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In Reply to: Ping Allen Wright - eliminating hum in phono stages posted by Doug Flynn on February 11, 2006 at 21:13:56:
>I changed one part - I use 3V batteries to bias the 6DJ8 grid, so it's probably not fair to call it a FVP5A.....<That's no problem - battery or LM329 - that doesn't disquilfy it from being a FVP5A.
But there is a probem with your powersupply - and it's not one for this reason.
1/ The big bird flapping it's wings - if you look at the FVP-5A map again. you'll see it shows a SuperReg smack dang in the middle. This device has an output impedance of around 3 milliohms all the way down to DC - but your supply, no matter how many LC sections you have, has rather high Z at DC and in fact significant OPZ at all lowish frequecies and I doubt if you can stop the flapping without a SuperReg. Just try it.
The cascode first stage has zero powersupply rejection ratio )PSRR)- so anything on it goes right into the second stage's grid. This stage has some PSRR but not much - so it will kick the rail a bit and this goes right back to the gid and woop, woop woop etc.
2/ Hum. I cannpt belive you have any 50/60Hz on that B+ rail with all that filtering - so it has to be earth loops in the input wiring. You will justhave to work at correcting them...
Allen
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- )eliminating hum in phono stages( - Allen Wright 11:10:58 02/12/06 (0)
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