceramic cap to ground on the filament ground of the input tube.
Added BCP-15+ for plate chokes, at this point I'm at 6mV and 7.2mV hum. Took the padding off the volume pot and put 33 ohms on the input of the TRS jack (all I had in the garage). Still have a little hum in the quiet passages and when input is paused. Ordering more resistors to find the right listening level.
Follow Ups:
Is the input wiring shielded?
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Buy Chinese. Bury freedom.
Yes,
The input wire is a shielded twisted pair with the shield wire connecting RCA inputs ground on one end and star ground on the other.
Well, I think I fixed the issue but I didn't solve the issue. I added a -8db pad onto the TRS jack (38K in series, 49K to ground) that cured all the hum problems. Completely black background on my Oppo PM3's, no hum anywhere.
The pad is a little too much, my first 5 clicks on the volume pot are useless and the max volume is just below the "rocking out" level I want so I'm going to try a -1 or -3db when the grid chokes I ordered come in. I know this is another source of possible hum but I never thought this would be easy :)
Going to try something I saw on the Bottlehead big headphone amp and route the inputs thru grounded copper pipes and relocate my setting switches as far away from the power transformer and the 404's as possible, shouldn't hurt and will make the amp pictures more interesting.
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